Quote:
Originally Posted by dtrewren
Yes, there is a critical range for focus which is intimately dependent on your telescope optical speed, below is a table giving the critical depth of focus for scopes of various speeds.
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Thanks. Airy disks hey.
Quote:
Originally Posted by wasyoungonce
Daves info is spot on but I'll add some. You can check out ccdcalc.exe to find how various scopes and CCD combo's go together and part of the info given is Critical Focus Zone (CFZ) per combo.
As Dave said shorter FL scope have smaller CFZ...so you want lots of steps from your focuser in this zone...not too many as it will be verrrrry slow! Longer FL scopes...not so critical. |
I have a Celestron 9.25", so will need to work out the pitch of the focus screw (should not be to hard). But I would say a factor 10x of 20x less then the critical focus zone should provide plenty of resolution.
Thank you both, it does help.
cheers,
Bram