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C14 edge HD nominal back focus w & w/o reducer

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#1 Gert

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Posted 22 September 2019 - 11:00 PM

Hello All,

 

I am getting through the learning curve of a used C14 edge HD (with Celestron 0.7 reducer) that I picked up recently. So Celestron publishes this 'Nominal Back focus' (NBF) for the C14 (and same for C11) of 146mm from the rear flange of the OTA. General interpretation of this is that at this focus position the movable primary is in the optimum position (least aberrations) with resp. to all the other optical components (of which in the edge hd there are plenty).

 

So I cobbled together extension tubes and eyepiece to put the EP field stop right at these 146mm then focused the scope and admired the sharp image.

 

I unmounted the extension without changing it, added the reducer which (interestingly) Celestron says has the same NBF of 146mm, added my eyepiece holder and looked. The focus was way of. (about 1-2inch). So I am adjusting the scope focuser until I have a sharp image in the EP again (now with reducer and my extensions in the optical train).

Asking myself this. When w/o the reducer the optical parts where at their perfect design position, then they can't have been with the reducer installed. So the OTA will have introduced aberrations. Now should that lead to the conclusion that the reducer not only reduces the FL but also un-does the just introduced aberrations? Or does Celestron simply ignore that as the refocus is assumed small (is 2inches small?) and the change in aberrations can be neglected?

 

What's the verdict of the experts?

 

Clear Skies,

Gert

 



#2 jhayes_tucson

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Posted 22 September 2019 - 11:57 PM

Yes...when you add the reducer, the position of the primary has to be adjusted to bring the image into focus at the proper position behind the reducer.  The primary-secondary spacing was used as a degree of freedom when the reducer was designed so that's the way it is supposed to work.  It sits at a fixed position relative to the secondary and it changes the optical power of the whole system.  Remember that the Celestron reducer is not a third-party add on.  It was designed as a part of the whole optical system.

 

John


Edited by jhayes_tucson, 22 September 2019 - 11:58 PM.

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#3 Gert

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Posted 23 September 2019 - 12:03 AM

Thank you, John.

 

In the design process it might actually have been beneficial to change the M1 M2 spacing. Maybe that just introduces aberration with opposite sign as the reducer such that both cancel in the end. Making this a catadioptric system (alas it already was one to start)

 

Thanks & Clear Skies,

Gert



#4 jhayes_tucson

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Posted 23 September 2019 - 02:30 PM

Most 3rd party reducers, will reduce the back focal distance.  In this case, when you insert a reducer with positive power with a requirement that it has the same back focal distance, you've got to change the mirror spacing to make that happen.  Then, you optimize the design for image quality (and probably a bit for image position as well.)

 

John




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