[daip] Question about AIPS task RM {External}

[egreisen]

On 2022-09-21 06:39, ailingZeng via Daip wrote:
> Dear friend,
> 	I am using a task called RM in AIPS to do a Faraday rotation measure
> map. I use VLBA data at 5GHz and 8.4GH. When I run RM , MSGSRV window
> shows : ' TOO FEW POINTS ON f AXIS FOR RM FIT,  Purports to die of
> UNNATURAL causes. ‘
> 
> 	first, I use COMB to combine Q and U image at 5 GHz. And do the same
> again for 8.4GHz data.
> 	then, use FQUBE to combine these two cubes derived by COMB.
> 	next, use TRANS to put frequency at first dimension.
> 	finally, use RM, and failed.
> 
> 	I want to ask that if RM can deal with a cube image with only 2
> frequencies, I mean, it need more than 2 frequencies so it failed and
> said too few points on frequency axis. Or maybe I carried on something
> wrong so it failed. Hope to get some suggestions from you.
> 
> 	Thank you very much for your kind consideration and I am looking
> forward to your early reply.
> 
> Sincerly, yours
> Ailing Zeng

The help file states that at least 3 frequencies are required for the 
task
RM.  With only 2 frequencies, COMB itself has an 'RM' option.  The help
file is less than helpful for this option which takes the angle 
difference
between two images of polarization position angle.  It fiddles around 
with
+- 180 and 360 to see what angle difference is closest to the expected
difference (APARM(3)) and returns an RM in suitable units (rad/m/m).

Eric Greisen