Peter,

Your pixel scale of 0.22 arcsec/pix is well in excess of what you need
to sample the image. I have a camera with the same pixel size as you
(3.8u) and a focal length which is shorter (2.8m) and I tend to bin by
4. That gives a pixel scale of 1.12 arcsec per pixel. On good nights
near the zenith I get stars with an FWHM of around 2.0 - 2.5 arcsec. In
my case I bin by a factor of 2 at capture and then a further factor of 2
during calibration and this means I can get back to a scale of 0.55
arcsec/pix if the the night was good and the object needs it. Most of
the time though 1.1 arcsec/pix is fine and the images are a quarter of
the size.

The photometric aperture in Astrometrica is (n+1/2)*scale so if you bin
by 4 and get 0.9 arcsec/pix then use a radius of 10 to get 9.5 arcsec.

Nick.

On 11/11/2024 11:05, Peter Tickner - peter.tickner1472 at btinternet.com
(via baa-comet list) wrote:
> Nick
>
> Thank you - I managed to image 29P through high cloud as well and it is
> looking very Pac-Man like.
>
> Astrometry.Net tells me my image scale last night was 0.225 arc secs per
> pixel.  On that basis am I right in assuming for 9 arc secs I'd need to set
> the aperture value in Astrometrica to 40 pixels?
>
> At the moment I'm restricted to unguided tracking until I can find the right
> spacers to get the camera parfocal with my off-axis guide camera.    The
> clear patches last night didn't last long enough to find the right
> combination.
>
> Peter