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1. Instrument Validation

Integration Time & Time Averaging

The correlator integration time (Tint) should be appropriate for the array configuration, your scientific goals, and observing mode (i.e., on-the-fly-mosaicking, subarrays). Make sure you have short enough integration time in order to reduce time averaging losses and to offset atmospheric/ionospheric phase winding. The minimum possible integration time depends on the total number of channels produced in the correlator and the maximum data rate and data volume. The smaller the integration time, the larger your data set will be. Thus, making the integration time unnecessarily small should be avoided. Current integration time defaults and guidelines can be found in the Time Resolution and Data Rates section of the current VLA OSS.

Consider that time averaging causes some smearing of sources away from the phase center of the image; the further away from the phase center, the more smearing there is. This effect cannot be remedied by self-calibration and it persists after deconvolution. Therefore, it is important to devise observing strategies that hold the distortions down to acceptable levels. For more details, refer to the Time-Averaging Loss section in the current VLA OSS.

 

Continuum

Most observers will be able to utilize the NRAO's default continuum resources located within the Instrument Configuration tool (also known as the Resource Catalog Tool (RCT)). When choosing one of the defaults, be sure to select the resource according to the array configuration in which the SB will be observed. The defaults are organized by array configuration and corresponding correlator integration time. For example, A/Any config will contain 2s integration time resources and DCB/Any config will contain 3s and 5s integration time resources.

Did you choose the correct default resource?

While checking the Reports tab of the SB in question, the first table of that page summarizes the resource(s) utilized within in the SB. The NRAO defaults adhere to the following naming scheme: the first letter denotes the receiver band, the number following signifies the total number of subbands, the letter following denotes the polarization (f=full and d=dual), and the last number indicates the correlator integration time in seconds. Some defaults contain extra letters at the end, these signify the preferred array configuration for its integration time.

Default Examples:

K64f2 = K band, 64 subbands (3-bit), full polarization, 2s integration time
X16f5DCB = X band, 16 subbands (8-bit), full polarization, 5s integration time, D, C, or B configuration

The distinction between 3-bit and 8-bit resources can be made through the number of the subbands: 8-bit resources, regardless of the receiver band, have 16 subbands. The 3-bit resources have 64 subbands at K, Ka, and Q-bands, 48 subbands at Ku-band, and 32 subbands at X and C-bands. In order to see a detailed overview of a resource within an SB, click on Show All Subbands located in the first table of the Reports tab.

If a given default resource is not exactly what is required for the science (i.e., the default center frequency is not ideal), then the observer can either create a new continuum resource from scratch or copy/paste a default resource into their own catalog and then make adjustments to the resource as needed.

For very detailed guidelines on how to create continuum and spectral line resources, please read through the Creating a Personal Resource Catalog within the OPT Manual.

 

Spectral Line

As noted above, please read the Creating a Personal Resource Catalog within the OPT Manual in addition to reading through the Spectral Line section within the Guide to Observing with the VLA.

If you are setting a spectral line resource that requires narrow subbands, you may want to use Doppler Setting. Details on this are available in the Spectral Line observing guide and the Spectral Line Resource of the OPT manual. With velocity resolution, make sure the spectral resolution is enough to resolve your line (take subsequent Hanning smoothing into account) and possibly over-sample by a factor of 4 to be able to correct for Doppler Setting effects. Also, make sure you have enough line-free bandwidth to measure the continuum flux.

During the process of building your spectral line resource be aware if, at the time of line placement, you see the following warning message in the pop up window: Warning: The subband overlaps a forbidden region of the baseband. When this happens, the 128 MHz boundary condition overrules the sky frequency of the subband and the subband gets placed off from the desired value because it cannot be on the boundary itself (note, this process is not reversible). The only way to address this is to shift the baseband frequency (within the Baseband tab).

When you change the baseband frequency, make sure to re-generate the subbands containing the spectral lines which had the above noted warning. The original spectral lines have been forced to a different frequency due to hitting the 128 MHz boundary.(Note, be sure to remove any incorrectly placed spectral lines within the subbands.) Then update the scheduling block (SB) with the new resource, and finally verify that the subbands are as expected in the Reports tab of the OPT (explained below).

Validity Check

Once the resource has been created and imported into the SB, we recommend performing a check on the spectral line sky frequencies and Doppler Settings (if applicable) to verify that everything is ok.

  1. Within the OPT, go to the SB containing the resource in question and then go to the Reports tab. At the top of that page, insert an LST date and start time in the assumed LST start fields. (LST dates for a given month can be found on the Graphic Monthly Schedules page located at the bottom of the Schedsoc home page or use the figure in the Scheduling Constraints page.) Click Update.
  2. In the Instrument Configuration Summary table,  find the resource of interest and find the Doppler line, velocity, position, etc columns. If they contain dashes instead of values, then the resource has not been set up to apply Doppler corrections at the time of observing and all frequency information for this resource relates to sky frequencies. Assuming Doppler corrections are essential, switch them on in the resource (bottom of the Baseband tab), change the resource name and re-import the new resource to all the scans with the old resource (using the Scheduling Block Bulk Edit Scans tab) and check these columns in the resource again.
  3. Click Show All Subbands for the resource of interest. Here you will see the calculated sky frequencies of all subbands (frequency range, subband center, etc...) for the assumed date of the above step. With Doppler Setting active, these frequencies will change with date and time of observation as they are not fixed sky frequencies.
  4. Select one of your lines to compare the subband center frequency with the value generated through the Dopset tool. In the Dopset tool, populate the appropriate fields, including the LST day and time you used in the OPT. The frequency reported by Dopset and the OPT may not be identical but it should be very close (within a kHz). However, if Doppler Setting was not used in the resource, then there would be a noticeable difference in the frequencies between Dopset and the OPT. For such cases, make sure that your spectral line of interest is well within the subband even if it is not at its center.
  5. Repeat step 3 for other spectral lines in both the A/C and B/D basebands.
  6. Repeat steps 3 and 4 for the other LST start dates/times to make sure the frequency values in the table will change as expected due to Doppler and are consistent with Dopset.

If you have questions or need assistance, please submit a ticket to the NRAO Helpdesk and include the URL of the resource in question along with the project code.

2. SB Validation

In this section we provide practical considerations to ensure that your VLA scheduling block (SB) adheres to the discrete requirements for successful observations. Please follow this list during the preparation of your SB and before submitting it through the Observation Preparation Tool (OPT). If you wish to check the validity of the correlator setup of your observations, please visit the Instrument Configuration Verification page.

It is essential that one checks their SBs carefully prior to submitting. Although the OPT has several features to deal with logistical observing issues, it will not catch everything. The same goes for the SB verification that NRAO staff performs after submission; SBs are checked only for common errors. Items like source positions, Doppler-setting frequencies, choice of calibrators, etc., and anything particular that refers to the science goal of the observations cannot be verified by the staff. Remember that the final responsibility for a correct observing schedule remains with the observer.

SB Information Tab

  1. Appropriate wind and API constraints: Important because most of VLA observing is dynamic.
  2. Be aware of selecting "avoid sunrise/sunset": In most cases this is not necessary and can decrease the chances of observing. For more details, refer to the Low Frequency and Very Low Frequency strategy guides.
  3. Duration of the SB: This is highly dependent on the observing priority of the SB. Please see the optimal duration question in the Frequently Asked Questions section. (Note, any length of SB is permitted, i.e., 33min, 1hr12min, etc..)
  4. Count: Should the SB be observed more than once?

SB Reports Tab

  1. Setup scans: These are needed for observations with both the 8-bit and 3-bit samplers. Please see the 8/3-Bit Attenuation and Setup Scans section for more details (note there are special instructions for S- and P-band observations).
  2. Start-up time: This is typically 10-12 minutes at the start of the SB. It is primarily for the initial slew to the first source in the SB and for setup scan(s). These are followed by the first calibrator-intent scan for low frequency observations or the first reference pointing scan for high frequency observations.
  3. Scans in Duration (LST) only: In this mode, scans include both slew and on-source time.
  4. Antenna Wrap Requests: While checking the elevation, make sure to set the LST start range to avoid antenna wrap issues and request an antenna wrap when appropriate. Note, not all SBs will require an antenna wrap request.
  5. Elevation Limits: Ensure that all scans (for all LST start times) have appropriate elevation limits.
  • High frequency (> 15 GHz) observing: avoid elevations < 30° (if possible) and > 80° (required).
  • Low frequency (< 15 GHz) observing: avoid elevations < 10° (recommended) and > 85°.
  • S-band (2-4 GHz) observing: avoid elevations < 20° (recommended to avoid ground spill-over).
  • The VLA's hard lower elevation limit is 8°.
  • Step through the LST start range to validate elevation limits for all scans.
  • Double check the science target source coordinates are correct and are using the J2000 epoch.
  • Include all necessary calibrators: complex gain, flux density, bandpass (and delay) and, if applicable, polarization.
    • Bracket the science target scans with the complex gain calibrator and use the appropriate cycle time: This is especially important for high frequency observing. An exception may be when the target can be self-calibrated. The final decision, however, is up to the observer.
    • If observing more than one frequency (LO/IF) setting - this being within a single receiver band or being through multiple receiver bands - and switching between them in the SB, there is no guarantee to return to the same phase state upon returning back to a given frequency setting. Therefore, in such cases it is extremely important to bracket the target with complex gain calibrator scans that use the same frequency setting as the target to avoid experiencing any phase jumps.
    • All scans have appropriate scan intents: If you wish to use the VLA Calibration Pipeline, a flux density and bandpass calibrator must be defined in the SB. (Note, do not use Determine Autophase or apply last Phase & Delay cal; these are reserved only for VLBI observations.)
  • Use and apply reference pointing: Interferometric reference pointing is needed during high frequency (> 15 GHz) observing with the solutions applied to the proceeding scans. For more details, see the Antenna Reference Pointing Calibration section in the Calibration guide.
    • Repeat reference pointing scans when the difference from the last pointing position is at least 20° on the sky in AZ or EL.
  • Minimum on source time: For regular (standard observing) scans, no less than 20 seconds on source after slewing (for flux density calibrator no less than 30 seconds on source after slewing); for reference pointing no less than 2.5 minutes on source after slewing. [On the Reports tab of the SB, step through the LST start range to verify sufficient on source time.]
  • Maximum total scan length: regular (standard observing) scans may be up to 10 minutes in duration; interferometric pointing (reference pointing) scans and OTF mosaicking scans may be greater than 10 minutes in duration.
  • Do not observe too close to the Sun, especially at low frequencies. This is not related to selecting the "avoid sunrise/sunset" option within an SB. For more information, refer to the Avoiding the Sun section.

    If you would like more help with validating your SB(s) (i.e., instrument configuration or appropriate calibrators), please submit your questions to the NRAO Helpdesk.