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Silly noobie question.

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#1 skookum

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Posted 20 September 2005 - 10:40 PM

Why is it that SCT's use focusers that move the primary mirror? Obviously there is a good reason for this, but I can't seem to figure out what it is. Is there a reason why a standard (newtonian) type focuser couldn't be attached to the rear of the OTA to move the eyepiece alone?

Thanks for being gentle on me.

#2 TeamGS

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Posted 21 September 2005 - 12:32 AM

I am not sure as to the why, and I am not really a noob. :D I guess it has to do with the range of travel needed.

You can, and many do indeed use a microfocuser on the back of the sct. In fact, Meade ships one as OEM on the GPS models.

Regards,

Gary

#3 matt

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Posted 21 September 2005 - 03:24 AM

I think the long travel is a good explanation, as well as the fact that rack and pinion focusers (I don't think there were a lot of Crayfords out there in the 70s when Celestron came up with the design) are affected by gravity when the scope is pointed upwards, the weight of the stuff attached to them, as well as they make cumbersome additions at the back of the scope.

Until there were CCDs, few people had noticed "mirror flop", let alone worried about it.

#4 Victor Kennedy

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Posted 22 September 2005 - 08:54 AM

I believe it was Mrs. Pomfrey's dog that had "flop-bot", not her cat (in All Creatures Great and Small). You can add a focuser to the back of your SCT that works like a refractor focusser, leaving the mirror steady. Here's an example.


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