Exposure and Overhead

Time on Source: Exposure Calculator

The PI is responsible for ensuring all calibrators required to properly calibrate and reduce the data are observed in the allocated telescope time. The total time a proposer requests has to include not only the time on the sources of interest, but also time spent observing calibrators, slew time between sources, and various types of setup times, known collectively as 'overhead'. The VLA Exposure Calculator Tool (ECT) is a web-based tool (https://obs.vla.nrao.edu/ect) to help observers to perform these approximate calculations.

Please read the instructions below on how to run the exposure calculator; if you encounter problems or need further assistance then please submit a ticket to via the NRAO Helpdesk (http://help.nrao.edu) to the VLA/GBT/VLBA Proposing department.

If you have an old JAVA version of the exposure calculator, please discard it and do not use it. Please make sure you use the latest web-based version of the calculator.

The Exposure Calculator performs the following three types of calculation:

1. given bandwidth, sky frequency, image weighting, number of polarizations, and RMS noise required the ECT will return time on source
2. given time on source, bandwidth, image weighting, number of polarizations, and sky frequency the ECT will return RMS noise
3. given time on source, RMS noise required, image weighting, number of polarizations, and sky frequency the ECT will return bandwidth

The calculator essentially solves the image noise equation given in the Sensitivity section of the VLA Observational Status Summary (OSS).

Running the Calculator

Important:

• The fields labeled Representative Frequency and then Bandwidth must be entered before the calculator will do anything else.
• The text field labeled Purpose of Calculation must be filled in before the output of the calculator can be saved.
• After entering data into a field a carriage return <cr>, a <tab>, or a mouse click outside the input field will submit the data to the calculator.
• By hovering the mouse over some fields, a tool tip with some helpful information is shown.

Two options are available for more complex observations, multiple sources or multiple frequencies:

1. By setting Number of Sources to > 1, the total time is calculated for a single-frequency observation of the selected number of sources; this gives a more accurate estimate of the overhead by only counting fixed overhead once. This assumes that all the sources will be observed in a single Scheduling Block, and the sources are not far apart on the sky.
   
2. By setting Number of Frequencies to > 1, the total time is calculated for the selected single-frequency in a session that will observe multiple frequencies; this gives a more accurate estimate of the overhead by distributing fixed overhead across the frequencies (including pointing calibration overhead if Session Includes Pointing is set to True). This assumes that all the frequencies will be observed in a single Scheduling Block. Also note that this mode gives the time for the Reciever Band requested, you must still do the ECT calculations for the other frequencies requested.
   

By leaving both Number of Sources and Number of Frequencies set to 1, the tool calculates the exposure time for a simple observation of a single source at a single frequency (i.e. the classical behavior of the ECT). It is not possible to combine both multiple sources and multiple frequencies in a single calculation.

The following screenshots show the various fields for the two options – multiple sources on the left (or on top) and multiple frequencies on the right (or below). A description of the fields is included below the screenshots.

 

Description of the various fields:

• Purpose of Calculation: This text box should be used to describe the calculation's purpose. Supply corresponding source/source-group and session names as in the PST, and any other information to pair this ECT output with a specific request in the proposal. This text box must be filled in before the output of the ECT can be saved and imported into the PST. The text should be between 8 and 200 characters long; if you hit 'Save' with an invalid text or no text you will get an error and your output will not save. Once the error is addressed you will be able to save your PDF.
• Array Configuration (A, B, C, or D): does not affect the RMS noise, but does indicate the brightness temperature sensitivity as well as the confusion level that will be reached (see the OSS for further discussion of confusion). In some bands, it will affect the calculation of overhead time.
• Number of Antennas: By default uses 25 instead of the full total 27 antennas to allow for the contingency that not all telescopes are in working order. For RMS calculations per baseline (e.g., for calibration) enter 2 for the number of antennas. See the Calibration section in the Guide to Observing with the VLA for more information.
• Polarization Setup: either single or dual polarization products. Typical value would be dual for Stokes I flux (density) measurements.
• Type of Image Weighting: this is the weighting of the data in the u-v plane during imaging. This affects the RMS noise sensitivity and the beam. Natural weighting is usually used to obtain the best sensitivity and a larger, somewhat worse beam, in terms of angular resolution and sidelobes. Robust (= 0) imaging gives somewhat less sensitivity but a somewhat smaller and better beam as compared to Natural. Use Natural for detection experiments.
• Representative Frequency: the observing frequency (not the rest frequency) that determines the observing band. The online dopset tool can be used to convert rest frequencies to on-the-sky frequencies at the VLA for certain observing dates.
• Approximate Beam size: This is the approximate synthesized beam size which is based on the frequency, the configuration selected, and the type of weighting. If the bandwidth divided by the frequency is greater than or equal to 0.25, a range of beam sizes is shown that correspond to the range in frequencies.
• Number of Frequencies: This is used to indicate the number of bands that will be observed in a session. The exposure time will be different for different bands, so ECT calculations should be performed for each band; setting this allows the overhead to be distributed between the bands so the total time when the ECT outputs for each band are combined is correct. This cannot be combined with more than one source so this option is grayed out if Number of Sources is greater than one.
• Session Includes Pointing: This is used when Number of Frequencies is more than one to indicate whether the session includes bands that require pointing calibration (see High Frequency Strategy), allowing this to be accounted for in the overhead calculation. As the overhead is distributed between all bands, this should be set to 'True' for all bands if there are bands that require pointing calibration in the session, including those bands that do not require pointing calibration.This is grayed out if Number of Sources is greater than one.
• Digital Samplers: 3-bit or 8-bit can be selected. If the bandwidth is wider than 2048 MHz, the calculator will automatically set the samplers to 3-bit, with 8-bit not selectable. Bandwidths up to 2048 MHz cause the calculator to default to 8-bit samplers, although 3-bit sampling may be selected if desired. For bandwidths up to 2048 MHz, 3-bit is less sensitive than 8-bit for a given time and requires more overhead. In general, the tool assumes a 15% sensitivity penalty when using the 3-bit samplers. For more information, see the VLA Samplers section in the OSS. 
• Elevation: the RMS noise, especially at high frequency, is worse at lower elevation mostly due to the atmospheric opacity increasing the system noise. We calculate the increase in system noise assuming an average opacity selected by season (see Average Weather below). The elevation goes into the calculation as the standard exponential of the opacity multiplied by the secant of the elevation. The four elevation selections for calculation purposes are:
  • zenith, elevation = 90 degrees;
  • high, elevation = 60 degrees;
  • medium, elevation = 40 degrees;
  • low, elevation = 18 degrees.
  • Note that for S-band (2–4 GHz), ground spillover results in higher system temperatures; S-band observations below 20 degrees elevation are not recommended.

• Average Weather: this entry selects the empirical opacity based on many years of tipping data at the VLA, as discussed in VLA memo 232 and EVLA memo 143. What is actually selected is the 22 GHz opacity; opacities at other frequencies are derived from this. The average weather corresponds to 22 GHz opacities as follows (no diurnal variations in the opacity are accounted for): 
  • Summer: 22 GHz opacity = 0.158
  • Autumn: 22 GHz opacity = 0.07
  • Winter: 22 GHz opacity = 0.045
  • Spring: 22 GHz opacity = 0.091

• Calculation Type: see the three main operation modes described in the ECT introductory section.
• Number of Sources: This can be used to specify that a number of sources will be observed within a single observing block, using identical instrument configurations such that they can share calibration observations. The calculation of overhead is made on the assumption that the fixed overhead (consisting of initial slew and flux/bandpass calibration) can be distributed between the sources; the fixed overhead per source is thus reduced. The variable overhead is calculated using the same overhead fraction as normal from the total time on source – no allowance is made for slewing between sources or any additional calibration needed. If Number of Sources is set to 1 (the default) then the calculations are performed for a single source in an identical manner to older versions of the ECT. This cannot be combined with more than one frequency so this is grayed out if Number of Frequencies is greater than one.
• Time on Source: input or output field. This is the time on the source of interest, not including overhead for calibration, slewing, etc. Also see the description of the next field, Total Time.
• Total Time: input or output field. This is the total time to be requested in the proposal, and includes typical additional calibration, slewing, etc., for the number of sources specified and for the single band specified. Different calculations of the overhead are implemented for observations of different lengths to ensure the best estimate is delivered. Earlier versions of the ECT used overheads derived from 2-hour observing blocks, severely underestimating the necessary observing time for shorter blocks. Here we describe the various implementations:
  • For 'standard' observations greater than 2 hours in length, default overhead factors, as a function of receiver band and array configuration, have been implemented into the time or noise calculations and are based on a 2-hour scheduling block as before. The overhead is calculated by multiplying the overhead factor by the time on source. The fixed startup overhead  is rolled into the general overhead factor rather than being explicitly accounted for.
  • For 'short' observations less than 2 hours in length, the (band-dependent) fixed startup overhead is split out and explicitly included, with the remaining (band and configuration dependent) variable overhead factor calculated to give the same result as the 'standard' calculation for a 2-hour scheduling block. The overhead is calculated as the fixed startup overhead plus the variable overhead (the product of the variable overhead factor and the time on source).
  • For 'very short' observations where the variable overhead (the product of the variable overhead factor and the time on source) is less than four minutes, the variable overhead is replaced by a fixed four-minute overhead. A warning is also given that small errors in the overhead calculation could lead to large changes in the exposure time. For such observations, the use of the OPT to create mock scheduling blocks is recommended in order to better estimate the overhead.

Additional output in the 'Overhead Explanation' box at the foot of the ECT gives information on the calculation used to estimate the overhead.

Both Time on Source (the time on a single source) and Total Time (including observing overhead and, if selected, on-source time for multiple sources) are displayed (and can be input) in the calculator. Time on Source or Total Time take various input formats, e.g., 20s (for twenty seconds), 10m 10s (for ten minutes, ten seconds) or 2.5h (for 2.5 hours). Time inputs can also be just seconds, minutes, or hours (e.g., 800s, 75m), or 2h 5m 35s (for 2 hours, 5 minutes, 35 seconds). Spaces between units are optional.

• Bandwidth (Frequency): the bandwidth in frequency units. For continuum use the full usable bandwidth (excluding RFI), up to 2 or 8 GHz. For spectral line observations, one can use the width of a channel or the width of many channels that define the RMS in the science goal. This can be the total anticipated width of a line or a fraction thereof. In any case, please mention and explain the bandwidth value that is used in the Technical Justification

  At lower frequencies, RFI can limit the amount of effective bandwidth for observing. The calculator does not take this into account in its calculations, so it is up to the observer to insert a reasonable bandwidth for continuum observation calculations. The maximum affected bandwidth for the lower frequency bands (L through Ku-band) is given below, and also as messages in the calculator:

• L-band (1–2 GHz):  maximum affected bandwidth 40% (i.e., 600 MHz is a reasonable effective total continuum bandwidth)
• S-band (2–4 GHz):  maximum affected bandwidth 25% (i.e., 1500 MHz is a reasonable effective total continuum bandwidth)
• C-band (4–8 GHz): maximum affected bandwidth 15% (i.e., up to 3.4 GHz is a reasonable effective total continuum bandwidth when using 3-bit samplers)
• X-band (8–12 GHz): maximum affected bandwidth 15% (i.e., up to 3.4 GHz is a reasonable effective total continuum bandwidth when using 3-bit samplers)
• Ku-band (12–18 GHz) maximum affected bandwidth 12% (i.e., up to 5.28 GHz is a reasonable effective total continuum bandwidth when using 3-bit samplers).

• Bandwidth (Velocity): the bandwidth in velocity units (assuming the line is at redshift zero). For continuum, use the full usable bandwidth (excluding RFI) up to 2 or 8 GHz in the field above (Bandwidth Frequency) instead of this field. For line observations, one can use either the field above in frequency units or this field in velocity units (at z=0). See Bandwidth Frequency above for what to enter.

• RMS Brightness Temperature: the conversion to RMS brightness temperature from RMS flux density depends on the size and shape of the synthesized beam. Since details of the actual (u,v)-coverage are unknown, we have to make certain reasonable assumptions about the beam shape; here we assume that the beam is Gaussian and round. This means the derived RMS brightness temperature is approximate only. More details about this conversion can be found on our mJy/beam - Kelvin conversion page.
• Confusion Level:  Given the array configuration (i.e. synthesized beam), and the sky frequency, the confusion level is calculated.  This confusion level is displayed here.
• Overhead Explanation: Gives details of the overhead time and how it was calculated.
• HI Column Density: this feature is only for science projects targeting neutral Hydrogen and is shown in the calculator when the frequency is at L-band and < 1500 MHz. The calculation assumes a rectangular line shape of width given in the bandwidth entry and calculates an HI column density based on the RMS value.
• Help: clicking this button brings you back to this page.
  
• Save: a screen capture to a PDF file is available (the Save button at the very bottom of the calculator). This PDF can be uploaded to the proposal's Technical Justification in the Proposal Submission Tool (PST). Before uploading, please check the PDF file for any errors (which may be caused by timeout of the web based exposure calculator tool) and compare the numbers in the PDF with the text input fields in the proposal Technical Justification and possibly the Scientific Justification.

Known Issues

• Purpose of Calculation: The text for the 'Purpose of Calculation' should be between 8 and 200 characters long and an error message is shown if you attempt to save the output of the calculator with invalid input in this box. This error message will not vanish after the issue is addressed if you hit save directly after editing the text box (although it will if you go to one of the other input fields) but your output will save correctly.
• Digital Samplers: A bug can sometimes cause the automatic switch to 3-bit for wider bandwidths to fail, with the 8-bit remaining selected even though it is not selectable. If this happens, you should select 3-bit manually by clicking on the 3-bit radio button.

A Special Case: P-Band

The exposure calculator has been updated for P band (224–480 MHz) observations. Elevation and seasonal differences have been turned off for this frequency. The noise calculation for P-band, however, is done assuming reasonably high Galactic latitude (greater than 60 degrees). At low Galactic latitudes, the Galactic sky background dominates the system temperature and the exposure calculator does not take this into account. We have made some initial tests on how much time over what the exposure calculator reports is needed for low Galactic latitudes. These are rough estimates: for Galactic latitudes below 30 degrees one should increase the time request over the exposure calculator by a factor ~2; for latitudes between 30 and 60 degrees by a factor ~1.2. For observations at the Galactic Center (a special place), the time might need to be increased by a large factor ~30. We are continuing to try to improve the time estimates and noise calculations for P-band. Questions about P-band observing should be directed to the NRAO Helpdesk.

A Special Case: 4-Band

The exposure calculator as of January 2020 assumes a 50 kJy SEFD (T_sys = 1780 K, antenna efficiency 20%) for 4-band (70 - 82 MHz). This value was estimated from commissioning observations of a subset of the installed modified J-poles (MJP; EVLA Memo #172). Note, this value is only valid for Y-polarization, while X polarization is worse by about a factor of two. Please refer to EVLA Memo #190 for further information on sensitivity estimation.

The VLA is now fully equipped with MJP dipoles. Current estimates show that the VLA is now about a factor 2-4 better in sensitivity at 4m-band than the pre-EVLA system. Some caveats remain regarding the imbalance between the two polarizations and differing beam patterns. To obtain sensitivity estimates for standard SRO 4m band observations we recommend using a center frequency of 76 MHz and a bandwidth of 12 MHz, which corresponds to the most sensitive range of the system. Also make sure to use a single polarization setup to obtain the correct value for Y polarization. If there are further questions regarding sensitivity calculation, please submit an NRAO Helpdesk ticket.

 

Overhead and Total Time

Every proposal needs to specify the total amount of time requested which includes setup scans, slewing, and observations of calibrators. Using the information supplied in the input fields, the exposure calculator first derives a Time on Source. It then uses the array configuration and the observing band to make a best estimate of the required overhead to arrive at a Total Time for a phase-referencing type of observation to reach the sensitivity requested. The sum of the Total Time with the fixed scheduling block (session) overhead, and any additional overheads described below, is the observing time request that should be specified in the proposal.

