[Kal (Andrew)]

Jacquie - I think the main point to consider in your application is airy disk size. All telescopes (within reason) will see stars as pinpoints of light, but those pinpoints will have the light spread out within the airy disk (and diffraction rings) at the focal plane. You want to have your optics grab the light from the star and contain it within 4 pixels. If the airy disk is too small, you will have all the light fall within 1 pixel which isn't enough, if you have the airy disk too large you will capture it on up to 9, or 16, or 25 pixels, which is too much.

In truth you need to take into consideration other things aside from the telescope optics - you need to factor in the pixel size of the camera, and seeing conditions.

Edit: Some more info of what I am trying to explain here