Facilities > VLA > Documentation > Manuals > OPT > Frequency Band Nomenclature and Using the Resource Catalog Tool (RCT)

Frequency Band Nomenclature and Using the Resource Catalog Tool (RCT)

« Return to page index

Observation Preparation Tool (OPT)

1. Orientation and Nomenclature of Frequency Chunks in the OPT

Assuming you already have successfully logged in to the OPT web application, and assuming that there is no message in the Important message banner that makes you decide to abandon the OPT for the moment, look for the navigation bar at the top. If Instrument Configurations is not in bold face, but in normal font and underlined, click it with your mouse button to navigate to the RCT (Figure 3.1). To exit the tool properly use the Exit link in the upper right corner or with FILE - EXIT; do not kill the browser window/tab.

A short introduction to the layout of this tool's page has been given in the introduction (Chapter 1). There should be at least one NRAO Defaults catalog visible in the left hand side column, the catalog browser. Like for the SCT, for orientation and to get a feel for the tool(s), it is instructive to walk through this catalog. After this orientation it should be almost intuitive to create your own personal resource catalog(s) which you will use in your project's SB scans or help to understand how to use one of the standard wide band resources provided in the NRAO Defaults catalog. Note that a resource catalog for each of your successful proposals may be pre-filled (if you are the PI or contact for the proposal); it is important that you check the pre-filled information for correctness.

Figure 3.1: Web browser screen shot of the RCT opening page.

 

Example of a Resource Catalog: NRAO Defaults Catalog

Be aware that data from the WIDAR correlator is different from the old VLA correlator in the sense that data is always delivered in spectral line or pseudo-continuum mode, similar to Very Long Baseline Interferometry (VLBI) practice. When referring to continuum below, it is meant to refer to data taken for wide band observation purposes: the data itself is divided in frequency channels, but the scientific interest is in the data averaged over all channels and not in individual channels with line emission (or absorption). The latter is referred to as spectral line data. This is the difference in obtaining a two-dimensional image of the sky versus a three-dimensional image cube, where the data retains that different frequencies show different (two-dimensional) sky images.

The best continuum sensitivity is obtained using the maximum available bandwidth in the most sensitive part of the observing band, and thereby avoiding Radio Frequency Interference (RFI) as much as possible. The resource which gives the best performance in each observing band is defined in the NRAO Defaults catalog. To describe the setups, is it useful to understand how the basic generalized path of the radio frequency (RF) signals collected by the receivers in the antenna are delivered through the intermediate frequency (IF) electronics to the WIDAR correlator and where the correlated data ends up in a data set.

baseband

Figure 3.2: Simplified schematic of nomenclature and the involvement of the 8-bit and/or 3-bit sampler choice in yielding one 1 GHz or two 2 GHz basebands for each of the two independent IF paths (AC and BD) from the (up to) 5 GHz down-converted RF bandwidth delivered by the receivers. Per IF path an 8-bit or a 3-bit sampler can be chosen independently; a baseband pair consists of RCP and LCP signals (or linear polarization X and Y) at the same frequency. After being defined at the antennas, these baseband pairs (A0C0) or (A1C1 and A2C2) as well as (B0D0) or (B1D1 and B2D2) are then transported over optical fiber to the WIDAR correlator in the control building for processing.

 

Baseband Pairs

The receiver in the antenna passes (up to) 5 GHz down-converted frequency of the RF receiver bandwidth to four signal paths (Figure 3.2); two right circular polarization signals (RCP), labeled IF A and IF B, and two left circular polarization signals (LCP), labeled IF C and IF D. IF A and IF C (i.e., one RCP and one LCP) signals are tuned to the same RF frequency and thus may produce a Stokes I signal from the source. IF B and IF D are also tuned to the same frequency, which typically is not the same tuning as for IF A and IF C. These IF signals are then sampled independently using 8-bit samplers or 3-bit samplers. The Observational Status Summary Sampler page should aid in which sampler to use for your observations. The 8-bit samplers each yield a one 1 GHz wide frequency range containing a corresponding 1 GHz down-converted RF range. The 3-bit samplers each yield two 2 GHz wide frequency ranges containing two corresponding 2 GHz down-converted RF ranges. Per IF path (AC or BD) the two 2 GHz ranges from the 3-bit samplers must be within a total range of 5 GHz and are typically placed to yield a continuous 4 GHz RF bandwidth per IF path, or an 8 GHz RF bandwidth total.

The individual sampled frequency ranges are referred to as basebands, in particular baseband pairs when a combination of simultaneously tuned RCP and LCP signals is involved. The 8-bit samplers yield 1 GHz baseband pairs which are labeled A0C0 or B0D0, depending on the original IF path. The 3-bit samplers produce 2 GHz baseband pairs labeled A1C1 and A2C2 as sampled from IF path AC, or B1D1 and B1D1 if the signals are sampled from IF path BD. These baseband pairs are then transported over optical fiber from the antennas to the correlator.

Part of setting up the resource is to specify which samplers are used and to specify the baseband pair center sky frequencies.

Subband Pairs

When the basebands from each antenna reach the correlator room, they are fed in 128 MHz bandwidth intervals into station boards. This regular pattern of 128 MHz creates a fundamental interval boundary which cannot be observed, nor included in processing of nearby frequencies. Apart from the baseband edges, there are 7 of such unobservable frequency boundaries per 1 GHz (1024 MHZ) baseband when using the 8-bit sampler, and 15 per 3-bit sampler baseband (i.e., per 2 GHz, per 2048 MHz). Note therefore that, since this is an odd number, the chosen baseband center sky frequency never can be observed: do not place the baseband center at the frequency of your spectral line! From each 128 MHz chunk, the station boards determine which part (central frequency and frequency width) is forwarded to the correlator for processing. That is, per polarization for each 128 MHz bandwidth it is determined whether the signal should be forwarded to the correlator, and whether each 128 MHz bandwidth should be divided in powers of 2 and tuned to another center frequency within the 128 MHz range, provided that the frequency interval does not cross the boundary when forwarded to the correlator.

The filtered and tuned frequency ranges delivered by the station boards are referred to as subbands, in particular subband pairs for simultaneously tuned RCP and LCP signals. The individual subbands are at most 128 MHz wide, and independently tunable in frequency if reduced in width by powers of two without crossing the 128 MHz boundaries. Per resource, up to 64 subband pairs can be defined.