N.B. Starting with June 2023 version of the ECT, the overhead estimation has been improved by using different overhead factors for shorter and longer observing blocks.

Types of Overhead

The main types of overhead are:

Fixed.  The start-up scan sequence needs to be at least 10 minutes (lower frequencies) or 13 minutes (when reference pointing is used) for setup scans and for slew time from the previous (unknown) pointing. Another type of fixed overhead is that there usually needs to be one flux/bandpass calibrator scan per observation. This fixed overhead obviously affects the shorter scheduling blocks (0.5 and 1 hour) more than the longer ones.

Variable.  This is largely determined by periodic complex gain calibrator scans, slew time in between target source and complex gain calibrator, and slew time between target sources (if more than one). High frequency observations require more frequent source/calibrator sequences and therefore require more fractional overhead. They also require reference pointing scans, typically once every hour. This variable overhead is often taken to be linear with time on source.

Thus, for any frequency observing, the overhead in the ECT accounts for:

• An 11-minute block of setup scans at the start of the observation to ensure to get on source;
• A flux/bandpass calibrator scan of duration 2–4 minutes, depending on its brightness at the frequency of interest;
• Complex gain calibrator scans, each long enough to detect it (typically ~1 minute duration), and often enough for phase coherence, e.g., once every 8–15 minutes for the low frequencies (see Calibration Cycles in the Calibration section of the Guide to Observing with the VLA) and as fast as every 2 minutes at the higher frequencies;
• Slew time between source changes, that is, twice during a cycle time;
• a 30 second requantizer scan whenever a scan uses a 3-bit resource different from the resource used in the previous scan (whether 3-bit or 8-bit).

For high frequency observing, in addition to all points above, we also have:

• A reference pointing scan near the flux/bandpass calibrator and an initial reference pointing scan near the target;
• Further periodic reference pointing scans, dependent on the length of the scheduling block.

There are special cases not covered by the ECT, e.g., polarization calibration, multiple bandpass calibrator scans, multiple targets in a single observing block, multiple frequencies in a single observing block, or the possibility of self-calibration on the target.

Estimating Overhead

Based on the array configuration and the observing band, the Exposure calculator multiplies the Time on Source with a multiplication factor to give an overhead time, according to the following table:

These numbers were determined by creating realistic scheduling blocks with the following assumptions:

• Scheduling block duration: 2 hours. Overhead will decrease slightly for longer scheduling blocks, but can increase substantially for shorter blocks.
• Referenced pointing for Ku and higher frequency bands.

Note: every case is different, and the entries in the table (and therefore the Total Time reported by the Exposure Calculator) are only guidelines. Our recommendation, especially for higher frequencies, is to determine your overhead empirically by creating a realistic test scheduling block in the OPT with the on-source time to achieve the science goal. Experiment with different LST start times and use the most reasonable total time reported by the OPT.

For observations shorter than two hours, where the overhead substantially increases, the fixed and variable overhead are separated out. The Exposure Calculator Tool calculates the variable overhead according to the following table:

This is then added to the fixed overhead for the band, estimated according to the following table:

Subreflector move times

The subreflector on the antennas change position in both focus and rotation when changing bands. The amount of time needed for the position change varies when changing between bands (see Table 8.2 below). When changing from P-band to either L-band or S-band the subreflector requires the most time to get into position and may affect your overhead calculations. 

Note that if you slew either 20 degrees in Azimuth or 10 degrees in Elevation (or greater) then this slew time will absorb the required time for any subreflector positioning. If the slew is less than above, or you are staying on the same source and just changing bands, then you should account for a little extra time when going from P-band to L-band or S-band. 

 

Data Volume

Using the total estimated observing time (on source + overhead) and the data rate which is given by the PST, the total data volume of the proposed observations can be computed. The VLA OSS provides more details on the data rates and limits.