K/Q-band Scheduling and Proposal information

If you have K/Q-band time scheduled, please allow plenty of time to prepare your observe file. You will need to obtain the latest version of OBSERVE or JOBSERVE (a GUI-based Java-version of the OBSERVE program). When submitting your observe file to the VLA operators (observe@nrao.edu, email as body of the text) it is recommended to route a copy to the Data Analysts (analysts@nrao.edu) who will look over your file and, if necessary, get back to you with advice. Please consider the following items before submitting your schedule.

1. Observing strategy (includes example OBSERVE files)
2. Referenced pointing
3. Phase stability and fast switching
4. K/Q-band calibrators
5. Absolute gain calibration
6. Opacity corrections and tipping scans
7. Spectral line issues

Observing strategy:

The observing strategy will differ depending on whether your sources are strong or weak, what configuration the VLA is in at the time of your observations etc. If your source is strong you can apply self-cal on the source and hence there is no need for fast-switching. However, if the source is weak and an external phase calibrator is needed, you could do fast-switching (especially if VLA is in its A- or B-array).

To determine the observing strategy suitable for your observations, we recommend that you look at the collection of example files. For those we describe the type of observations, outline the observing strategy and show the resulting observe file. Parts of those files are highlighted with corresponding explanations - those are things you need to check extra carefully before submitting your schedule! For a detailed description on how to interpret the OBSERVE files, check the System File and Observer Source Card Reference.

If this is not enough for your observations, please feel free to Claire Chandler (cchandle@nrao.edu) or Mark Claussen (mclausse@nrao.edu) in advance of your observations for advice on observing strategy.

Referenced Pointing:

Because the systematic pointing errors at the VLA generally are 10-20 arc seconds (but can be as bad as one arc minute), they are a significant fraction of the primary beam at 43 and 22 GHz (FWHP about an arc minute at 43 GHz and about two arc minutes at 22 GHz). Therefore, high frequency observers will want to make frequent pointing measurements in their observations. The basic procedure is as follows:

1. Determine pointing offset by pointing up on a nearby, bright point like source. This is done at X-band and full bandwidth to give maximum sensitivity.
2. Tell the on line system to apply those corrections to any number of subsequent scans.
3. Repeat (1) when the pointing has changed significantly, for instance the source may have moved to a different AZ and EL. New pointing corrections should also be done when there is a change of temperature which could alter the shape of the dish (e.g. sunset and sunrise). As a role of thumb, point up every hour or so (except when temperature is changing rapidly).

The pointing calibrator should be at least 0.5 Jy and be close to the target. A pointing scan should not be applied to sources more than 30 degrees in AZ or EL from the pointing calibrator - often the phase calibrator is suitable also for a pointing calibrator. Avoid observing sources within 10 degrees of the zenith where changes in AZ are too rapid to calibrate. The dwell time should be long enough to allow a five-point cycle, which means it should be at least 2.5 minutes. The integration time should be a multiple of 10 seconds (not less, since the sensitivity then will be to small to derive solutions).

In OBSERVE, for ObservingMode on the Source Page specify "Determine ref pntng" for a primary pointing scan. Do not specify "Ref. Pointing" (further down on the Source Page); this should instead be specified in the following scans since it determines whether any pointing offsets are applied. Use the NRAO Default XX correlator mode to get full bandwidth at X-band. A much more detailed description can be found on the referenced pointing page.

The results of each reference pointing scan are printed on hard copy at the site. If you are not present for your observations, ask the operator to send the output to you (either by email observe@nrao.edu, or ask for it under "Special Instructions" in the OBSERVE file). It can be a helpful diagnostic of your run.

Phase stability and fast switching:

Due to variations in the water vapor content of the troposphere, there will be an additional source of phase noise when observing at high frequencies. If your source is stronger than 0.1 Jy, you can deal with this by applying self-calibration.

For weaker sources a "fast-switching" calibration method has been implemented at the VLA. This fast switching method involves normal phase calibration using external calibrators, only with a calibration cycle time short enough to 'stop' tropospheric phase variations. Normally, there is a 20 seconds start-up time each time the VLA reads a new source card. The fast switching mode avoids those overhead 20 seconds by using an "Offset Card", and thus allows a much shorter cycle time (down to 40 seconds). The fast switching mode also has the advantage of a much shorter observing file, which is easier to alter in real-time than a normal observe file (which has a long series of source cards). The fast switching mode requires that the correlator mode must be the same for the source and for the calibrator, e.g. one cannot use a narrow line mode on a maser source for phase calibration, then increase the bandwidth for the target source. Please read the fast switching page for all details.

If your target source contains a maser source (e.g. SiO), you may want to exploit the technique of self-calibration by monitoring the atmospheric phase fluctuations using the maser in one IF and applying the solution to data taken in the other IF. This can be done either in line or continuum mode. In continuum mode one would use a very narrow bandwidth centered on the maser in one IF and a 50 MHz bandwidth for source continuum observation in the other IF. For more information contact Mark Claussen (mclausse@nrao.edu) or Claire Chandler (cchandle@nrao.edu).

K/Q-band calibrators:

Select your phase calibrators from the VLA Calibrator Manual. Many high frequency calibrators have only been measured at Q-band. However, you can still use those as K-band calibrators, estimating their flux by interpolating between X/U- and Q-band.

For a 0.25 Jy source the expected residual rms phase variations (after calibration) due to sensitivity alone are about 25 degrees, using an averaging time of 10 seconds and full VLA bandwidth. This rms decreases as 1/(source flux density). Sources between 0.25 Jy and 0.5 Jy are recommended only for phase calibration. If the phase calibrator is weaker than 0.5 Jy, then one must observe a stronger calibrator every hour or so to track amplitude variations.

Absolute gain calibration:

Absolute gain calibration is difficult in the largest arrays at the highest frequencies, principally because both 3C48 and 3C286 are resolved. Currently the recommended primary flux calibrators are: 1331+305 (3C286) and 0713+438. If you cannot get to either of those, 0410+769 and 0319+415 (3C84) are also acceptable, although 0319+415 may be variable, and 0410+769 has uv restrictions (see the VLA Calibrator Manual) and may also be variable. A- and B-array clean component models of 3C48 and 3C286 are currently being improved by Claire Chandler. Those FITS models can be downloaded from her calibration web page or from the high frequency data reduction page. Please also check Steve Myers VLA/VLBA Polarisation Calibration Page, which includes flux density monitoring of a number of compact calibrators (C, X, K and Q band).

Opacity corrections and tipping scans:

One effect at high frequencies is the variation of total opacity as a function of elevation. Under good conditions a typical K-band zenith opacity could be 0.05, but varies a lot depending on the weather. The sky opacity at Q-band is dominated by the atmospheric oxygen line at 60 GHz, and so unlike the phase fluctuations which are determined by water, the atmospheric attenuation does not vary greatly under different weather conditions. At 43 GHz the typical zenith opacity is 0.06, and for 49 GHz it is about 0.15.

Ideally you would want to observe the flux and gain calibrator and your source at the same elevation. This is not always possible, and opacity corrections may need to be made. In the 31Dec2002 version of AIPS, FILLM automatically loads weather data recorded at the VLA, and you have the option to apply an opacity correction. FILLM determines the zenith opacity as a function of time from the surface weather data on the archive tape. At K and Q bands the opacity is a function of the surface ambient and dew point temperatures. See VLA Scientific Memo 176 for the function for K- and Q-bands.

Another way to apply opacity corrections is to use the task CLCOR, which given a value of the opacity applies the corrections. You can either take a standard value (see above), or derive the opacity by including a tipping scan in your observations. By using tipping scans the observer can achieve the zenith sky opacity at the time of the observations. By default, each tipping pass consists of 7 samples in elevation, spaced uniformly in air mass between 23 and 55 degrees elevation. For a tipping scan the specified RA is the azimuth (in degrees) at which the tip should be performed. Do not tip at 180 degrees or along any of the arms, to avoid shadowing (especially in the compact C and D configurations). A good choice is 13h for the "RA" and 20 deg for the "Dec". For a range of suitable azimuths, see Fig. 2 in VLA Scientific Memo 170 by Bryan Butler. A tipping scan is specified in the OBSERVE by setting ObserveMode in the Source Page to "Tipping Procedure". Allow around 10 minutes for a tipping scan. See also the JOBSERVE information about how to schedule a tipping scan.

Spectral line issues:

Integration time: The minimum integration time at which all data can be written to tape is a function of the total number of channels of data produced by the correlator. This means that if you are observing spectral line you need to check your minimum integration time. This can be estimated via
t_min = N (N+1)/2 * N_ch/10 000 sec
where N is the number of antennae and N_ch is the total number of correlated channels. For more details please read the time resolution chapter of the VLA Observational Status Summary.

Fast Switching & Doppler Tracking: Doppler tracking is only done at the beginning of a scan, as part of the 20-second overhead you're avoiding by using fast switching. For spectral line experiments you should check that the change in frequency from scan to scan is much less than a channel. Thus, you may need to setup several shorter fast switching scans instead of one long. To check on this you can either run dopset, or give a series of short scans in OBSERVE and check to see how the frequency corresponding to a constant velocity changes with time (you may find the Frequency Report helpful in this regard).

Also, be aware that Doppler tracking is performed for the "primary" source (the one given on the first (SU) card) only; the LO settings are then left fixed for the "secondary" source (the one given on the //OF card). Generally this means you will want the source you are actually interested in to be the primary, and the calibrator to be the secondary. As a bonus though, this does mean that your phase calibrator & source are automatically observed with identical LO settings. You have to be sure to put an initial calibrator integration at the beginning of src/cal cycles so that your observations will start with a calibrator.

Before submitting your schedule: Once you set up the observe file to have the right sequence of events, then write out a frequency report. Note: Calibrator frequencies are all different by sometimes a large amount because they are in a different part of the sky. Go through as see how much the frequency changes over the course of the observation in the source scans. If it is less than, say 1/3 of a channel, then take the average source frequency and edit ALL separate calibrator scans to have that frequency. This is particularly important for bandpass calibrator scans -- you don't want the frequency to be off by more than a relatively small fraction of a channel.

Double check the OBSERVE output: Please also note that once you start changing frequencies etc. OBSERVE may get confused; for instance about what a Q-band default is. Many times you just have to check ALL numbers and edit them if they are wrong, which means you have to know what is right. This includes the fluke settings in the LO card -- make sure the cal values are the same as the source values.

Bad LO settings - avoid those: If the IFA L6 frequency on the LO card is between 3750 and 3800 MHz -- change it! This value gets 1000 MHz added to it before it goes into C band and there is a birdie at 4800 in C band that can destroy your observations.