[daip] Question about AIPS task RM {External}

[ailingZeng]

Thank you for your kind help! Hope you have a nice day.


> 2022年9月21日 下午11:55，egreisen <egreisen at nrao.edu> 写道：
> 
> 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