Re[2]: [ATM] Back Focal Distance design considerations

[Richard <cnc@cncservo.co.uk>]

Hi Michael,

Monday, February 14, 2005, 1:03:32 AM, you wrote:


MP> You're talking about a Cassegrain, right? One reason commercial, and for
MP> that matter professional Cassegrains focus by changing the 
MP> primary-secondary separation is the back focal length measured from the
MP> secondary vertex changes by approximately m^2 mm. for every mm. change in
MP> mirrror separation (m is the secondary magnification).

It's a 16" Ritchey-Cretien with a corrector just below the
primary. Fp=3, m=2.8. I've been ray tracing your suggestion of
focusing with the secondary and unfortunately the corrector
doesn't allow this method. Changing the primary-secondary
distance and secondary-corrector distance to compensate messes
up the spot size significantly. Shame :(

MP> I don't understand what you're getting at here, assuming I'm remembering
MP> right that you're building a Cassegrain. I'd attach the focusser directly
MP> to the rear mirror support, then use a standard 2" or 1.25" star diagonal
MP> with eyepieces or remove it for imaging. Then the length of the racked in
MP> drawtube would be limited by the distance to the back of the primary or the
MP> first mechanical obstacle it encountered.

Again, the position of the corrector doesn't allow the focuser to
attach behind the mirror cell, which is a great shame. Looks like
the gain in image quality from the corrector is causing me other
problems now.

As an aside - how do people get on with a focuser and eyepiece
on opposite sides of a diagonal?. I've not tried it, but I think
I would find it odd to have the eyepiece move sideways when
focusing.

-- 
Best regards,
 Richard                            mailto:cnc@cncservo.co.uk

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