[ATM] Back Focal Distance design considerations

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

Hi ATMers,

I'm designing the back end of my telescope now and finding it
hard to reach a compromise focal plane position.

>From the small amount of research I've done, it looks like
eyepieces, when seated, can have the focal plane anywhere from
50mm below the focused seat for long focal length large eyepieces
to 6mm above the focal plane for short focal length high power
eyepieces. If I then consider my digital camera, it needs the
focal plane 44mm above¹ it's flange which becomes the standard
55mm with a T mount fitted. This suggests a focusing range of
105mm which I cannot see as being feasible without a very long
BFL.

If you take a Crayford² or FeatherTouch³ type focuser with a
body height of, say, 30mm or more sensibly 40mm, then the
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
trying to achieve, routinely must be achieved. Should I just have a
very long BFL (hence larger secondary obstruction or larger F
number) or do people use insertable spacers for eyepieces which
is removed/replaced for a camera hence allowing a shorter
focusing range, or... ?

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.

¹ http://www.a1.nl/phomepag/markerink/mounts.htm   (Canon EOS)
² http://www.mindspring.com/~davebevel/focuser/focuser.html
³ http://www.starlightinstruments.com/


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

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