[evlatests] VLA image -- and an interesting clue ...

[Barry Clark]

Great job George; unfortunately the bottom line is that we have once
again ruled out a hypothesis that might explain our problems.

George Moellenbrock wrote:
> Folks,
> 
> Rick, Michael, and Barry encouraged me to calibrate and image in
> CASA the blank field (near 0217) in the L-band data
> (256 chans/subband) taken Jul 10.  The intent here is to isolate
> the details of Rick's reductions in AIPS from intrinsic problems
> with data itself, as regards recovery of the weak (10s mJy) background
> sources in the "blank" field, and to better characterize frequency-
> and bandwidth-dependent effects in the imaging.
> 
> SUMMARY:  I believe I have pretty much reproduced Rick's results.
> Namely, the blank field image is poorer than what Rick achieved
> with VLA data;  I (barely) recover a background source 692" West
> and 92" North of the blank field phase center (I've NOT made any
> correction to the global phase sign).  I have not verified the location
> of this source with Rick's reference VLA image. For narrow (1 MHz)
> channels near the band center, the strength of this source is ~0.45%
> of 0217 itself.   For wider bandwidths (8 and 64 MHz), the strength of
> this source decreases, though not as dramatically as reported
> by Rick.  Also, 64 x 1 MHz and 8 x 8 MHz cubes (middle 50% of
> the 1436 MHz subband) show that the apparent strength of this source is
> a significant function of frequency, tending to decrease away from
> the band center.  There is no convincing evidence of systematic
> motion in the source position as a function of channel, but the peak
> posiiton fluctuatates a bit in the 1 MHz resolution image due to
> the general crappiness of the image.
> 
> DETAILS:
> 
> 1. Entire reduction done in CASA, starting with the MS
> generated by the archive (from Michael).  I split out the middle 2 subbands,
> which are the cleanest and most sensitive.  I've calibrated both subbands,
> but imaged only the first one centered at 1436 MHz). For calibration, I
> assume a 1 Jy point for 0217, and make no attempt to scale this to proper Jy.
> 
> 2. I quacked the first 15 seconds in each scan, along with several dead minutes
> at the beginning of the first scan.  Also flagged channels 29 and 72 (counting
> from zero) on all baselines in the 1436 MHz subband; these channels contain
> birdies (the one at ch 72 is the 1408.25 MHz birdie reported by Rick
> on Monday).
> Otherwise the data are very clean.
> 
> 3. As there is no formal delay calibration in CASA (yet), I relied on bandpass
> calibration from scan 9 (fifth 0217 on-source scan) for all freq-dep
> calibration.
> The solution interval was the whole scan (the time-dep phase variation was
> relatively tame) and the solution was normalized in amp and phase post-solve.
> 
> 4. Gain calibration was determined (using the bandpass obtained above) from
> the on-source scans both per-integration and per-scan.  The per-integration
> solutions reveal considerable time-variation in the phase (up to ~rad) on
> timescales shorter than the scans; the long-timescale phase variation is much
> smaller.  Closely spaced antennas show very similar phase fluctuations, as
> expected.
> 
> 5. Both the bandpass and gain calibration were applied to all of the data
> (the gain calibration differently for the two fields as noted below).  I had
> no trouble calibrating the bulk delay (cf Ed), but I note that the constant
> delay calibration performed here leaves behind a residual of a few 10ths
> of a nsec that fluctuates on timescales of a few seconds to minutes.  This
> residual is nominally too small to explain the freq-dep effects discerned
> from the images and described below.
> 
> 6. 0217 on-source imaging:  Calibrated with the per-scan gain solution, the
> dynamic range is ~500 per 1 MHz channel and ~constant  over the 128 channel
> cube.  This image is limited by poor phase calibration.  Calibrated with the
> per-integration gain solution, which tracks the detailed phase fluctuations
> much better,  the dynamic range improves dramatically at the band center,
> and deterioriates toward the edges.  E.g., 32 MHz from the band edges, the
> (1 MHz) dynamic range is 1100-1200, and it increases to 1600 at the center.
> Closer to the edges (including where the bandpass drops precipitously), the
> dynamic range drops to 460 at one edge and 700 at the other.  Even at the
> band center, I see no background sources in the 0217 field.
> 
> 7. Blank field imaging:  The only reasonable option for calibrating
> the blank field
> is to use the long-timescale solutions.  I used my per-scan solution (Rick used
> a constant solution, smoothed from a short-timescale solve, in fact).
> There is no
> doubt that there are more rapid phase fluctuations that remain uncorrected in
> this data.  In the resulting images (1 x 64 MHz, 8 x 8 MHz, and 64 x 1 MHz),
> I consistently find a background source 692" W and 92" N of the phase center
> (which is probably backwards due to the phase sign convention error).  This
> source is probably strongest in the narrowest central channels at
> 0.45% of 0217.
> At 8 MHz resolution in may be a little weaker (0.4%); at 64 MHz, even
> more (0.36%).
> (Rick saw more substantial decreases with bandwidth, I think, which may
> have to do with the fact that he averaged the data in frequency before
> gridding.)
> More obvious is the decrease in power away from the band center, which is
> most significantly detected in the 8 MHz/ch image.  This variation is
> presumably
> the origin of the deterioration with increasing bandwidth.   I haven't
> looked carefully
> enough to find any additional sources; I gather Rick expects we should
> find many based on the VLA imaging.  They are not at all obvious....
> 
> That's all for now.
> 
> -George
> 
> 
> 
> 
> 
> 
> 
> On Fri, Jul 10, 2009 at 5:20 PM, Rick Perley<rperley at nrao.edu> wrote:
>>    I calibrated the VLA data, taken simultaneously with the WIDAR test
>> data.
>>
>>    The images are very nice, with loads of the expected background
>> sources.
>>
>>    I then returned to the WIDAR images, specifically the set of 'noise'
>> images (blank field).  There are three, made with 1, 10, and 100 MHz BW
>> each.
>>
>>    The peak background source in the 1 MHz wide field is 11.2 mJy
>>    The same object in the 10 MHz field is now only 7.1 mJy
>>    The same object in the 100 MHz field is now essentially invisible,
>> at 1.2 mJy!
>>
>>    The VLA dataset shows this background source at 17  mJy.
>>
>>    For the VLA correlator dataset and the WIDAR 100 MHz dataset, the
>> expected thermal noise limits should be about the same  -- and they are,
>> at 0.15 mJy.
>>
>>    From this, I'm guessing that the act of coherently summing over the
>> bandpass is actually destroying the signal (but not the noise).
>> Something is backwards?
>>
>>
>> _______________________________________________
>> evlatests mailing list
>> evlatests at listmgr.cv.nrao.edu
>> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests
>>
>>
> 
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests