Casapy and miriad creating different results!?
Posted: Thu Sep 06, 2012 9:03 pm
Hello,
I've got a problem, once again.
I noticed that there is something wrong when I checked results of the cleaning of ATCA Cabb data. I tried it in miriad and casa. The first part, the initial calibration, of it was the same for both. I exported the miriad files to uvfits, imported it to casa, did the calibration. Then I imaged the data in casa. For miriad, I exported it again to uvfits and imported it to miriad. There I also imaged it. And now the big surprise, at least for me, is that stokes Q and U have completely different values. Which is bad if one plans to do rotation measure synthesis on this data.
To verify this I created a simulated observation with uvgen. I used a 10 Jy point source with 10% linear polarization. After using invert and just taking the central pixel of the dirty map, I got the following values for i,q,u,v:
10 Jy, 1027.46 mJy, -2.9 mJy, 15.77 mJy.
and for casa (using 0 iterations to get the dirty image):
10 Jy, -13.35 mJy, 547.04 mJy, 16.43 mJy.
Curiously, stokes V is very similar in both, but Q and U are completely different. Also, the PI in the miriad image is 10%, and in miriad only about 5%.
Now I would say that at the moment I cannot say which of these results is more reliable. I guess that invert would probably create the "true" result since the data was created in miriad. But it would be good if someone could clear that up. Is there some flaw in casa and linear feeds? or am I missing something?
edit: I just checked if the uvfits import/export might be the reason. I exported the testdata to uvfits, and then imported it right back to invert it again. Stokes I, Q and V are the same (in Q there is a very slight difference), but U changes from -2.9 mJy to 8 mJy for the central pixel. Maybe the uvfits format is not properly exported?
Arpad
I've got a problem, once again.
I noticed that there is something wrong when I checked results of the cleaning of ATCA Cabb data. I tried it in miriad and casa. The first part, the initial calibration, of it was the same for both. I exported the miriad files to uvfits, imported it to casa, did the calibration. Then I imaged the data in casa. For miriad, I exported it again to uvfits and imported it to miriad. There I also imaged it. And now the big surprise, at least for me, is that stokes Q and U have completely different values. Which is bad if one plans to do rotation measure synthesis on this data.
To verify this I created a simulated observation with uvgen. I used a 10 Jy point source with 10% linear polarization. After using invert and just taking the central pixel of the dirty map, I got the following values for i,q,u,v:
10 Jy, 1027.46 mJy, -2.9 mJy, 15.77 mJy.
and for casa (using 0 iterations to get the dirty image):
10 Jy, -13.35 mJy, 547.04 mJy, 16.43 mJy.
Curiously, stokes V is very similar in both, but Q and U are completely different. Also, the PI in the miriad image is 10%, and in miriad only about 5%.
Now I would say that at the moment I cannot say which of these results is more reliable. I guess that invert would probably create the "true" result since the data was created in miriad. But it would be good if someone could clear that up. Is there some flaw in casa and linear feeds? or am I missing something?
edit: I just checked if the uvfits import/export might be the reason. I exported the testdata to uvfits, and then imported it right back to invert it again. Stokes I, Q and V are the same (in Q there is a very slight difference), but U changes from -2.9 mJy to 8 mJy for the central pixel. Maybe the uvfits format is not properly exported?
Arpad