[evlatests] C and X-band observations from Thursday night

[Rick Perley]

    Three hours of 'RSRO' observing on Hercules A was taken Thursday 
evening. 

    The observing setup is fairly demanding:  Three different frequency 
setups organized this way:

    4.5 and 5.5 GHz in C-band in AC and BD IFs
    6.5 and 7.5 GHz in C-band in AC andBD IFs
    8.5 GHz in X-band, for both IFs. 

    For all these, the subband width was 128 MHz, full polarization, 
with 2 MHz channel width.  1 second averaging, as usual.  This made for 
BIG databases, (about 50 GB for each of the three) so all processing 
steps are sssslllloooowwww...

    Hercules A, a nearby calibrator, and the (far away) flux and 
polarization calibrator 3C286 were all observed with all three tuning 
combinations.  All durations on each source/band/tuning setup were no 
longer than 2 minutes.

    'Dummy scans' in OSRO mode at the beginning were utilized to setup 
the attenuators. 

    Below are some top-level comments. 

    1) Getting the data into AIPS. 

    Two routes were explored
   
    A) The 'standard' route via the archive interface to generate UVFITS 
files on /home/e2earchive, followed by FITLD and INDXR. 
    B) Bill Cotton's 'direct to AIPS' OBIT filler program. 

    Route a was a little bit slow.  The request to the archive was 
submitted Friday morning.   It spent 2 hours in the queue, then took 28 
hours to complete the conversion (this for 3 hours of real time 
observing!).  Three passes of FITLD were then required -- each took 
about two hours of clock time.  The last step was INDXR -- about 20 
minutes each. 

    By contrast Bill's direct translator is much, much faster -- about 2 
hours to copy the SDM file from the archive, and about 1.5 hours to do 
the translation.    However, there was an issue with the first attempt 
to use this route which required Bill to clarify -- by the time this was 
done the 'standard' path had completed, so the notes below are all based 
on the data copied via Route 'A'. 

    2) Phase stability. 
   
I have reported separately on the 'jumping antennas' -- antenna 26 on 
the AC side, and antennas 18 and 27 on the BD side are jumping phase (by 
typically a radian or more) on scan changes (which include a frequency 
re-tuning).  Phases are stable within a scan. 
    Antenna 5 has large phase changes at X-band, but these are related 
to amplitude changes -- see below. 
    Antenna 3 has very unstable phase (not obviously associated with 
scan changes), but only at the high-frequency C-band setting (6.5 and 
7.5 GHz). 
    Otherwise, phase stability seems about normal for the 
frequencies/configuration/time of year. 

    3) Amplitude stability. 

    For most antennas, the amplitude stability is fairly good (< 10% 
variations in amplitude gain).  Some antennas are notably poor. 

    Antenna 5, at X-band only, on both AC and BD IFs, is having repeated 
1-db gain changes throughout the observation -- at least nine times 
(these from the calibrator alone -- I haven't checked the target source 
yet). 

    4) Amplitude level.

    By this I mean the observed 'fringe amplitude', factored out by 
antenna.  The desired level (in the units provided by AIPS) is about 
5.0.  Some antenna/IFs are very low.  I give below those whose apparent 
fringe power (factored out by antenna) is more than a factor of two in 
amplitude too low (or 6 dB in power units). 

    a)  X-Band:  Antenna 6 -- in all IFs, 8 dB low
                                       10 -- in all IFs, 8 to 10 dB low 
-- the amplitude was steadily changing throughout. 
                                        13 -- on all IFs, 4 dB low (but 
IF A is especially low, and is notably time variable).
                                       16 -- 11dB low on IF A, 8 dB low 
on IF B.
                                        23 -- 5 dB low on A and C. 

    b) C-Band:  Antenna 1, in LCP only, is spectacularly low:  14 dB at 
the low frequency (4.5 and 5.5 GHz) , and 18 dB at the high frequency 
setting (6.5 and 7.5 GHz). 
                        Antenna 6, in IF D, at 7.5 GHz -- 7 dB low
                        Antenna 24, in IF D, at 7.5 GHz  6 dB low
                        Antenna 26 has special problems in RCP:  7 dB 
low in IF A at 4.5 GHz, and about 14 dB low in both IFs A and B at 6.5 
and 7.5 GHz. 
                        Antenna 27 is 10 dB low in IF 'B' at both 5.5 
and 7.5 GHz. 

    5)   Failures to Fringe:  This means that when we returned to a 
particular band/frequency, no fringes were seen. 

    a) No failures were seen at X-band.  All scans fringed as expected. 
    b) Numerous failures were seen at C-band.  (Although 'numerous', 
these events are still rare, as the number of commanded changes is very 
high).  Some failures affected single IF pairs, some affected both.  
Rather than clutter up this report, I'll offer to send a 'failure log' 
to anybody who asks ...

    6) Delays.  All delays were less than 6 ns, and all were stable. 

    7) Bandpasses.  Except for antennas 16 and 17 (which have the old 
style C-band polarizers), bandpasses looked good.  I have not yet tried 
a bandpass stability analysis.  (Very slow with 150 GB of data ...)

    8) RFI.        All is perfectly clean at X-band.
                       The lower pair of C-band frequencies (4.0 to 6.0 
GHz) is very good, with an important exception:  4.0 to 4.2 GHz is 
completely obliterated for Hercules A and its nearby calibrator.  
However, it is perfectly clean for 3C286.  The difference is direction 
-- Herc A is adjacent to the geostationary belt of satellites, while 
3C286 is about 30 degrees north of this.  It makes a Big Difference! 
                      The higher pair of C-band frequencies (6 to 8 GHz) 
was bothered only by the microwave links that are already known:  near 
6.0, 6.25, and 6.7 GHz.  I see no evidence for strong RFI in the 7.0 to 
8.0 GHz range.