Using Retrofitted EVLA Antennas with the VLA

Last Revised December 28, 2007, 2007

Previous revisions of this document can be accessed via the EVLA Returns Archive.

While NRAO is in the process of building the EVLA, EVLA antennas are being retrofitted and returned to the operational VLA. These antennas are operated in a transition mode that does not yet have all VLA features available, and the antennas may be less reliable than unmodified VLA antennas. As information about the inclusion of EVLA antennas into the VLA becomes available it will be posted at the links below and updated on a regular basis.

NEW More information on the aliasing problem (December 17, 2007)

NEW Latest news on 25 MHz spectral line correlator offset problem (November 16, 2007)

Modcomp computers replaced (October 19, 2007)

Including Retrofitted EVLA Antennas in Observations (December 28, 2007)

Known Problems and Solutions/Workarounds (October 2, 2007)

Extended C-band coverage available on EVLA antennas (October 23, 2007)

Extended K-band coverage available on EVLA antennas (January 12, 2007)

Guidelines for post-processing EVLA data (October 5, 2007)

General description

Impact on observing

1. Whereas the actual line emission may not suffer from aliasing directly, the phase and bandpass calibrators are continuum sources, and will therefore be affected

   Since the calibrators are continuum sources, aliasing will occur, and potentially affect the calibration. It is therefore important that in determining the calibration, all data from EVLA-EVLA baselines be ignored. This can be done by flagging all EVLA-EVLA baselines prior to calibration, e.g. by using UVFLG with OPCODE='FLAG' and a unique choice for the adverb REASON. Note that in this way EVLA antenna gains are still being determined, but based on VLA-EVLA baselines only. After the full calibration (including bandpass) is complete, the EVLA-EVLA baselines need to be unflagged, again using UVFLG, now with OPCODE='UFLG', and the same value of REASON as was used when flagging them. For subarrays containing only EVLA antennas the above will clearly not work, and methods for calibrating these data are under investigation.

2. In most cases, the field of view will contain both line and continuum emission, and removing the continuum requires extra care

   The aliased continuum can be subtracted in the visibility plane using the AIPS task UVLSF, just as has been common practice in the past for data not affected by aliasing. However, in order to fit the shape of the aliased continuum, a high order fit is needed. The latest version of UVLSF now supports orders up to 4, and users will need to make sure they have run a Midnight Job since October 31, 2007, to obtain this version. It is our experience that a fourth order fit is required and sufficient to properly represent the aliased continuum. Note that while the line signal can be recovered this way, this is not the case for the continuum as there is no way to separate the aliased part of the continuum from the unaffected part. We have received reports, though, that running BLCAL can improve the resulting continuum, but this requires frequent observations of a strong calibrator.

   Clearly, a higher order polynomial fit will only work if there is a sufficient number of line-free channels at either end to base the fit on. We recommend that at least one-quarter of the total bandwidth at either end is line-free. In other words, the total number of channels with line emission should not exceed 50% of the total number of channels.

   A successful fit requires aliased signal to base the fit on. This may not be the case if the field of view does not contain strong continuum sources. We are considering implementing AIPS software that will perform the UVLSF fit on calibrators only, and subtract a properly scaled version of this aliased response from the source data. In anticipation of such a task, we strongly recommend all spectral line observers planning to use narrow bandwidths to give higher weight to strength than to vicinity to the source, when deciding which phase calibrator to use.

   Note that it is almost certainly impossible to stitch together narrow bands to obtain wider frequency/velocity coverage for wide spectral lines, because there is no way to subtract the aliased continuum signal in this case. Also, the line emission below baseband will be aliased into the band in an unpredictable way depending on the shape and strength of the emission line.

3. In all cases, noise is aliased into the band and will decrease the effective sensitivity, with the channels closest to baseband being the most affected.

For observations taken before that date, the effects of the offset can be removed during post-processing provided there is no emission of interest in the center of the field. See the relevant section in the post-processing chapter for more details.

While we have done all we can to ensure that the most commonly used modes work under the new system, we ask our users to inspect their data extra carefully, and let us know of any unexpected effects as soon as possible

A number of problems or limitations related to having two different systems controlling the array have disappeared:

• Now that only one system controls both VLA and EVLA antennas the original reason that Doppler tracking was not supported on VLA - EVLA baselines has disappeared. However, the presence of phase jumps on VLA - EVLA baselines at frequency changes remains, and we still don't recommend Doppler tracking unless one observes EVLA - EVLA baselines only.
• When moving to the first source in an observation, VLA antennas and EVLA antennas will now choose the same wrap
• For non-dynamic observations, durations are an acceptable alternative again to stop times. Note that for dynamic observing, durations continue to be mandatory

A few observing modes are not supported yet, but we intend to have them operational by the time scheduled observations require them. These include support for planetary observations, and phased array observations. Multiple subarrays are supported now, but all subarrays must have the same correlator mode (e.g. 2AC), bandwidth, integration time, etc. In case of a conflict, the settings of the subarray with the largest number of antennas will be used for all others.

SUMMARY

As of 1 August 2006 we changed the mode by which EVLA antennas are given to observers. Beginning on that date, all observing programs have been given the EVLA antennas that have returned to operations, and are listed in the table below. We believe that the antennas returned to operations will likely give reasonable data most of the time, if the guidelines appearing on this page are followed; but this is not guaranteed. For a list of the main known problems in using the EVLA antennas and their solutions/workarounds click here.

• The following retrofitted EVLA antennas are currently available (or, if in red, are scheduled to be included shortly), for use in the following frequency bands:

• EVLA antennas will be automatically included by default in an observing program. If EVLA antennas are not wanted, then an explicit message to the VLA operator should be put in the comment field at the beginning of the observe file, such as "Please do not include EVLA antennas in this observation."
• The observe file remains the principal way for observers to instruct the array what to do and when. In particular, the //LO card continues to be used to tune the local oscillators. For the EVLA antennas, which have a completely different LO chain, the LO card settings are used to derive an observing frequency, from which optimal EVLA settings are derived. This is transparent to the observer.
• Observe files should be sent to the data analysts at least one week before scheduled observing to be checked to make sure they command the EVLA antennas properly.
• The sensitivity of the EVLA antennas may differ from VLA antennas by tens of percent. Therefore, observers should take extra care to calibrate the data weights (DOCALIB=2 in AIPS, actually any DOCALIB > 0 and < 100 in 31DEC06)) whenever using EVLA antennas. (We recommend this step as well even when EVLA antennas are not used.)
• The EVLA bandpass is enough different from that of the VLA that about a 6% closure error is present on EVLA-VLA baselines in continuum modes using 50 MHz bandwidths. This closure error will be larger in continuum modes at narrower bandwidths. The closure error can be mitigated by observing a strong source with known structure and using the AIPS task BLCAL to determine baseline-based closure corrections. For observers who are interested in continuum observations for bandwidths of 25 MHz or narrower we recommend the use of spectral line mode (i.e. pseudo-continuum) rather than continuum mode to be able to correct for these closure errors.
• In principle, under the new Modcomp-free system it would be possible again to get meaningful data on crossed (VLA - EVLA) baselines while Doppler tracking, but for the fact that the potential for phase jumps at small frequency changes remains. Therefore we continue to recommend to use fixed frequency observing for those projects that need the crossed baselines (when using EVLA - EVLA or VLA - VLA baselines only, Doppler tracking is supported, as before). See the documented Doppler workaround recommendations and/or the Online Dopset Tool . We suggest that, after reading these documents, line observers consult with a VLA staff person before submitting their first observe file. For questions or concerns about creating Observe files using EVLA antennas (whether line or continuum), please e-mail our support staff.

  GENERAL ISSUES

  • Users should inspect data from the EVLA antennas particularly carefully, since not all the online system flags are currently operating for these antennas, and some of the other problems mentioned under Current Known Problems can result in data that need to be flagged. At present, flags are set for antennas that are off source, mispositioned subreflectors, and synthesizers that are out of lock.
  • The new online system should no longer erroneously flag good data. Some observers may want to inspect all data regardless of flagging by the online system by using CPARM(2) = 1 and CPARM(3) = 16 in FILLM.
  • We strongly recommend that Observe files (for the VLA or the EVLA) should be written entirely in J2000 coordinates. However, B1950 coordinates work and have been tested for the EVLA antennas.
  • Observe files for regularly scheduled projects can now be in stop times or in durations. Observe files for dynamically scheduled projects must be in durations
  • The planned availability of the various bands on the EVLA is shown in this graph. Interim L and interim C-bands are not equipped with Ortho Mode Transducers (OMT), and therefore have poor polarization properties outside the 'old' L and C band ranges (1.25 - 1.75 GHz for L-band, and 4.5 - 5.0 GHz for C-band)
  • No EVLA antennas have the VLA U band (15 GHz) frequency available. U band receivers will begin to be installed on EVLA antennas in 2011.

  SPECTRAL LINE OBSERVATIONS

  • Though under the new Modcomp-free system, cross-correlations between EVLA and VLA antennas with online Doppler tracking have been shown to work again, we still cannot recommend this since the potential for phase jumps at frequency changes remains. Therefore we continue to recommend to observe in fixed frequency mode unless only VLA or only EVLA antennas are being used. Note that we strongly recommend a calibrator scan between any change in frequency
  • Doppler tracking corrections can be applied to data taken in fixed frequency mode using the routine CVEL in AIPS.

  OBSERVATIONS AT MULTIPLE FREQUENCIES

  • The phase jumps on VLA - EVLA baselines introduced by changes in observing frequency and/or observing bands should be tracked by observing a phase calibrator every time there is a frequency or band change in the observing program. Generally, source scans should be bracketed by a phase calibrator scan using the same band, local oscillator, and Fluke settings.

  OBSERVATIONS AT K AND Q BANDS

  • The EVLA high-frequency performance is not as good as that of VLA antennas, due to a combination of issues that have not been completely resolved which includes pointing, subreflector positioning, and focus.
  • Observers at high frequencies should take the following into account:
  • Tipping curves for opacity determination are not implemented on the EVLA antennas. In most cases this will not affect users, who at this point should probably be using the opacity corrections derived from the weather data that are calculated by FILLM in AIPS when running with the default BPARM=0.
  • Antenna gain curves as a function of elevation are not yet available for the EVLA antennas, and subreflector focus and rotation with elevation is not yet implemented. Thus the gain response versus elevation is likely to be significant, and to change on relatively short timescales for these antennas until the retrofits are complete. These effects can to some extent be taken into account by observing your primary flux calibrator at an elevation as close as possible to that of your phase calibrator, so that its flux can be bootstrapped while minimizing systematic elevation-dependent offsets. The routine ELINT in AIPS can also help to alleviate elevation-dependent gain offsets between your source and phase calibrator.
  • Secondary reference pointing was implemented and tested on EVLA antennas in mid-July 2006. Observers should pay careful attention to their secondary reference pointing results on EVLA antennas, and report any apparent problems.

  AVAILABILITY OF OTHER OBSERVING MODES

  • Primary and secondary reference pointing work with EVLA antennas
  • Fast switching works with EVLA antennas.
  • Multiple subarrays are now supported. Note that all subarrays must have the same correlator mode, bandwidth, integration time, etc. In case of a conflict, the settings of the subarray with the largest number of antennas will be used for all others. For example, if the largest subarray has correlator mode 2AC and 6.25MHz bandwidth, all subarrays will use mode 2AC and 6.25MHz bandwidth.
  • Use of B1950 coordinates has been tested and is currently working
  • Observations of moving objects (such as planets) with the EVLA antennas is currently not implemented but efforts to make it available are underway
  • Solar observing with the EVLA antennas is currently not implemented
  • It is now possible to include EVLA antennas as part of the phased-VLA for VLBI. Observers should be sure to phase up often during a phased-array experiment, and to inspect their data carefully during data reduction. Since it takes a minimum of 30 seconds to phase up, phasing scans should be 30 seconds plus the amount of time it takes to detect the source for VLBI purposes.
  • The High Time Resolution Processor is not available with the EVLA antennas

  OTHER IMPORTANT ISSUES FOR OBSERVERS

  • The EVLA L band (20 cm) and C band (6 cm) feeds are new. Their properties are not yet well determined. In particular, the variation of polarization across the antenna beam has not been measured.
  • Use of the round-trip-line-length monitor data is not implemented. The net effect is a diurnal term in the antenna phase. Not a problem (except for astrometry) with reasonably frequent calibration.
  • Bad data may not be flagged, especially near scan start. Observers can expect the need to examine the baselines including EVLA antennas much more carefully than is needed for unmodified VLA antennas, and they may need to flag their data more extensively (perhaps using the task QUACK in AIPS) at the beginning of each scan.
  • EVLA antennas do not control feed heaters. Expect problems with frost or dew on the feed covers if relative humidity approaches 100%.

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  Known Problems and Solutions

  The following lists the main known problems in using the EVLA antennas and their solutions/workarounds:

  Known Problems (in red) and Solutions/Workarounds (in black)

  
  
  
  1. During the period June 27 to August 29, reference pointing solutions were not applied to antennas 1 - 9
     
     In high-frequency observations taken during this time period antennas 1 - 9 should be excluded when deriving the flux calibration in GETJY
  
  
  2. Antennas sometimes fail to get organized during the first scan.
     
     For any observation, add a dummy first scan. Typically, this would be a short scan on the first source, and would be followed by the actual scan on that source. Though the first 'dummy' scan could in principle be of arbitrarily short duration, JObserve will insist on a non-zero dwell time for that scan
  
  
  3. The online system that runs the EVLA antennas does not know, a priori, where the antennas will start out at the beginning of an observation. This means that if an observation begins in the overlap region of the azimuth wrap (-85 to 85 degrees, or 275 to 445 degrees) the VLA and EVLA online systems may choose different wraps. Once both sets of antennas have observed a source in the south, or if an antenna wrap card (//AN) specifying explicitly the wrap to use is encountered in the observe file, they will be synchronized again. For more on antenna wrap, click here.
     
     To avoid the potential loss of synchronization at the beginning of an observation that starts in the overlap region, and the subsequent need for one set of antennas to "unwind" at some point during the observation, add an antenna wrap card to the first scan of the observe file by selecting either the Right or Left wrap on the scheduling page in JObserve. Be aware that if the previous observation ended in the overlap region of the azimuth track on the opposite wrap from that selected for the first scan of your observations the antennas will have to unwind, which could take 10 minutes or more. This extra time needs to be included in the first scan.
  
  
  4. Changes in the (VLA) fluke synthesizers are known to cause phase jumps on VLA - EVLA baselines and are therefore a transition issue
     
     On any frequency change, however small, bracket the source scan by calibrator scans at exactly the same frequency (fluke settings). Note that this also precludes use of Doppler tracking as the resulting frequency shift will cause phase jumps.
  
  
  5. Closure errors of up to approximately 8% on EVLA-VLA baselines in 50 MHz continuum mode due to non-matched bandpass shapes. The closure errors on narrower continuum bandwidths are larger.
     
     Observe a strong source with known structure (could be your amplitude calibrator) and use the AIPS program BLCAL to determine baseline-based closure corrections. Preliminary tests using this method reduced the closure errors for the EVLA-VLA baselines by an least an order of magnitude (in 50 MHz continuum mode). For narrower band continuum observations, it is probably best to observe in spectral line (pseudo-continuum) mode.
  
  
  6. EVLA antennas lack Tsys values
     
     In order to obtain a Tsys value for EVLA antennas use the VLA back-end Tsys, which can be obtained by setting CPARM(2) = 2 in FILLM.
  
  

  Finally, we encourage users to give us feedback as soon as possible after their observations. Any questions prior to observing, and feedback afterward, can be e-mailed to our support staff.

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  Extended C-band coverage available on EVLA antennas

  New C-band frequencies

  Most EVLA antennas currently in the array (see the table in the relevant section for the current status) have the new wide-band C-band receivers installed, which can be tuned outside the traditional VLA C-band range (4.5 - 5.0 GHz). Tests show that meaningful results can now be obtained from 4.2 - 4.5 GHz and from 5.0 - 7.7 GHz as well. For this reason a special call for exploratory proposals to use this new C-band capability has gone out with an April 9, 2007 proposal deadline. The rms (in arbitrary units) as a function of frequency between 5.5 and 8.0 GHz is plotted here. The features at 6.0, 6.4, and 7.9 GHz are resonances which we expect to go away once the Ortho Mode Transducers (OMTs) are installed some time in 2008.

  A plot showing the K value against frequency is shown here at higher frequency resolution than the figure referred to above. This plot covers in detail the range from 5900 to 6700 MHz. This range was chosen since it is not only there that the resonances occur, but it also contains the familiar OH lines at 6031 and 6035 MHz and the Methanol line at 6668 MHz. See the Observational Status Summary for a definition of the K-value.

  Note that at outside the traditional C-band range polarization information is lost, as illustrated by this figure showing the ratio RL/RR for two baselines for an unpolarized source. Note that only between 4.5 and 5.0 GHz this ratio is close to zero, as expected for an unpolarized source. Polarization observations won't be able to exploit the extended frequency range until the installation of the OMTs.

  To illustrate the opportunities this new frequency range offers, we observed a number of maser lines in W3OH. In figures 1 - 3 we show their scalar cross-power spectra on one of the EVLA - EVLA baselines. Figure 1 shows the 6030.747 MHz OH line, figure 2 the 6035.092 MHz OH line, and figure 3 the 6668.5192 MHz Methanol line. The flux density scale is indicated below the plot; because of a lack of accurate calibrator flux densities at these frequencies we did not perform any flux calibration; we expect true flux densities to be about 40% higher than the y-axes indicate.

  Those interested in observing outside the traditional VLA C-band should be aware that only data from EVLA antennas can be used - VLA antennas will not produce useful data at these frequencies. Also, no polarization information can be obtained. Finally, JObserve does not support the creation of the required observe files; see the following section for more information.

  Observe file preparation specifics

  Since observations at the new C-band frequencies can only be done with EVLA antennas, there is no danger of introducing phase jumps caused by the small frequency changes while Doppler tracking, on VLA - EVLA baselines. Under these circumstances, Doppler tracking is possible again.

  When Doppler tracking, the EVLA LO chain is tuned based on the sky frequency derived from date and time, rest frequency, and velocity. In that case, values on the //LO card will be ignored; in fact, the whole LO card can be deleted. (In fixed frequency observing on the other hand, values on the LO card are required in order to calculate the sky frequency).

  Since JObserve does not support the new frequencies, we suggest preparing the observations using rest frequencies JObserve does allow, and modify the observe file at a later stage. The steps are:

  1. using JObserve or observe, prepare the complete observe file using, e.g., the 4829 MHz formaldehyde line
  2. choose 'write observe file' and exit JObserve
  3. using the editor of your choice, replace all occurrences of the 4829 MHz rest frequency in the observe file with the one you want. The format is very specific, so make sure the new frequency appears in exactly the same fields as the old one
  4. Don't use 'no change' as a synthesizer setting option, as it will cause the frequencies of the data to be mislabeled. If the difference in sky frequency is not important, use Doppler tracking instead, with the same velocity as used for the source. If the difference in sky frequency is important, use Doppler tracking with a different velocity which leads to the same sky frequency as the source. This velocity can be determined using our Online Dopset Tool .
  5. it is highly recommended to run the observe file by our support staff before submitting it
  Notes:
  • After editing the observe file by hand, do not read it back into JObserve, since JObserve will not be able to handle some of the modified entries in the observe file
  • if for some reason you do not want Doppler tracking, you will need to specify the appropriate LO settings directly in the observe file. If this is the case, please contact our support staff

  Subarrays

  Support for multiple subarrays under the Modcomp-free system has been implemented as of mid-September, 2007. Note the following:
  • for dynamically scheduled observations, use of only one subarray is allowed
  • When observing in line mode under the new system, the restrictions have become more stringent: for all subarrays, the correlator mode, bandwidth, etc. must be identical. If they aren't, the settings of the subarray with the largest number of antennas will override the settings for all other subarrays. For example, if the largest subarray has correlator mode 2AC and 6.25MHz bandwidth, all subarrays will use mode 2AC and 6.25MHz bandwidth. If you plan to use multiple subarrays in line mode we strongly encourage you contact a local expert.

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  Extended K-band coverage available on EVLA antennas

  Most EVLA antennas currently in the array (see the table in the relevant section for the current status) have the new wide-band K-band receivers installed, which can be tuned outside the traditional VLA K-band range (21.2 - 25.2 GHz). Recent tests show that meaningful results can be obtained from 18.1 - 26.5 GHz, with a small penalty in performance at the extreme ends, as shown in this figure, which plots the rms (in arbitrary units) averaged over the four IFs as a function of frequency at the band edges. Note that at 18.1 and 18.3 GHz not all IFs led to a solution. Between these edges, the rms rises linearly with frequency, with a slope largely determined by the frequency dependent antenna efficiency.

  Those interested in observing outside the traditional VLA K-band should be aware that only data from EVLA antennas can be used - VLA antennas will not produce useful data at these frequencies. Also, JObserve does not support the creation of the required observe files. Please refer to the 'observe file preparation specifics' and 'subarrays' subsections in the extended C-band section above.

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  Guidelines for post-processing EVLA data in AIPS

  Since August 1, 2006 EVLA antennas have been included in observations by default. During the post processing of any data including EVLA antennas there are a number of caveats observers should be aware of. In any case we strongly recommend users to use the latest version of AIPS (31DEC07), including running the AIPS midnight job regularly

  EVLA antenna positions

  Since their inclusion in the array by default, we have obtained improved EVLA antenna positions over time. This means that the position of the EVLA antennas (and VLA antennas, for that matter) in your data may not be the best available one, which can lead to phase errors. Especially for EVLA antennas, this error may be substantial. The AIPS task VLANT determines the antenna position corrections applied by the VLA operations staff after your observation that apply to that observation. It copies an input CL table and applies these antenna position corrections to the antenna phases. It is important that VLANT be run on CL table version 1 immediately after running FILLM, and prior to any other operation on the CL table. Note that since VLANT is a new task you must have 31DEC07 AIPS to run it.

  Closure errors

  As advertised, 50 MHz continuum observations show closure errors of around 8% on VLA - EVLA baselines due to non-matched bandpass shapes between VLA and EVLA antennas. This error becomes much more severe at narrower bandwidths, and we recommend observing in spectral line mode for those bandwidths, in which case an initial bandpass calibration can correct for the non-matched bandpass shapes.

  It is common practice to use 'channel 0' as created by FILLM in the initial calibration stage. On data with VLA - EVLA baselines this practice is strongly discouraged since the 'channel 0' data now contain closure errors on VLA - EVLA baselines. Our recommended solution is to create an alternative 'channel 0' from which closure errors have been removed. There are two ways to do this: an easy one which requires an AIPS midnight job (MNJ) after October 1, 2007, and a more complicated one for older versions of AIPS. The difference is that in the latest versions of AIPS the task AVSPC allows application of calibration, which reduces the number of steps required.

  • AIPS with no MNJ after October 1, 2007

  1. Run BPASS on the LINE multi-source dataset, creating a bandpass (BP) table. Make sure BPASSPRM(5)=0 (the default)
     
     
  2. Apply this BP table using SPLAT, creating a multisource output file. In SPLAT, make sure to specify DOCAL=0 (which causes CL table version 1 to be copied to the output file) and all sources
     
     
  3. Add an index (NX) table to the SPLAT output either by copying the NX table from the SPLAT input to the SPLAT output using TACOP, or by running INDXR on the SPLAT output
     
     The first option is easiest, but requires an equal number of visibilities in input and output; it fails when application of the BP table in SPLAT leads to visibilities being flagged
     
     The second option is tricky when pointing scans are included: standard application of INDXR will merge calibrator scans at either side of a pointing scan into one, with a possible phase jump within that scan. In that case, scan boundaries need to be explicitly specified in INFILE (see EXPLAIN INDXR)
     
     
  4. Use AVSPC to average a number of spectral line channels together into an alternative 'channel 0' dataset (FILLM uses the central 75%, which is also the default in AVSPC, but other choices over a flat section of the bandpass are fine, too). Channel specification in AVSPC is flexible allowing the user to omit channels contaminated by line emission or absorption in the calibrator passband
     
     
  5. Continue the calibration as one normally would (i.e. run CALIB etc) on the newly created 'channel 0' data
     
     
  • AIPS with MNJ after October 1, 2007

  1. Run BPASS on the LINE multi-source dataset, creating a bandpass (BP) table. Make sure BPASSPRM(5)=0 (the default)
     
     
  2. Use AVSPC to average a number of spectral line channels together into an alternative 'channel 0' dataset (FILLM uses the central 75%, which is also the default in AVSPC, but other choices over a flat section of the bandpass are fine, too). Channel specification in AVSPC is flexible allowing the user to omit channels contaminated by line emission or absorption in the calibrator passband. Apply the BP table using DOBAND=1 and make sure to specify DOCAL=0 (which causes CL table version 1 to be copied to the output file) and all sources
     
     
  3. Continue the calibration as one normally would (i.e. run CALIB etc) on the newly created 'channel 0' data
     
     

  Doppler tracking

  After the Modcomp retirement Doppler tracking works again for all baselines. However, the potential of phase jumps on VLA - EVLA baselines, caused by the small frequency changes, remains. Our recommendation not to use Doppler tracking therefore remains in effect. If you did manual Doppler tracking (i.e. calculate the appropriate sky frequency for every individual scan), no resampling in frequency is needed. In that case you should have put calibrator scans around each scan on source to deal with the phase jumps associated with frequency changes on VLA - EVLA baselines.

  During the transition, online Doppler tracking is not recommended in many cases, for a variety of reasons. In that case, you probably observed all or most scans with the same frequency, and you may need to resample your data to a common frequency/velocity axis using the AIPS task CVEL, and enter the velocity manually using SETJY. Note that if the line width is narrow compared to the channel spacing, resampling may result in unwanted sidelobes in frequency (Gibbs effect), and off-line Hanning smoothing may be needed to suppress these sidelobes at the price of a loss in frequency resolution.

  Removing 25 MHz spectral line artifacts

  Data taken in spectral line mode at 25 MHz bandwidth between August 2006 and November 8, 2007 and suffering from the correlator offset problem can be corrected during post-processing as long as there is no significant emission in the center of the field.

  To this end the AIPS task UVMTH, written in 1989 and then mostly forgotten, was resurrected. UVMTH time-averages the visibilities in a uv data set on each baseline and correlator and adds them to, subtracts them from, multiplies them by, or divides them into the visibilities in another data set. It was recently revised to apply all of the data selection, calibration, and flagging options to the first input uv data set while averaging each baseline and correlator over all TIMERANG. The result is applied to the second data set which is read, modified, and written with no application of data selection (other than sub-array and frequency ID), flagging, or calibration. For the VLA correlator offset problem (and a similar one sometimes seen in GMRT data), the user should specify the same data set for both inputs. SOURCES should select those pointings with no significant source at the center of the field and FLAGVER should select flags for at least these sources, but all calibration should be turned off. The default subtraction operation should be selected.

  If you have comments/problems/questions about data from the EVLA antennas please contact our support staff.

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