[daip] Iring

[Helen Thomas]

Dear Sir/madam,

I have been using iring to work out the HI flux in a given annulus in
order to calculate the surface density of HI in some of my VLA maps.

However, I don't understand how it works. Given the radius, inclination
angle, and the PA of the annulus, does it fit an ellipse to the image and
then add up the flux with a cos(i) correction? If so, how does it
calculate the minor axis of the ellipse? I am concerned that iring is not
including all the flux within a given radius.

I have written a program in fitsio which just fits an ellipse (which I
specify) over the inclined galaxy and then works out the flux within that (with a cos(i)
correction). My program does not give the same results as iring.

Also the PA has to be specified E of N. Presumably this is measured
anticlockwise from north on a VLA map (just in case I am inputting this
parameter incorrectly!!!!)

I would be grateful for any help/advice!

Helen.

E-mail:hct24 at mrao.cam.ac.uk