Part of setting up the resource is to specify the frequency tuning and frequency width of the subbands that are to be used.

correlator

Figure 3.3: Simplified schematic of nomenclature of correlator components. Up to 64 baseline board pairs process the baseband pair streams from the antennas as formatted by the station boards in four quadrants (Q1-Q4). A resource in the RCT defines the output of the station boards (after defining the baseband pairs at the antennas) and the assignment of the available baseline board pairs for processing to yield up to 64 independently configurable subbands with spectra. These subbands will end up as a simultaneously observed subset of spectral windows (SpWs) in the visibility data. At most one subband (of 128 MHz or less bandwidth) can be processed per baseline board pair, but more than one baseline board pair can be used to process the same subband (called baseline board stacking), yielding a larger number of channels to obtain an increased spectral resolution over the bandwidth of that subband (in the case without recirculation).

 

Baseline Board Pairs

The resulting subband pairs per antenna and IF path are presented to one of the four correlator quadrants for processing by pieces of hardware known as baseline boards, or baseline board pairs when the subband contains both RCP and LCP signals (Figure 3.3). There are 16 baseline board pairs per correlator quadrant.  A single baseline board pair can only receive data from a single subband for processing. Per baseline board pair 256 correlation products can be computed, where the number of products is the number of polarization products (1, 2 or 4) times the number of spectral frequency points (256, 128 or 64). Within the limits of the number of baseline boards in a correlator quadrant, more than one baseline board can be assigned to process a single subband pair (thus up to 16) at the cost of processing other subband pairs. Assigning more than one baseline board to a single subband is referred to as baseline board stacking. Without recirculation, the combination of subband width, number of polarization products and number of baseline boards determine the channel frequency width of the data in the subband. The Observational Status Summary contains more details about the WIDAR correlator.

Part of setting up the resource is to specify the distribution of the computing power of the baseline board pairs over the active subbands.

Spectral Windows and SDM/BDF Data Archive

The correlated data consists of up to 64 independently tunable (center frequency and frequency width) and configurable (polarization and spectral frequency points) subbands per observing resource. This data is written as Binary Data Format (BDF) files to the archive, together with header and auxiliary information defining the corresponding Science Data Model (SDM) for the observation. Multiple resources can be used during an observation, and therefore many more than 64 subbands can be in the data; subbands contained in the SDM/BDF are called individual spectral windows (SpW) in CASA (or IFs in AIPS). CASA can process non-homogeneously configured spectral windows simultaneously, but care must be taken in the interpretation of spectral windows versus subbands when referring to an observing resource: any resource can have up to 16 or 64 subbands (for 8-bit and 3-bit respectively) but a data set may contain hundreds of spectral windows (from multiple resources).

OPT figure 2.2

Figure 3.4: Web browser screen shot of the RCT when NRAO defaults is selected.

NRAO Defaults

The NRAO Defaults catalog (Figure 3.4) is a collection of hardware and instrument configurations (front-end receivers, correlator integration time plus observing/subband bandwidth and frequency channels, frequency tuning, etc.). They are expected to be good standards for wide band continuum observations using the VLA.

The NRAO Defaults catalog is in red italics and has a plus-icon in front of it. Remember that this means that this catalog is read-only and has groups. Very much like was explained for the SCT tool in a previous section, if you click the plus-icon (xpnd) or "NRAO Defaults" these groups will appear in the catalog tree.  Similarly, clicking "NRAO Defaults" differs from clicking the plus-icon in that it will expose the total content of the catalog in the main window, with 25 sources per page, starting with a pointing resource group. As there are more default resources than 25 for the A/Any config group, the top and bottom of that table displays a small page navigation menu.

Pre-defined resource groups in the NRAO Defaults catalog are Pointing setups and a number of array configuration dependent groups.  During the commissioning of the correlator, different commissioned resources were tied to the array configuration dependent groups. When a group is highlighted or selected using the mouse button, the right-hand side window with the contents will only show (filter) the resources which were grouped in this sub-catalog. For example, selecting the A/Any config group will now only list the NRAO default resources for wide band observations in the A array configuration (any band). Similarly, the Pointing setups will show the NRAO default resources for pointing scans in C and X band (see below).

Each line in the table represents one resource with a name and some descriptive information. A line starts with a tick-box and an edit icon (Edit Source). The tick-boxes can be used to select one or more entries in the catalog for copy/paste as described in the SCT catalog chapter. Selecting and copy/paste has to be redone for every page. The Edit Source edit icon is used to access the details of the resource entry in the catalog, i.e., the specifics of the resource of interest. Here it will be a NRAO default resource, but later this might as well be the specifics of your scientific target resource, and the information contained in these entries therefore may be slightly different from entries in a personal source catalog created by an observer or the automatic PST to OPT pre-filler.

Click NRAO Defaults in the left-hand side column to return to the NRAO Defaults catalog.  The basic catalog rules, use of icons, browsing, table viewing, and the mechanics of creating and editing of source catalogs is almost identical to that of the SCT tool. So to access the details of a single resource, click the edit icon (Edit Source).

Default Wide Band Continuum Resources

Continuum observations are generally performed using the maximum available bandwidth to obtain the best signal to noise ratio for a signal that is (mostly) independent on frequency. The receivers for the upper three receiver bands (> 18 GHz: K, Ka, Q) cover more than 8 GHz bandwidth. To obtain maximum instantaneous sensitivity it is therefore possible, with the 3-bit samplers, to observe a full 8-GHz wide bandwidth for continuum purposes. On the other hand, signals obtained with the lower frequency receivers, where RFI is apparent and the receiver coverage is less than 4 GHz, are better sampled with the 8-bit samplers covering up to 2 GHz bandwidth. For C, X and Ku bands, one has to choose. Below, examples of a 3-bit and an 8-bit sampler default wide band resource are shown.

High Frequency 8 GHz Wide Band Continuum (3-bit, K, Ka and Q band)

As an example of a 3-bit wide band continuum resource, select the K64f3 wide resource in the NRAO Defaults catalog (in group DCB/Any config).  Click on the (Edit Source) edit icon (with fly-over help tool-tip Show/Edit properties for this catalog entry) to see the hardware and instrument options used in this resource.

Figure 3.5: Web browser screen shot of the RCT when the default setup for K band is selected.

