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8 inch sct focal reducer questions

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#1 richard hoyt

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Posted 06 January 2013 - 01:09 AM

I have a 8 inch celestron sct and I removed the visual back and replaced it with a 2 inch crayford from opt as they recommended. The question I have is do I just unscrew the focuser add the reducer, put the focuser back on and call it a day? I just read a different post where someone said you needed extensions for spacing. Do i need any and what are they for?

The last question I have is regarding guidescopes. Right now I have the celestron 80 guide scope and have it piggybacked to the top of the c8 (or at least I will once I decide on a guide camera). What is this that I read about flexation and needing to lock the mirror. Is the celestron 80 to big for a c8 and should I go with he orion 80? (I can still return the celestron)

#2 mclewis1


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Posted 06 January 2013 - 09:42 AM


With any focal reducer you will need to maintain a certain amount of spacing behind the reducer. If you are talking about the f6.3 SCT reducer/corrector it will require about 110mm to achieve the full .63x reduction factor. You can reduce the spacing down to about 75mm or so (with the corresponding lowering of the reduction factor - to about .8x or so).

If you put the focuser between that reducer and the eyepiece you may have too much spacing.

If you put the reducer on after the focuser you'll need both 2" barrel to SCT adapter and spacers (or a diagonal) but you will also have to watch the amount of focus travel you still have. In some configurations you may find that some eyepieces won't come to focus behind the reducer if the reducer is moved back from it's position right against the rear cell.

#3 richard hoyt

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Posted 06 January 2013 - 10:08 AM

Yes it is a 6.3 reducer (celestron brand). So it sounds like the best way if I read this correctly is to have the focuser on first then the reducer then my 2 inch diag and eyepiece. How will I know if I have too little spacing or too much? Will it be obvious as in not being able to focus on the object?

#4 *skyguy*



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Posted 06 January 2013 - 10:47 AM

I'd return the celestron 80 guide scope ... provided it doesn't cost you ... and replace it with the smaller Orion 80mm Short Tube for your guidescope ... with the Orion SSAG camera for guiding. I've used an 80mm f/4 refractor as a guidscope ... with a SSAG ... for many years on my 12" SCT @f/6.3 (1920mm FL) with great results.

#5 richard hoyt

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Posted 06 January 2013 - 11:25 AM

Good to know. I was concerned with the celestron just because it was so long.

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