Antenna Gains

Unfortunately, neither mfcal nor smamfcal has been implemented for solving antenna gains from multiple calibrators. However, it is possible to obtain an antenna-gain table from multiple calibrators. Here is a procedure to produce a merged gain table from different calibrators based on the data taken from the observations of HII region NGC 7538 on 2005-9-11:

1. Split the data into single-source files

Assuming the multiple source uv file 050911H30a_rx0.usb.tsys.bp.bl has been calibrated for bandpass. Two gain calibrators 2202+422 and 0102+584 were interleved. Here is the observing pattern of the target source interleaved with the two calibrators:

05SEP11:05:26:19.3 2202+422         
05SEP11:05:32:00.0 n7538          
05SEP11:05:42:50.5 2202+422     
05SEP11:05:48:31.2 n7538       
05SEP11:05:59:21.6 2202+422   
05SEP11:06:05:02.3 3c454.3  
05SEP11:06:15:47.6 2202+422
05SEP11:06:21:28.3 n7538             
05SEP11:06:32:18.7 2202+422     
05SEP11:06:37:59.4 n7538             
05SEP11:06:48:49.9 0102+584          
05SEP11:06:54:30.6 n7538             
05SEP11:07:05:21.0 2202+422          
05SEP11:07:11:01.7 n7538             
05SEP11:07:21:52.2 0102+584          
...
05SEP11:12:24:53.5 n7538         
05SEP11:12:35:44.0 2202+422          
05SEP11:12:41:24.7 n7538            
05SEP11:12:52:15.1 0102+584          
05SEP11:12:57:55.8 n7538             
05SEP11:13:08:46.2 2202+422          
05SEP11:13:14:27.0 n7538             
05SEP11:13:25:17.4 0102+584          
05SEP11:13:30:58.1 n7538             
05SEP11:13:41:48.5 2202+422          
05SEP11:13:47:29.2 n7538           
05SEP11:13:58:19.7 0102+584         
05SEP11:14:04:00.4 2202+422

The gain calibrators need to be splitted:

uvsplit vis=050911H30a\_rx0.usb.tsys.bp.bl \
        select='source(2202+422,0102+584)' \
	options=nowindows                                  
mv 2202+422.230409 2202+422.gtest
mv 0102+584.230409 0102+584.gtest

The calibrators 2202+422 and 0102+584 are splitted into two separate uv data sets and renamed the two single-source data files as 2202+422.gtest and 0102+584.gtest.

2. Solving for antenna gains from indvidual calibrators

Now, the antenna gains can be solved from the individual files using smamfcal:

smamfcal vis=2202+422.gtest refant=3 interval=1 \
       options=nopassol
smamfcal vis=0102+584.gtest refant=3 interval=1 \
       options=nopassol

Note that the default for the input Keyword flux is used here so that the rms gain amplitude is set to 1 and no flux density scaling effect is applied to the visibilities in this process.

3. Merge the indvidual gain tables into a joint gain table

Then, the disjoint gain tables can be merged into a joint one, sorting into the ascending time order, with smagpplt:

smagpplt vis=2202+422.gtest,0102+584.gtest \
	device=/xw yaxis=amp,phase \
	options=gains,merge polyfit=8 \
        nxy=1,6

The merged gain table is placed under 0102+584.gtest, replacing the old gain table. Fig.3.10 and Fig.3.11 shows the ampltidue and phase curves computed from the merged gains (red dots). For some reasons, the antenna gains derived from the two calibrators show systematic offsets. The magnitude of the offsets varies with antennas and time. smagpplt also provides orthogonal polynomial fits to the gains in an order of polyfit=8 which can be specified by users. The polynomial fits would be helpful to minimizing the systematic errors. Users might wish to use the polynomial fits instead of the original merged gains. Then, please go back to step 2 to recalculate the antenna gains for 0102+584.gtest and redo smagpplt with options=gains,merge,opolyfit instead of options=gains,merge:

smagpplt vis=2202+422.gtest,0102+584.gtest \
        device=/xw yaxis=amp,phase \
        options=gains,merge,opolyfit polyfit=8 \
        nxy=1,6

The fitted gain table is placed under 0102+584.gtest. The tradeoff in using the polynomial fits is that the detailed structure in the original gains might be lost.

Figure: This plot shows the gain amplitude computed from the merged antenna gains (red dots). The blue curves are the polynomial fits (8th order).

Figure: This plot shows the gain phase computed from the merged antenna gains (red dots). The blue curves are the polynomial fits (8th order).

4. Gpcopy the merged gain table back to the multiple-source file

The mergered gain table in 0102+584.gtest is then copied back to 050911H30a_rx0.usb.tsys.bp.bl using gpcopy:

gpcopy vis=0102+584.gtest out=050911H30a_rx0.usb.tsys.bp.bl \
	mode=copy options=nopass