The information displayed (Figure 3.5) in the top graphic is the receiver band coverage, one color per IF path, four in total. Furthermore the nominal (green, 1dB sensitivity drop) and extreme (white, 3dB) receiver coverage ranges are shown as vertical dashed lines. A small table shows a summary of correlator resources used for this setup, which will update when further specification is made. Note that this non-editable default resource uses the maximum of 64 baseline board pairs to cover 8 GHz of bandwidth within the allowed data rate. Below the graphic is a window with six tabs: Basics, Lines, Basebands, Line Placement, Subbands and Validation. For simple wide band observations ignore the Lines and Line Placement tabs.

The Basics tab displays the name (K64f3 wide), receiver band (K) and the correlator integration time (3.0s to remain within the allowed data rate).

The Basebands tab summarizes the samplers in use and the central sky frequency to which each of the four (2 AC + 2 BD) 2-GHz wide baseband pairs are tuned, with their individual sky range bandwidths.

The Subbands tab lists, for each baseband under a different tab, the subbands as configured for the baseband. In this case there are 16 subbands of 128 MHz per baseband, each distributed over a single correlator quadrant (displayed by different colors, see also at the bottom of the Validation tab). Each subband will yield 64 spectral frequency points at full polarization. This setup thus will generate 64 spectral windows in the data; each 128 MHz wide divided in 64 2-MHz wide channels and 4 polarization products.

The Validation tab summarizes the setup in receiver band and correlator integration time, baseband properties in the next table, and subband properties. Note that because the "yellow" baseband is centered at 19.000 GHz, and the baseband is not 2.0 GHz wide, but slightly wider at 2048 MHz, some (24 MHz) of the baseband is actually below the official 1dB limit of 18.0 GHz. This generates a warning, but in practice is not as serious as it appears.

Navigate back to the NRAO Defaults catalog either by clicking NRAO Defaults in the catalog column tree, or by clicking Return to NRAO Defaults (or DCB/Any config, depending on how you got there) at the top of the page. Please allow the web application to finish its operation and do not use the browser back button.

Low Frequency 1 and 2 GHz Wide Band Continuum (8-bit, P, L, and S band)

As an example of an 8-bit wide band continuum resource, open the S16f5DC resource in the NRAO Defaults catalog (in group DCB/Any config).

Figure 3.6: Web browser screen shot of the RCT when the default setup for S band is selected.

The information in the top graphic (Figure 3.6) now shows only two colors, one per IF path. This is a direct result from choosing the 8-bit sampler in the Basebands tab. The number of baseline board pairs used in this setup is only 16 to cover 2 GHz of bandwidth.  The Basebands tab lists two (1 AC + 1 BD) 1-GHz wide baseband pairs with their tuning centered in S band.  Through the Subbands tab, and the tab per baseband it is seen that there are 8 subbands of 128 MHz per baseband, each distributed only partly over the available correlator quadrants (per color used) to yield 64 full polarization spectral frequency points. This setup generates 16 spectral windows in the data, the number of colored items in the correlator quadrant summary under the Validation tab. Here also the subbands are 128 MHz wide divided in 2-MHz wide channels.

 

Spectral Line Observations

There is no separate example of a spectral line resource in the NRAO Defaults catalog. The WIDAR correlator writes all its data in spectral line format, meaning that the continuum resources described above are already spectral line resources with 2 MHz frequency channels. However, when the scientific interest is in a specific line, typically one would want to use a spectral resolution that is better than the 2 MHz channel width in the continuum resources above. The correlator would be set up to provide data with narrower frequency channels than 2 MHz, tuned to the line frequency when corrected for (approximate) Doppler shift. Note that the channel frequency width is not the final spectral resolution in the data cube as this depends on the actual data processing and whether, e.g., Hanning smoothing was applied in post processing. Smoothing will decrease the spectral resolution in the data. When planning on smoothing, be aware that the channel width only approximates the best spectral resolution available without smoothing, at about 1.2 times the individual channel separation. To get a particular spectral resolution with Hanning smoothing make sure that the line is oversampled with at least a factor of two (i.e. double the number of channels that you need for that spectral resolution). There are four possibilities to reduce the width of the frequency channels to obtain frequency channels narrower than 2 MHz for spectral line work; they can be used independently or simultaneously.

  • The number of polarization products may be reduced from 4 to 2 or 1 to obtain contiguous 128 MHz subbands with a spectral channel separation of 1 or 0.5 MHz respectively.
  • The subband bandwidth of 128 MHz can be reduced in factors of two to obtain factors of two narrower channel separations in that subband.
  • When the subband bandwidth is reduced, processing capacity becomes available to process more lags. This is known as recirculation. The product of bandwidth and recirculation factor should be less than 128 MHz per subband.
  • Processing of fewer subbands than the maximum of 64 allows the use of additional baseline board pairs to produce more channels. This is known as baseline board stacking.

See the Observing Guide, Spectral Line section, and the Observational Status Summary, WIDAR section, for more information, but also read the below..

2. Creating a Personal Resource Catalog

2.1. Introduction and Wide Band Continuum Resources

It is possible that NRAO has already been able to retrieve your resources from what you specified in your proposal.  If this is the case you will find these in a resource catalog labeled with the project name in the column at the left.  However, you should follow the examples below to get a feel for what is in your resource catalog. You will want to check the entries in your resource catalog, especially if you require specific frequency settings, and the examples will help you check and/or modify the content.

When you are planning simple wide band observations at default VLA frequencies only, and thus all anticipated resources are already defined in the "NRAO Defaults" catalog, it is still strongly suggested that you make separate personal catalogs for your different PBs/SBs, e.g., in the catalog with the proposal ID generated automatically. Simply copy the resource of interest (including pointing resources if needed) from the "NRAO Defaults" catalog to your project catalog.  Not only will the reason become clear later on, it also creates an opportunity to get used to the tool and you will find that scheduling using small catalogs will be faster over the web interface.  If you plan a spectral line observation, it is instructive to read the wide band case first as this saves repeating information.  Resources for wide band and spectral line observations may be mixed in the same resource catalog and resource group.

Wide-band resources

The observer has the following choices, described in the next two sections:

    Please consult the Guide to Observing with the VLA about setup-scans and 3 bit resources.

    Using an NRAO default

    Suppose your SB consists of VLA default frequency full polarization wide band observations in C band and in Ka band, and that you want to populate your personal catalog in first instance with these default (polarization) wide band resources. For this particular example, this is what to do: Option 1 uses the Resource Wizard, Option 2 uses predefined NRAO resources..

    • Make sure you have navigated to the RCT, shown in the navigation strip as Instrument Configurations.
    • From the top menu strip, select FILE - CREATE NEW - CATALOG; you can skip this step (and the next step) if the catalog you want to use already exists and is writable (i.e., the catalog name is not in slanted red font), e.g., the catalog automatically generated with your project ID.
    • Your new catalog with the default name "[New Catalog]" appears in the main editing window. Change the name of the catalog to something useful (to remind you of its purpose) by navigating to the Properties tab.
    • Optionally add the names of coauthors that you want to share the catalog resources with and who may edit the resources in the catalog.
    • At this stage you can opt to group your resources. This is not necessary, but convenient if you are going to have many resources. If you want to group resources in this catalog, select FILE - CREATE NEW - GROUP, and name your group under the Properties tab.
    • Click to navigate back to the first tab: Resources.

    Option 1, using the Wizard:

    • Select from the top menu strip FILE - CREATE NEW - 8/3-BIT INSTRUMENT CONFIGURATION
    • For 3-bit configurations, a pop-up will request inputs for the band, array configuration and polarization products, for 8-bit configurations you are taken directly to the Basics tab to select the band and the correlator integration time.
    • Upon selecting, name the resource, (select band) and confirm the integration time (which is set to the default) in the Basics tab.
    • Optionally change the Baseband center frequencies in the third Basebands tab and validate in the Validation tab.

    Option 2, copying an NRAO template:

    • Select the NRAO defaults resource catalog and e.g., D/Any config group.
    • In the resource table to the right (main editing window), check C16f5 wide and the Ka band wide band option you want (i.e., Ka64f3 for 8 GHz full polarization, or perhaps Ka16f3 for 2 GHz bandwidth). If you don't know which Ka band resource to select, study the details of each before selecting one, or simply select all. If you selected a group first (and not the whole NRAO Defaults catalog), you may not be able to select both bands in one go; simply repeat these steps. If the resources are not on the same page when selecting the whole NRAO Defaults catalog, simply change the number of resources per page from 25 to 50 or 100.
    • From the top menu strip, select EDIT - COPY - INSTRUMENT CFGS..
    • Select your newly named resource catalog (or group within it).
    • From the top menu strip, select EDIT - PASTE - INSTRUMENT CFGS.. The resources now show up on the right hand side.
    • This can also be achieved by copy/paste of entire groups and/or entire catalogs using the top menu strip options or the menu icons at the top of the (left hand side) resource catalog column. Use the fly-over tool-tip help to identify the proper icon for each action.
    • Maybe you want to check the resource properties using the Show/Edit icon for each catalog entry, especially if you copied one of the Ka band resources as you would probably want to check the polarization or channel separation for each. You can also reorganize your resources by adding groups (FILE - CREATE NEW - GROUP) and move your resources around using the column icon menu, or using EDIT in the top menu strip. Unwanted resources can be deleted using EDIT - CUT - INSTRUMENT CFGS. and similarly unwanted groups or catalogs can be removed using EDIT - CUT - CATALOG/GROUP or by using the scissors in the icon menu.

    If you are unhappy with the name of the catalog or group you can always rename it by selecting it and then clicking on the Properties tab. If you want to change some parameters, choose the relevant items in the next list. Refer to Figure 3.4.

     

    Create your own resource from scratch

    • Make sure you have navigated to the RCT, shown in the navigation strip as Instrument Configurations.
    • From the top menu strip, select FILE - CREATE NEW - CATALOG or select an existing personal catalog.
    • Your new catalog with the default name "[New Catalog]" appears in the main editing window. Change the name of the catalog to something useful (to remind you of its purpose).
    • Optionally add the names of coauthorss that you want to share the catalog resources with and who may edit the resources in the catalog.
    • At this stage you can opt to also group your resources. This is not necessary, but convenient if you are going to have many resources. If you want to group your resources in this catalog, select FILE - CREATE NEW - GROUP,  and name your group under the Properties tab.
    • Click to navigate back to the first tab: Resources.
    • From the top menu strip, select FILE - CREATE NEW - 8/3-BIT INSTRUMENT CONFIGURATION. You will be presented with, e.g, the 3-bit resource page for which an example is shown in Figure 3.7:

    Figure 3.7 - 3-bit resource page. Note how the four 2GHz basebands cover 8GHz in K-band

    • In the first (Basics) tab, name your resource, select an observing band and enter some descriptive information in the comments field at the bottom of the page (not shown here).
    • Choose a correlator visibility integration time, in integer seconds with a minimum of 1 second, or use 5 seconds if you don't really care; the latter will reduce the size of the data set at the cost of time-averaging smearing away from the phase center. The correlator integration time may have to be adjusted later to remain within the maximum data rate for the specific setup you are creating.
    • Move to the Basebands tab (i.e., skip the Lines tab).
    • For the high frequency bands (K, Ka, and Q) the default is to use 3-bit samplers covering 8 GHz bandwidth using four 2 GHz basebands, but you may select the 8-bit system to use two 1 GHz basebands. For the low frequency bands the default is to use the 8-bit setup.
    • For each baseband select the baseband center frequency if the defaults won't work for you. If you plan on doing this remember the restrictions: for 3-bit the baseband centers for A1C1 and A2C2 must be within about 2.5 GHz (same for B1D1 and B2D2), and in cases where AC and BD are spread over a large range usually AC needs to have the upper center frequencies. Ka band has extra restrictions (see below or the OSS). Ignore the Doppler table below the baseband center frequency table.
    • Continue to the Subbands tab (i.e., skip the Line Placement tab).
    • Under every baseband tab (i.e. repeat this four or two times) click Fill Subbands. Each defined subband will fade a fraction of the color of the baseband in the graph on top.
    • If you are unhappy with any of the subbands you can delete individual subbands with the delete icon (delete) at the right hand side of the subband table. Mulitiple subbands can be selected and deleted with the Delete Selected Subbands button.
    • Subbands can be changed individually, or be selected and bulk-edited using Bulk Edit Selected Subbands. Note that editing is limited to the current baseband tab only; if you, e.g., want to change the polarization of the whole observation you have to repeat the editing in each of the baseband tabs. You can only enter the center frequency for subbands of 16 MHz and narrower, provided that they do not cross a 128 MHz boundary; wider subbands can only be selected from a fixed drop-down list.
    • You may sacrifice subbands in favor of more correlator products (channels or polarizations) in another subband. More of this will be described in the Spectral Line Resources section below.
    • If you want to start anew (using the already specified baseband centers), click Clear All Subbands to remove all subbands in all basebands, or delete the resource (or Group, Catalog) completely using the menu strip or icon menu.
    • Finally use the Validate tab to inspect your setup in terms of frequencies and correlator resources (i.e., baseline board pair allocations). Check your resource, from top to bottom. If you create more than one resource, check each of the resource properties using the Show/Edit icon for each catalog entry. You can also reorganize your resources by adding groups (FILE - CREATE NEW - GROUP) and by moving your resources around using the column icon menu, or using EDIT in the top menu strip. Unwanted resources can be deleted using Cut.
    • If you are unhappy with the name of the catalog or group you can always rename it by highlighting it and then clicking on the Properties tab.

    Incorrect or unfinished template resources — the ones which generate red errors in the interface feedback strip — will be saved for future use, when you exit and log in again. They will however not be usable when assigned to a scan in the OPT. The resource first must be fixed in the RCT after which it can be assigned to a scan in the OPT. Resources with warnings can be assigned to a scan, but the warning should be understood before continuing with OPT scheduling. This behavior also applies to sources in the SCT.

    Regardless of how you create (or how NRAO fills) your resource catalog entries, make sure they are correct before you continue with using them in the OPT. The OPT does not use global resource properties; when you have modified a resource you have to use the OPT to reassign the new resource separately to every scan that needs it.  For this a global edit has been implemented in the OPT. Check your catalogs before making scans!

    Options from the menu strip and icon menu

    In the previous recipes some usage of the options in the menu strip were given (e.g., FILE - CREATE NEW - CATALOG). The names of the actions are quite self-explanatory, similar to the menu strip options in the SCT:

    FILE CREATE NEW CATALOG EDIT [ADD TO GROUP]   -> [[group name]] HELP ABOUT THE RCT
    GROUP [REMOVE FROM CURRENT GROUP] ABOUT ME
    8-BIT INSTRUMENT CONFIGURATION CUT CATALOG NEW FEATURES
    3-BIT INSTRUMENT CONFIGURATION GROUP DOCUMENTATION
    EXPORT ...
    INSTRUMENT CFGS. CONTACT SUPPORT
    IMPORT COPY CATALOG
    EXIT GROUP
    INSTRUMENT CFGS.
    PASTE CATALOGS
    GROUPS
    INSTRUMENT CFGS.

    Menu strip options may act on both items in the left hand side column as well as items in the main editing window, and are grayed and striked out if the item is not active/highlighted (e.g., the catalog instead of the group). The icon menu was introduced in the previous chapter. Here the menu icons behave exactly the same as in the SCT.  The fly-over tool-tip help will remind you of their actions.

    2.2. Spectral Line Resources

    Creating a spectral line resource is similar to creating wide band resources as outlined above, except for the more advanced specification of the subbands and subband frequency tuning and possible Doppler setting of the frequencies.  Eventually, resources with the requested correlator settings will be pre-filled from information submitted to the PST during the observing time allocation procedure.  However, at this stage not much of this has been implemented.

    The NRAO Defaults resource catalog contains full polarization dual IF pair spectra-polarimetry resources (the wide band continuum resources). If they appeal to you, you can copy/paste them in a personal catalog just as for the wide band resources above and edit them as needed. Check the spectral line resource properties very carefully as the spectral line resources in the NRAO Defaults have a fixed sky frequency whereas you probably want to use a rest frequency in combination with Doppler setting.  Most likely, however, you will opt to create your own resource from scratch, just like creating a wide band resource previously.  Some items that need extra attention are described below, step-by-step, but first a small detour to outline the options for creating large numbers of narrow frequency channels.

    Recirculation versus Baseline Board Stacking

    Spectral line observations are typically constrained by the requirement to have the best spectral resolution (i.e., narrowest spectral channel width) combined with the best velocity coverage (i.e., widest observing bandwidth), the latter perhaps also for calibration purposes (as gain calibration is done per subband).

    The maximum subband bandwidth is 128 MHz and can be decreased by factors of 2 to 31.25 kHz(*). Total bandwidths wider than 128 MHz are achieved by placing subsequent bandwidths next to each other with the caveat that the few channels next to a subband edge should be considered lost for line work (on either side of the boundary, for continuum work as well). To obtain a contiguous bandwidth which is near-homogeneous in sensitivity and without any subband boundary gaps, another baseband would be placed some MHz offset (e.g., half a subband bandwidth) from the original baseband to enable subband stitching in post-processing. Note that this limits the number of available basebands for other line settings.

    A single Baseline Board Pair (BlBP), out of 64 available BlBPs, can handle 256 spectral points divided over the polarization products (polProd). That is, it can deliver 256 spectral channels in single polarization, 128 spectral channels in dual polarization, or 64 spectral channels in full polarization. The channel width (which is slightly less than the spectral resolution) is then simply the subband width divided by the number of spectral channels.

    With one BlBP per subband as standard, the selected subband bandwidth is thus limited to 256/polProd channels which may not be narrow enough to achieve the desired spectral resolution. There are two ways to overcome this limitation: Baseline Board Stacking uses more of the limited amount of hardware and Recirculation uses limiting the total bandwidth per subband and CPU cycles. Both have their disadvantages and the choice depends on the science requirements. If either can be used, we recommend using Recirculation.

    Baseline Board Stacking uses additional BlBPs to compute extra channels in a subband (of any width up to 128 MHz). Each additional BlBP increases the number of spectral channels by 256/polProd. As there are only 64 BlBPs total, and as every subband uses a minimum of one BlBP, Baseline Board Stacking reduces the number of subbands that can be observed to less than 64, and in the extreme case to a single subband of 128 MHz or less. When most of the 64 BlBPs are being used and all of the subbands are required (instead of some being recognized as less important and thus "desired" versus "required"), the observations will not take place if one or more BlBPs are inoperable. The 3-bit NRAO default setups use 64 BlBPs with the subbands at the baseband edges "desired" to allow continuation of operations when not all BlBPs are available.

