How can I determine the diameter of the image circle (i.e. the diagonal size of CCD required to fully utilise a telescope's optics) at near-infinity focus? I have looked around online and found this calculator, but I am more interested in how the figure was calculated than knowing the figure itself. 

Background: I am new to astrophotography and recently bought a Celestron Nexstar 130 SLT telescope with a Barlow lens, T-adaptor and T-ring to connect it with my Nikon D80 to take photographs of the night sky in prime focus. The telescope dimensions are 130mm aperture and 650mm focal length. With the 2x Barlow lens (required to diverge the focus enough to mount the camera outside the optical tube assembly) it effectively becomes a telescope with 1300mm focal length. I can measure a picture of an object with known angular size (e.g. the moon) and use crop factors to establish a ratio, but this method doesn't help me understand how a bigger/newer telescope would fare on the same CCD. 

Edit:
Since this question now has a bounty, I would like to emphasize the real question is How to calculate the size of the image circle at infinity focus?, and not the follow-up question of 
Would the image circle size decrease if I was able to ditch the Barlow lens?


Question: Convert the_geom to text and then transform
Answer: You can use ST_Transform in your first query too. 

select sitename, st_astext( ST_Transform( the_geom, 4326 ) ) from sites_tbl;