Imaging and aperture

Hi all,
Just getting into ccd imaging. I understand how aperture affects visual astronomy, but not clear on how aperture affects imaging as an indepenent variable. I understand that for imaging the f-ratio is "king", so assuming 2 scopes have the same f-ratio does the aperture affect the image.

For example, comparing:
scope 1 8"f10 with focal reducer to f5 has a focal length of ~1000.
scope 2 ST80 f5 has a focal length of 400

The aperture results in a different FOV between the 2 scopes, but are there other affects of aperture I have missed, or is this all there is to it?

Thanks
John

Hi John,

For imaging, f-ratio will determine how quickly extended objects such as galaxies and nebulae will be recorded. So, if you have say a 16" f/5 and an 8" f/5, they will record extended objects equally fast. However because the *actual* focal length of the 16" will be longer, it will have a smaller field of view but better resolution.

For point sources such as stars, the f-ratio makes no difference. Here aperture rules. The 16" will record fainter stars than the 8" for the same exposure.

hope this helps.

Maurice

Hi John,

the exposure time will be the same because the light received in the bigger FOV is the same like light recieved from smaller FOV.

The smaller FOV will of course have a better resolution, if the seeing is allowing for that.

In deed, because f/ratio is "king", you can make superb astro images with smallest scopes and hence wide fields, i.e. the 60mm Baby Takahashi:

http://www.licha.de/AstroWeb/gallery_fullsize.php3?iSOId=4&iBPId=97

Even Telephoto lens can achieve amazing image quality.

Cheers
Tom