    The Recirculation option uses software to compute extra channels. For this, CPU cycles are "freed up" by limiting the subband bandwidth fed to the BlBP to less than 128 MHz to obtain more lags (in factors of two), running the data through the board for a second, third, etc., time; hence Recirculation.  As subband bandwidths (and CPU cycles needed to process them) can be decreased by factors of 2, each halving thus allows a doubling of the number of channels in the subband, currently up to a factor of 64. This does not require additional BlBP hardware and thus retains the possibility of using all subbands, albeit at less subband and less total bandwidth. Currently, subbands of 128 MHz must use Baseline Board Stacking to achieve more than 256/polProd channels.

    Baseline Board Stacking and Recirculation can be used simultaneously in the same subband (if less than 128 MHz), and configurations with multiple subbands configured with either or both are allowed. Note that here a correlator setup can still request all 64 BlBPs and thus designating some subbands as "desired" is still highly recommended, but Recirculation gives the option to use less than 64 to achieve the number of spectral channels.  However, requesting a large number of channels, whether or not with Baseline Board Stacking and/or Recirculation, yields higher data rates than normal with the default integration times. To remain within the limits set by the observatory, longer integration may be needed which has an impact on time averaging smearing in the larger array configurations and thus on the field of view.

    The choice for one or the other, or even for less channels than anticipated, depends on the trade-offs that can be made for the science goals and remain a responsibility of the observer.

    (*) When observing at such narrow subband bandwidths it is good to check with the NRAO helpdesk. There are other operational constraints, in particular the F-shift in bringing down the baseband, that need to be considered and that may decrease the usable subband bandwidth to much more than a (symmetric) few channels compared to the general case described above.

    Creating a new Resource

    First, navigate to the Resource Catalog Tool (RCT).  Then:

    • From the top menu strip, select FILE - CREATE NEW - CATALOG or select an existing personal catalog.
    • Your new catalog with the default name [New Catalog] appears in the main editing window. Change the name of the catalog to something useful (to remind you of its purpose).
    • Optionally add the names of co-workers who you want to share the catalog resources with and can edit the resources in the catalog.
    • At this stage you can opt to group your resources. This is not necessary, but convenient if you are going to have many resources. If you want to group resources in this catalog, select FILE - CREATE NEW - GROUP, and name your group under the Properties tab.
    • From the top menu strip, select FILE - CREATE NEW - INSTRUMENT CONFIGURATION. You will be presented with a resource page an example of which is shown in Figure 3.8 below.


      Defining the newly created Resource

      The instrument configuration is built up by going through 6 steps, represented by the 6 tabs in Figure 3.8, and summarized in the table below. If you know what you want exactly, e.g. because you have a fixed (sky) frequency for your line or have calculated your Doppler frequency by hand and want to include that in a fixed frequency resource, skip the Line and Line Placement tabs and tune the basebands (with Doppler setting switched off) and subbands directly in the Basics, Basebands and Subbands tabs. Before continuing you may want to read up on Doppler Setting and Dynamic Scheduling.

      TabFunction
      Basics Name resource, specify band and integration time
      Lines Enter all lines and their properties that you'd like to cover
      Basebands Tune basebands to opimize your line coverage
      Line Placement Generate template subbands to cover the lines
      Subbands Inspect subbands from previous step and modify if necessary
      Validation Validate for correctness

       

      Basics

      • Choose a correlator visibility integration time; the default integration times are given here, where the larger values will reduce the size of the data set. The correlator integration time may be adjusted later (upward) to remain within the maximum data rate for the specific setup you are creating.

      Figure 3.8 - Screenshot of the RCT in the 8-bit case, Basic Tab.  Note the two independently tunable basebands.  The 3-bit equivalent has four such basebands

      Lines

      • Specify the sky position for which your observations should be Doppler set (tracked); you can use the Import Source Position button to use a predefined source from one of your source catalogs in the SCT. Note that any velocity specified for a source is not used in the resource; this is specified here in the Lines and Baseband tabs. If you have more than one source, you can specify an average velocity, a separate line for each source velocity or create a new resource per source. Do not specify a position if you are specifying an exact sky frequency for your lines.
      • In the Lines table, click on Add Line. A row with label L1 will appear. The next step is to specify the specifics of the subband that will cover this line.
      • Name your line and specify the line rest frequency, line-of-sight velocity at which the line should be observed and the rest frame and convention to be used in Doppler calculations. Furthermore, specify the minimum velocity range that should be covered. Note that a first order bandwidth for this range will be calculated using the line frequency and displayed below the input field. This will set the subband bandwidth to the next wider possible value. Try to keep the bandwidth for the range as narrow as possible for your science to maximize flexibility in placing the 128 MHz boundaries between your anticipated subbands; you can widen them later. In general pre-defining subbands wider than 32 MHz will often give problems in the initial placing of the subbands as the final baseband center frequencies are not well determined. Similarly the (maximum) channel separation input field will display the velocity width converted to frequency below the input field, and set the channel separation in the subband to the next possible narrower value. Finally choose the polarization products required for this line. By default the option to use Recirculation when possible (over Baseline Board Stacking) is switched on, and unchecking the box will consider Baseline Board Stacking only when configuring the subband for this line setting.
      • Build your list of lines by adding additional lines and repeating the previous step. You can copy the last line to copy the items and make your editing easier using the Copy Last Line button. If there is a specific frequency that you want to Doppler set (track) on, but is not a line that you want to observe as such, e.g., a baseband center frequency, include it in this list. To remove an unwanted line click the delete icon (delete) to the right of the row that needs to be deleted.
      • Note the summary of the baseline board pairs (Bl.BPs) in the top table next to the baseband graph. Only 64 baseline board pairs can be used and this number will be updated in the following steps when generating and updating subbands.

      To prevent having to do the work again for a next resource with similar lines there is an option to download or import the line details at the bottom of the lines tab. See Section 6.5 in the OPT manual for the syntax.

        Figure 3.9 - The Line Tab of the RCT after adding two lines.  Note how neither baseband covers a line

         

        Basebands

        • For the high frequency bands (K, Ka, and Q) the default is to use 3-bit samplers covering 8 GHz bandwidth using four 2 GHz basebands, but you may select the 8-bit system to use two 1 GHz basebands. For all other bands the default is to use the 8-bit setup. If 3-bit selection is not possible, copy (and edit) a resource from the NRAO Defaults catalog. Ignore the message that there are no subbands defined yet (which is done in a next step).
        • For each baseband select the baseband center frequency if the defaults won't work for you. Remember that the exact baseband center itself cannot be observed as it falls on one of the 128 MHz boundaries. Try to place your lines as much as possible in the middle of the available basebands, or alternatively in as few basebands (i.e., one if possible) as long as your lines are not very close to the baseband edges and not close to the 128 MHz intervals within each baseband. If you plan on doing this, remember the restrictions: for 3-bit the baseband centers for A1C1 and A2C2 must be well within 2.5-3 GHz (same for B1D1 and B2D2) to pass through the 5 GHz down-converted frequency bandwidth at the antennas. In cases where AC and BD are spread over a large range (>4 GHz) usually AC needs to have the upper center frequencies and the maximum span of all frequencies in any band must be less than 12 GHz. Ka band has extra restrictions (see below or the OSS).
        • Once the baseband centers are set, typically after the subband step below, select for each baseband the Doppler setting characteristics. That is, from the list of lines specified in the Lines tab define the line that will be used to calculate the Doppler shift of the entire baseband for the sky position entered in the Lines tab. Formally your Doppler frequency will be correct for only one line per baseband at the start of your observation, but in practice the differences between the lines in a baseband usually are small enough to correct for in post-processing. This Doppler setting "line" does not have to be observed as that line, but it needs to be specified as a line option (with velocity and definitions) in the Lines tab. For example, one can specify a pseudo-line which is the baseband center frequency, skip generating a subband for it (as that won't fly because it falls on a 128 MHz boundary), but select it as the Doppler setting frequency.

        Figure 3.10 - The Baselines Tab of the RCT after shifting both basebands such that both lines are covered

         

        Line Placement

        • The lines you specified earlier will show up in a summary table. For each of the lines you want to observe hit the Generate button, which will pop-up a dialog window for confirmation. This will generate a subband with a subband bandwidth which covers at least the velocity range requested, with enough baseline board pairs assigned (within the recirculation factor allowed). It aims to cover the bandwidth with spectral channels that are at least as narrow as the requested separation with the specified polarization characteristics. When created, the part of the baseband around the line will be shaded lighter to show the allocation in the baseband (though it may be too narrow to be distinguished). If a line can be observed with more than one baseband, there is the option to select the baseband. If you hit the Generate button more than once you will generate identical subbands without actually increasing the sensitivity for that line.

        Figure 3.11 - The Line Placement Tab.  Clicking Generate creates a suggested subband setup which can be inspected in the following tab: Subbands

         

        Subbands

        • Every baseband tab will now show a table with the subbands that were generated during the Line Placement step which covers each individual line. The color assigned to a subband indicates which of the four correlator quadrant's baseline board pairs are assigned to it (in continuum typically 16 subbands have the same quadrant color) and currently for 8-bit setups you may generate up to 32 subbands in either baseband.  You have the freedom to modify the subband bandwidths, but note that each doubling of the bandwidth requires a doubling of the number of baseline board pairs or recirculation factor to retain the channel frequency width (the default behavior is to keep the number of baselineboard pairs and recirculation constant, doubling your channel separation with each doubling of the subband bandwidth). Changing the number of polarization products has a similar effect. If subband bandwidths of less than 128 MHz are used, enable Recirculation to reduce the use of the limited amount of baseline board pairs (see above).
        • The Snap to Grid and Fix to Baseband checkboxes are unchecked and/or checked by default, depending on the situation. Unchecking Snap to Grid allows for using more correlator resources to define a flatter bandpass filter and unchecking Fix to Baseband keeps the subband tuned if you decide to recenter the baseband respectively. For now, the best option is to keep them with the default at generaton: both unchecked for line work and both checked for subbands intended as continuum. Note that these items are tools to figure out the best placement of the baseband center frequency for your lines and do not really do anything during the observations and will be ignored after the frequencies are defined.
        • Under every baseband-tab (i.e., repeat this four or two times) you want to make sure you have enough continuum sensitivity (subband frequency coverage) to be able to calibrate your gains. You would want to include extra subbands you can use for this by clicking Add/Fill Subbands and placing them in a part of the observing band away from strong lines; whether they are your bright targets or RFI (for 8-bit samplers you may only generate 32 subbands). Each defined subband will fade a fraction of the color of the baseband in the graph on top so you can see whether they are avoiding your line. By default these subbands have the Snap to Grid and Fix to Baseband options reversed compared to the subbands containing your lines for flexibility.
        • If you are unhappy with any of the subbands you can delete individual subbands with the delete icon (delete) at the right hand side of the subband table. Multiple subbands can be selected and deleted with the Delete Selected Subbands button.
        • Subbands can be changed individually, or be selected and bulk-edited using Bulk Edit Selected Subbands. Note that editing is limited to the current baseband tab only; if you, e.g., want to change the polarization of the whole observation you have to repeat the editing in each of the baseband tabs. You can only enter the center frequency for subbands of 16 MHz and narrower, provided that they do not cross a 128 MHz boundary; wider subbands can only be selected from a fixed drop-down list.
        • You may limit the number of subbands (i.e., less than 64 total, 16 per baseband) in favor of more correlator products in another subband. The number of correlator products in a baseband is the number of spectral channels × the number of polarization products and each baseline board pair can produce 256 products. Doubling the number of baseline board pairs doubles the number of products at the cost of a subband per baseline board pair, but see above how to counter that with Recirculation. Keep an eye on the total number of baseline board pairs used in the top table (next to the graph); only 64 baseline boards are available. Also make sure that you do not exceed the maximum data rate (25 MB/s) for the sum of all subband resources.
        • If you want to start anew (using the already specified baseband centers), click Clear All Subbands to remove all subbands in all basebands, or delete the resource (or Group, Catalog) completely using the menu strip or icon menu.
          • Finally, judge the subbands: can they be merged, are they too close to a 128 MHz edge, are there left-over resources (baseline board pairs) to include more (e.g., continuum) subbands, to double the number of channels to increase the spectral resolution for one or more subbands, etc.?
          • The automatically generated subbands are made on a best effort basis; check the output!
          • When a subband is very close to a 128 MHz baseband edge (hover with your mouse over the yellow triangle to see how much), you may want to shift the baseband in question to a new center by returning to step 2; when doing so you should not have (any of) the generated subbands fixed to the baseband so they will not stay fixed close to the 128 MHz edge of the baseband (the default for "Fix to Baseband").
          • Adding 128 MHz continuum bands and/or switching on Doppler setting should be done after the baseband center frequencies are determined.
          • Each doubling of the subband bandwidth (e.g., when merging subbands seems useful) requires a doubling of the number of allocated baseline board pairs to retain the channel width originally requested.
          • Changing the individual subbands invalidates the input given earlier; in principle no information stays correct when stepping backward but more lines can be added to step 1 and be generated in the following steps when baseband and subband centers were calculated previously.
          • If your science has the flexibility, assign some subbands a priority other than "Required". This will allow a scheduling block with this resource to run on the telescope even if some of the hardware is inoperable or if one or more baseline boards cannot be configured at the cost of missing those subbands. The larger the number in the label, the less desired a subband is in case there are more choices that can be traded off. That is, the subband with the highest desired-number (64) will be left out first if such a need arises, the lowest (1) last, but Required subbands are never skipped.

