Re: [ATM] Back Focal Distance design considerations

[Michael Peck <mpeck1@ix.netcom.com>]

At 11:36 AM 2/13/2005, Richard wrote:
>Hi ATMers,
>
>I'm designing the back end of my telescope now and finding it
>hard to reach a compromise focal plane position.




>Current configuration uses the same standard thread as the
>larger SCT, ie 3.325" x 16TPI. From the rear of that threaded
>base the focal plane is 194mm.
>
>Any suggestions welcome.

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

>focuser tube needs to be 145mm long. When wound right in
>there's 105mm of tube to lose below the focuser bottom without
>hitting the diagonal!. What am I missing here, because what I'm

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

Mike Peck



------
Michael Peck
mpeck1@ix.netcom.com

_______________________________________________
ATM mailing list http://www.atmlist.net/