Hi Nick,

I still haven't got my act together re. photometry so it's nice to see "very faint" quantified!!!  A quick look at the AAVSO chart for this object suggests their photometric sequence only goes down to ~mag 14.  Is there a better sequence that reaches 17 - 18 where it is now?  I see you refer to 'UCAC4' in you image details...not something I'm familiar with...maybe you could point me in the right direction?

Thanks,

Mike.

Is the USNO CCD Astrograph catalog. It contains reference magnitudes for a lot of stars and Astrometrica uses these to determine the magnitude zero point for the frame. The frame is unfiltered but I have chosen R magnitudes as the reference to use since these most closely match the response of my camera. In this case the ZP is 25.75 which is about average for a 60s exposure with my camera and telescope. On very transparent nights I can get to 26.2 or so. The ZP is used to convert between counts and magnitudes so it is then possible to measure the magnitude of any object in the field by clicking on it in Astrometrica. 

Thanks Nick, that's really neat.  Something to look into on cloudy nights...

Mike.