          Figure 3.12 - The Subband Tab of the RCT, in this case showing baseband A0/C0. Note in the bottom panel where the selected bandwidth falls in the baseband

           

          Validation

          • This allows you to inspect your setup in terms of frequencies and correlator resources (i.e., baseline board pair allocations). Check your resource, from top to bottom. If you create more than one resource, check each of the resource properties using the Show/Edit icon for each catalog entry. You can also reorganize your resources by adding groups (FILE - CREATE NEW - GROUP) and by moving your resources around using the column icon menu, or using EDIT in the top menu strip. Unwanted resources can be deleted using Cut.
          • If you are unhappy with the name of the catalog or group you can always rename it by highlighting it and then clicking on the Properties tab.

          If there is a need to modify the existing resource, after generating a resource and attaching it to scans in a scheduling block in the OPT, rename or copy it to a new name and make the modifications in the resource with the new name. This adjusted resource has to be re-attached to the scans in the scheduling blocks, replacing the faulty resource. The easiest way to do this is with the Bulk Edit Scans tab on the selected scheduling block. Having a different name for the two resources is extremely useful in checking whether the edit was successful and whether scans with the old resource have been overlooked.

          Final Check

          After all this data entering, make sure you check your catalogs for correctness. It is important that your frequencies (with Doppler definitions) are correct before creating scans in the OPT, and before storing to disk or sharing your catalogs with your collaborators.

            3. Specific Resource Details

            The previous subsections on resources were dealing with resources defined to do the scientific astronomical observations you proposed for. However, to get the most out of your data, it sometimes is helpful to add some specialized scans to the SB in order to optimize the observations or to aid in the calibration of the instrument. Typical for high frequency (higher than ~ 15 GHz) are pointing scans and tipping scans. The observing mode for such scans (pointing or tipping) is selected at the scan level in the OPT. For pointing scans, typically one would use resources that are different from your scientific observation resources, e.g., a different bandwidth, correlator setting, or even a different observing band. We have added some of these resources to the NRAO defaults catalog, available to the OPT at the scan level or to copy/paste to your personal resource catalog.

            Pointing scans are used to improve telescope pointing accuracy which increases the sensitivity of the observations. As the instantaneous telescope pointing is only accurate to several arcseconds, this error may become a considerable fraction of the primary beam at high frequencies. Solving for this error is done using primary pointing scans on a strong source at X band, after which a secondary pointing may be performed at the observing frequency if deemed useful (the pointing resource in C band is not recommended). The actual pointing action is selected as Interferometric Pointing under scan mode in the scan details (see OPT), which may use the resources named Pointing presented in the Pointing setups resource group in the NRAO defaults resource catalog. It is important to use these pre-defined setups as they typically use a different frequency, bandwidth and integration time than the other (default) resources.

            Tipping scans are used to obtain a measurement of the atmospheric opacity at high frequencies, which allows for an estimate of the loss of sensitivity due to absorption of emission from the source of interest by the atmosphere. The actual telescope tipping action is selected as Tipping under scan mode in the scan details (see OPT). Because you typically want to do tipping scans at your observing frequency you would either use resources from the NRAO defaults catalog or you would reuse your own resource at the frequency you want; no new resources are needed. Tipping scans are currently disabled.

            Resources at Ka band: There is an issue with specifying the frequency of IF pair AC at Ka band. That is, tuning any part of the AC IF pair band below 32.24 GHz will not result in valid data, regardless whether this is A0C0 or any of A1C1 or A2C2. Only the BD IF pair can be tuned to frequencies below 32.24 GHz; use the BD IF pair instead of AC IF pair when you only need one IF pair for your resource with a frequency tuning below 32.24 GHz. If the OPT web application validation detects that any part of the bandwidth of IF pair AC is tuned below this 32.24 GHz it will try to swap the AC IF pair with the BD IF pair. If this is not possible, it will issue an error (in red font) in the interface feedback strip if this frequency is specified as a fixed sky frequency. It will issue a warning (blue font) for rest frequencies, as the particular tuning depends on the details of observing date, telescope pointing direction and source velocity definitions. Note that a rest frequency above 32.24 GHz may shift to below 32.24 GHz once it is assigned to a scan in the OPT. This should give you an error in the OPT; you should be aware of this possibility and pay attention to this. However, it is better to assign IF pair BD to the resource if you anticipate this might happen, if you still have this freedom in your resource of course.

            When observing very close to 32 GHz with both IF pairs, some combinations where frequency coverage of AC and BD are overlapping are not possible. Consult Helpdesk for options, preferably before submitting the proposal.

            The very wide bandwidth of the Ka band receiver, from 26.5 to 40 GHz, would suggest that IF pair separations of up to ~13 GHz are possible. Restrictions in the signal path, however, limit this separation to 10 GHz. The OPT web application validation will issue an error if the separation between IF pairs AC and BD is more than 10.5 GHz in sky frequency (with IF pair AC tuned to have the higher frequency centers). A separation of more than 10.5 GHz in rest frequency will result in a warning as, e.g., highly red-shifted lines may end up with less separation when the actual sky frequencies are calculated.

            Resources at K and Q band: If you choose the RF signals in the different IF paths to be separated by a large amount, it is possible that the OPT will only let you create a resource where the baseband frequency center(s) in IF pair AC is higher than the baseband frequency center(s) in IF pair BD, similar, but the reverse of the Ka band restriction above.