[BAA Comets] CCD astrometry and photometry

[Nick James]

Roger,

Example files (offset and nonoffset stacks + Astrometrica config for 
this system) are here if you want to have a go.

http://www.nickdjames.com/Transfers/photom/

These were taken under rather grotty conditions. I get a zero point of 
22.85 using Astrometrica using the nonoffset image. I then load the 
offset image into IRIS and use "noffset 0" to set the sky background to 
zero (this uses a global median estimate). Stretch the image to estimate 
the size of the coma (I get 250 pixels radius = 21.5 arcmin diameter). I 
then used concentric apertures of 250,350,500 radius and the previously 
measured zero point to get a magnitude of 4.2 (2015-01-22, 20:07).

Nick.

> Nick, could you let me have the images you used plus the necessary config file for Astrometrica so I
> can work through the process and then write it up for the Project Alcock website?
>
> I am reasonably familiar with IRIS so shouldn't be too much of a problem but could you please
> explain (from your previous posting);
>
> 3 - I then used a median estimator to get the sky background and
> stretched the image around this point to show the size of the coma.
>
> Not sure what a 'median estimator' is.
>
> I have done some work with a DSLR attached to a telescope and the astrometry and photometry looked
> quite reasonable but guess I need to do same for DSLR standalone.
>