invert/cgdisp coordinate issues near dec=0

[MarkWieringa]

Jess Broderick reports:

Imaging equatorial sources by Jess Broderick on Wed Apr 28, 2010 11:25 am

Hi,

I'm trying to make an image of a source at Dec -00:20:45. The data were obtained with the H168 configuration on March 23/24 at 5.5 and 9 GHz (project CX188). I have no problems with the calibration, but the dirty map produced with INVERT doesn't look quite right. First of all, the cell size changes across the image (at least according to KVIS); this happens if I let INVERT choose the default pixel size or I specify it myself. Secondly, there are some sources in the field whose positions I know from NVSS; they appear to have been shifted in my higher-frequency images.

Any suggestions? Even with the spur, is it possible to get a good image so close to the celestial equator?

Thanks, Jess Jess Broderick

[MarkCalabretta]

You can't expect to get reliable results from an east-west synthesis array for fields near the equator because the uv coverage is squashed into a line, i.e. there is essentially no resolution in the north-south direction. The NCP projection blows up because it attempts to rescale in declination by a large factor. Refer to a good text such as "Synthesis Imaging in Radio Astronomy", ASP, Vol. 180.

[MarkWieringa]

It turns out invert does the right thing (use a SIN projection instead of NCP) if you exclude antenna 6 from a hybrid array. Roughly, when the ratio of the N/S to E/W baseline lengths is less than 1/10 it uses NCP, otherwise SIN.