Hi All,
Another question (which I'm sure is very basic).
I've got 1934 data with the 6cm contiuum CABB data.
if I do
mfcal interval=3
gpcal interval=0.1 options=xyvary
Then plot real vs imaginary, I get bowties:
Also attached is a zip file containing the same plot over a range of frequencies. It shows what you'd expect (i.e. a spectral index from 6->4 with the scatter worst at the edge of the bands).
I guess I'm a little bit surprised that XX and YY have such bad scatter at the band edges. At first I convinced myself that this was just poorer system temperature at the band edges, but wouldn't that affect XY and YX too? If it's a bandpass effect, I'm surprised it doesn't affect the XY and YX in the same way....
1934 6cm bow-ties
Moderator: Mark.Wieringa
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1934 6cm bow-ties
- Attachments
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- 1934_reim_channels.zip
- Channel movie
- (245.07 KiB) Downloaded 9502 times
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- Posts: 10
- Joined: Tue Nov 29, 2011 1:55 pm
Re: 1934 6cm bow-ties
Hiya,
I worked it out, I think.
End result: the 5GHz bandpass phase varies substantially in 2 hrs. Regular observations of a bandpass calibrator are recommended, and time-dependant bandpass calibration is important
Below (and attached) are 3 scans of 1934, at 2am, 5am and 7am (roughly).
The mfcal based on a single scan results (mf1) are calibrated based on a single (5am) scan:
mfcal vis = ./1934-638.5500.mf1 select = time(10NOV26:05:00:00,10NOV26:06:00:00 interval = 3
The tdb ones are based on a time dependant bandpass with
mfcal vis=1934-638.5500 interval=3,3,3
Both ones have gpcal interval=0.1 options=xyvary applied
The bandpass phase clearly changes over 2 hrs in a way that gpcal cannot compensate for. Time dependant bandpass calibration fixes the problem more or less, although there are still a few bow-ties (or peanuts) happening, which might be due to the frequency-dependant Tsys.
Anyhoo, I hope this is useful for someone. Those commas in mfcal interval are important.
K
I worked it out, I think.
End result: the 5GHz bandpass phase varies substantially in 2 hrs. Regular observations of a bandpass calibrator are recommended, and time-dependant bandpass calibration is important
Below (and attached) are 3 scans of 1934, at 2am, 5am and 7am (roughly).
The mfcal based on a single scan results (mf1) are calibrated based on a single (5am) scan:
mfcal vis = ./1934-638.5500.mf1 select = time(10NOV26:05:00:00,10NOV26:06:00:00 interval = 3
The tdb ones are based on a time dependant bandpass with
mfcal vis=1934-638.5500 interval=3,3,3
Both ones have gpcal interval=0.1 options=xyvary applied
The bandpass phase clearly changes over 2 hrs in a way that gpcal cannot compensate for. Time dependant bandpass calibration fixes the problem more or less, although there are still a few bow-ties (or peanuts) happening, which might be due to the frequency-dependant Tsys.
Anyhoo, I hope this is useful for someone. Those commas in mfcal interval are important.
K
- Attachments
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- 7am scan. Calibrated with mfcal on the 5am scan.
- 1934-638.5500.mf1_7.png (11.01 KiB) Viewed 77134 times
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- 5am scan. Calibrated with mfcal on the 5am scan (itself)
- 1934-638.5500.mf1_5.png (10.78 KiB) Viewed 77134 times
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- 2am scan. Calibrated with mfcal on the 5am scan.
- 1934-638.5500.mf1_2.png (12.11 KiB) Viewed 77134 times
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- Posts: 10
- Joined: Tue Nov 29, 2011 1:55 pm
Re: 1934 6cm bow-ties
Posting again so I can get over the 3 attachment limit.
- Attachments
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- 7am scan. Calibrated with time dependent bandpass.
- 1934-638.5500tdb_7.png (9.95 KiB) Viewed 77134 times
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- 5am scan. Calibrated with time dependent bandpass.
- 1934-638.5500tdb_5.png (10.24 KiB) Viewed 77134 times
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- 2am scan. Calibrated with time dependent bandpass.
- 1934-638.5500tdb_2.png (10.13 KiB) Viewed 77134 times
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- Posts: 10
- Joined: Tue Nov 29, 2011 1:55 pm
Re: 1934 6cm bow-ties
Another thought:
Instead of solving for a time-dependent bandpass, one could split the data up into chunks in frequency (say, I dunno, 4 or 8?) and then calibrate them separately. That way gpcal will solve for (slightly) different phases on every chunk, which should get around this problem.
Instead of solving for a time-dependent bandpass, one could split the data up into chunks in frequency (say, I dunno, 4 or 8?) and then calibrate them separately. That way gpcal will solve for (slightly) different phases on every chunk, which should get around this problem.
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- Posts: 10
- Joined: Tue Nov 29, 2011 1:55 pm
Re: 1934 6cm bow-ties
Oh wait, use gpcal with nfbin>1