[evlatests] L-Band WIDAR Woes

[Barry Clark]

In re ea02*ea18 on E2, N16.

Numbers don't seem quite right, but Rick didn't give them very
accurately.  Might be worth checking.

Source 0217+738, time approx UT 0h, LST 11h20m

Baseline, in VLA centered coordinates = (2701, 552, -3994) in ns.

At that time, u was about 1520ns, or 1800 wavelengths (presuming
Rick was talking about the low band), fringe rate
37 mHz or a 27 second period.  This is the apparent fringe rate at
which a stationary interference source would show up.  Seems
significantly different from the 90s Rick quotes.

The position of the sun at 0h UT June 30 (or was it June 29 - unclear
from the message which day was meant) was 06h35m23s, +23d07'12.  For
the sun, u=3800 ns = 4600 wavelengths, and a fringe rate of 220 mHz;
subtracting 37mHz for the source gives a relative fringe rate of
183 mHz, or a period of 5.5 seconds, which might be Rick's 6s period.
Might be worth making a map at the above solar position (or at
06h31m40s 23d10'57" if the evening of June 28/29 was meant).


Rick Perley wrote:
>     I spent most of yesterday carefully working on our 1-hour L-band 
> dataset taken on the northern point-source, 0217+738, in the hope of 
> finding some sort of clues to help us figure out what is wrong. 
> 
>     I failed. 
> 
>     But perhaps the methods I used, and the results obtained, may be of 
> some interest or use, so I review these in some detail.   I apologize 
> for the length and depth of detail, but we probably need to record all 
> this to prevent future repetitions ...
> 
>     1) The duration of the test was about 1 hour, at HA = 9.  The source 
> was in the NW, at a low elevation (about 20 -- 30 degrees).  The data 
> were taken June 29 in a single block, with scans of length 1 minute.  
> The integration time was 1 second.  The source strength is 2.4 Jy -- a 
> strong calibrator, known to have no structure, in an empty part of the 
> sky (other than about 100 mJy of background sources within the 30 
> arcminute primary beam). 
> 
>     2) Four sub-bands, each of 1024 channels were set up. Two of these, 
> centered near 1050 and 1565, were so full of RFI that I made no attempt 
> to calibrate them.  The other two, centered near 1180 and 1950, are 
> relatively clean.  I used the program PHNEG to negate the phases for all 
> visibilities. 
> 
>     3) The basic spectra are exactly as expected, with RFI in all the 
> usual places, and 'open regions' contained stable data.  The delay 
> finding program FRING was run on all four sub-bands, with stable 
> results.  With antenna 1 as reference, antennas 2 and 9 both had large 
> delays (about 90 nsec!), all the others were less than 10 nsec.  The 
> stability of these delays over the length of the run, and between the 
> subbands, is very good -- about 1 nsec.  I used CLCOR to correct for 
> these delays, and checked that the resulting phase gradients were 
> negligibly small using POSSM.  I checked every single baseline to make 
> sure there were no correlator-based unique delay offsets.  All looks 
> good at this point. 
> 
>     4) CALIB was run, using a small (100 channel) subset of channels 
> taken from mid-band.  I limited the solution to long spacings,  as it 
> appears these are less susceptible to the issues discussed below.  Other 
> than some curious phase jumps within a minute of the start of the run, 
> all antenna phases are continuous and smooth.  There are no jumps or 
> ramps.  The amplitudes show numerous examples of 1db T304 attenuator 
> changes (these are *very* annoying!!!).   CALIB solutions were made 
> every second, the results were smooth with a 10-second boxcar, and 
> interpolated to the CL table in discrete time segments chosen to avoid 
> smoothing over the gain jumps.   The reasons for the smooth were to 
> avoid chasing noise, and to reduce the tendency of the antenna-based 
> calibration procedure to move correlator-based effects to antennas (a 
> problem that small-number arrays are especially vulnerable to). 
> 
>     5) BPASS was run to provide a normalized bandpass function for each 
> antenna.  A single solution, for each antenna and each sub-band was 
> generated.  These look entirely normal for the two RFI-free subbands. 
> 
>     6) I then generated three 'pseudo-continuum' databases from each of 
> the two RFI-free subbands -- of 1, 12, and 50 MHz width each, for 
> further analysis, using the program AVSPC.   UVPLTs of the three 
> databases from the high frequency sub-band showed clean visibilities 
> corresponding to a point source.  All visibility amplitudes were at the 
> expected level,and all phases centered near zero.  I had hopes of 
> getting clean images.  These were dashed upon viewing the results from 
> IMAGR.  The 1 MHz BW image (large enough to cover the entire primary 
> beam) gave a rather poor dynamic range (DR) of 2000:1.  This should be 
> enough to show the background sources (which are a factor of about 250 
> below the peak of the calibrator) -- but none were seen.  Only 'lumps 
> and bumps'.  The 12 MHz BW image had noise which was lower by the 
> expected factor (the DR is 7000:1), but *** still no background sources 
> *** -- these should be easily seen with at least 10:1 contrast against 
> the noise.  The 50 MHz BW image was not significantly different than the 
> 12 MHz BW image.  Although the map residuals gave no indication that 
> 'closure' problems are responsible, I ran the closure program BLCAL 
> anyway.  This made no perceptible different in the resulting image. 
> 
>     7)  The results from the low-frequency triplet of pseudo-continuum 
> databases gets more depressing, or interesting (depending on your point 
> of view).  The visibility plots immediately showed the present of *** 
> enormous *** deviant amplitudes, particularly on the short spacings.   
> These deviants are not 'spikes', but sinusoidally varying with 
> amplitudes up to 10 times the amplitude of the calibrator -- that makes 
> them 25 Jy.  These large deviant amplitudes are matched by similar 
> changes in phase, which are enough to rotate the phases continuously 
> around 360 degrees.  These deviations are strongest by far in the 1 MHz 
> BW dataset, but are still visible in the others.  The amplitude of these 
> oscillations are not constant through the hour -- they vary over a 
> factor of about ten in a rather continuous manner.  They are not due to 
> any 'stationary' external interference -- an image made at the north 
> pole shows no excess flux.  These excess visibilities cannot be due to 
> Cas or Cyg -- they are sufficiently far away to be attenuated by a 
> factor of at least 1000, more likely 10000, reducing them to sub-Jy 
> level.   It is perhaps possible that they are due to solar interference 
> -- the observations were taken in late afternoon -- except that some 
> baselines (2 x 18 -- E2 x N16 is an excellent example) show a clear 
> double beat, with a long period variation of period 90 seconds 
> superposed on a fast oscillation of 4 seconds.  I don't know how any 
> single external source can do such a thing.   In any event, I'm not 
> aware that the sun is up to anything recently -- if any of you have 
> information on this, let me know.   We'd have to be seeing nearly MegaJy 
> structure on the Sun in order to get ~25 Jy residuals on our northern 
> source, which is about 50 degrees away from the sun. 
> 
>     So , in sum, I'm completely baffled by what we're seeing here.  Note 
> that an observation of this source taken with the VLA correlator on June 
> 30 (the very next day!) provided a beautiful image, with no problems 
> whatever! 
> 
>     Some thoughts on what to try next:
> 
>     1) An observation of a good calibrator with accompanying 
> observations on nearby blank sky.  This will establish if we're seeing 
> an additive or multiplicative effect.  I note that the Cygnus A 
> observations seemed devoid of the fast oscillations noted above -- this 
> might argue for an additive effect, or that the problem was absent when 
> Cygnus A was observed.  (Note -- this was at night ...).    Although 
> 0217+738 is a good source, I think 3C147 (0542+498) may be better, as it 
> is 10 times stronger, and hence easier to calibrate in the face of RFI 
> and effects of undetermined origin. 
> 
>     2) I suppose it is somehow possible that some bizarre aliasing 
> effect is at work here -- it would have to alias the continuum via some 
> super-strong semi-continuous RFI spike (not seen in any spectra!).  We 
> could eliminate all this by band-limiting the RF signal prior to its 
> sampling.  Bob Hayward tells me he is pretty sure he can round up as 
> many as 10 RF filters with BW less than 100 MHz.   If we do this (and 
> doing so would clearly be an act of desperation ...), it will need to be 
> done on a maintenance day, and take some considerable organization.  
> Fortunately, we're in C-config now, so traveling amongst the antennas is 
> quick. 
> 
>    
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests