Collected Shreds of AIPS Wisdom: Joining Spectral Line Data with Multiple IFs

Occasionally I've encountered line data that are taken in 4-IF mode - that is, where you have 2 IFs, each tuned to different frequencies, that overlap on their edges. I found these to be a pain to combine the last time I worked with them (in my first Astronomy research project ever!), and they have resurfaced again now.. but I've discovered a much easier solution: UJOIN!

UJOIN takes a data set with 2 IFs tuned to different frequencies (as from 4-IF mode at the VLA), and joined them together. It even deals with re-weighting the overlap regions because now there's more data there! You will want to use UJOIN on data that has already been calibrated (gain AND bandpass).

Here are the inputs:


AIPS 1: UJOIN     Converts IFs to additional spectral channels
AIPS 1: Adverbs     Values                 Comments
AIPS 1: ----------------------------------------------------------------
AIPS 1: INNAME     ' '                     Input UV file name (name)
AIPS 1: INCLASS    ' '                     Input UV file name (class)
AIPS 1: INSEQ         0                    Input UV file name (seq. #)
AIPS 1: INDISK        0                    Input UV file disk unit #
AIPS 1: OUTNAME    ' '                     Output UV file name (name)
AIPS 1: OUTCLASS   ' '                     Output UV file name (class)
AIPS 1: OUTSEQ        0                    Output UV file name (seq. #)
AIPS 1: OUTDISK       1                    Output UV file disk unit #.
AIPS 1: CHANSEL    *all 0                  Begin, end input channels,
AIPS 1:                                    begin output channel each IF
AIPS 1: OPCODE     ' '                     'DIFF' to output the vis
AIPS 1:                                    difference in the overlap
AIPS 1: DOWEIGHT      1                    0 => delete spectrum if any
AIPS 1:                                    IF or channel is flagged
AIPS 1:                                    < -0.5 => keep data even if
AIPS 1:                                    one of the IFs is flagged
AIPS 1:                                    > 0.5 => delete channel if
AIPS 1:                                    one of the IFs is flagged

The main thing to set is CHANSEL. This is a 6-element array:


chansel(1) - first channel to use from IF 1
chansel(2) - last channel to use from IF 1
chansel(3) - output channel that corresponds to 1st input channel from IF 1 (ugh)
chansel(4) - first channel to use from IF 2
chansel(5) - last channel to use from IF 2
chansel(6) - output channel that corresponds to 1st input channel from IF 2 (ugh again)

The AIPS help file on UJOIN has some helpful information on how to set these values properly, but I will reproduce what I did to set these - it's not completely straightforward.

Using UJOIN on your own data
First, you should apply all your calibrations in SPLIT. Make sure that when you SPLIT off your science source, you keep BIF and EIF set to 0 - this will create a single source file with both IFs.

Then, you will need to figure out the frequency and reference pixel of each of your observations (f_1, p_1, f_2, p_2). I did this by SPLITting each IF off separately from the LINE data and then running IMHEAD on each. You should also record delta_f (from IMHEAD again - this should be the same for each IF). In my case, f_2 was bigger than f_1 - if this is different for you, you'll have some sign differences somewhere and CHANSEL will be in a different order.

Next, figure out if any of the channels in your IFs are on the edges of the bandpass. You will not want to keep these channels as they will add noise into the final, combined UV data. Typically, look at the region where the bandpass is flat-ish, and use that. To find these, you will want to use POSSM to take a quick look at your bandpass solutions for each antenna:


default possm
aparm = 0, 1, 0.7, 1.3, -180, 180, 0, 2, 0, 0
source [bpass calibrator]
solint -1
nplots 9
dotv 1
bpver 0

Find the first and last channels of the flat-ish region for each IF (n_first,1; n_last,1; n_first,2; n_last,2).

Now you can start setting CHANSEL with the first and last channels for IF 1:


chansel(1) = n_first,1      # first channel from flat-ish part of bpass, if 1
chansel(2) = n_last,1       # last channel from flat-ish part of bpass, if 1
chansel(3) = 1              # ichansel(1) goes into output channel 1
chansel(4) = n_first,2      # first channel from flat-ish part of bpass, if 2
chansel(5) = n_last,2       # last channel from flat-ish part of bpass, if 2

Now we move on to chansel(6), which is not nearly as easy as the above part was. Here you have to figure out which output channel corresponds to chansel(4). You'll have to take into consideration all the other ichansel values. Here is the equation I came up with for ichansel(6):

You can find the derivation at the end of this post if you're interested or getting incorrect answers. (As Martha Haynes once said, "You get what you pay for and this is free!")

As a reminder, here's what all of the variables stand for:


        n = this is the value for ichansel(6)
    f_1,0 = reference frequency of IF 1
    f_2,0 = reference frequency of IF 2
  delta_f = frequency steps between channels
n_first,1 = first channel of IF 1 used in output
n_first,2 = first channel of IF 2 used in output
      p_1 = reference channel of IF 1
      p_2 = reference channel of IF 2

Testing that UJOIN did the right thing
You'll probably want to check that this actually worked correctly. The UJOIN help recommends that you use POSSM to check that the spectrum of the phase calibrator is flat.

- First, SPLIT off the phase calibrator separately.

- Next, run UJOIN on this phase calibrator file with the same inputs as for the galaxy.

- Check that the spectrum of the UJOIN data is flat in POSSM:


default possm
aparm 0
solint 0
nplots 0
source [phase calibrator]

This should plot the phase calibrator spectrum with data from all antennas averaged together.

My derivation of the ichansel(6) equation:
To figure this out, I wrote some equations to find the frequency at a given channel (f1, f2):

These tell you the frequency (f_1, f_2) at an arbitrary channel (n_1, n_2) for each IF. f_1,0 tells you the frequency at reference pixel p_1 for IF 1. f_2,0 tells you the frequency at reference pixel p_2, for IF 2. delta_f is the frequency steps between each channel.

Now we can write an equation to for the frequency of the output channels:

We can sub in for f_out,0 because we know what channel we're setting as the first output channel (ichansel 1!), and we know its frequency from the above equation for f_1 (n_1) for IF 1: