But isn't the image at prime focus really tiny ?

Depends on the instrument that delivering the image. My C-14 @ f/11
delivers a very respectable image scale whereas my ED80 @ f/7.5
doesn't.

The focal length is what determines the image scale you'll get. Prime
focus is pure but adjustable via optical amplifiers (which make it
less pure). If the image is too small, then you'd use a Barlow or
PowerMate to increase the effective focal length but the power of the
amplifier. Conversely, if the image scale is too large for your tastes
you'd use a focal reducer.

There are practical limits to how far you can push a telescope and
whether the image scale it's delivering is optimal for the chip
capturing the light.

So, can you tell me in more detail what the parameters are ? I have an f/5 12"
reflector and an EOS 350D (sensor size 22.2 x 14.8 mm), is this a reasonable
combination of focal length and sensor size ?

And how exactly is focussing done, trial and error or what ?


Cheers,

Boo