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Focal reducer

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3 replies to this topic

#1 Darwin T

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Posted 06 December 2018 - 05:54 PM

I have a Celestron CPC1100, which is f/10

 

If I use a f/6.4, would the image be brighter as if it were a true f/6.4 telescope?

 

Thanks.


Edited by Darwin T, 06 December 2018 - 09:20 PM.


#2 bobito

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Posted 06 December 2018 - 06:00 PM

It will be brighter with any given focal length eyepiece as the exit pupil will be larger.  But for any given magnification it will be equally bright at f/10 or f/6.3 since the exit pupil will be the same size.

 

Bob



#3 Eddgie

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Posted 06 December 2018 - 07:28 PM

Since you have a camera in your sig, then the full answer (added to the last post) is that photographically, yes, it will be as bright as the faster scope but the image scale will be smaller and that is the tradeoff.   You get the shorter exposure time for the same brightness or a brighter image for a given exposure time, but .64x the size. 

 

With eyepieces though, it is exactly as Bob describes.  You did not mention eyepieces though so I could not be sure that this is what you meant.


Edited by Eddgie, 06 December 2018 - 07:30 PM.


#4 Darwin T

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Posted 06 December 2018 - 09:20 PM

Thanks to both of you!

 

I lost my focal reducer a few years ago and just found it. I use my telescope mostly to photograph the ISS. A focal reducer would make it easier to keep the ISS in the field view of my camera, with a smaller image, of course.

 

But I also want to use it for other things as well, and would welcome the lower f ratio for that.




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