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Replacing a SCT hyperbolic secondary with an elliptical flat

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#1 BumbleBeeArray

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Posted 24 May 2024 - 06:56 PM

Is it possible/practical to provide a "Newtonian" option on a SCT (I'm thinking an Edge HD) so I can position a reducer, like the Hyperstar, where it might allow the addition of, for example, a filter wheel?



#2 Keith Rivich

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Posted 24 May 2024 - 07:05 PM

https://www.cloudyni...ain-telescopes/


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#3 Scott E

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Posted 24 May 2024 - 11:53 PM

If you position the Hyperstar corrector at the correct distance it shouldn't matter that there's a flat in between. The Hyperstar is not so much a reducer, just a corrector. But there may not be enough backfocus for a filter wheel, no matter where you put it. These distances can't be changed. But the bigger problem is the size diagonal mirror needed. Most of these primaries are in the neighborhood of f/2. That means a minor axis larger, and maybe MUCH larger than 50% obstruction. Probably a deal breaker.


Edited by Scott E, 24 May 2024 - 11:55 PM.

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#4 BumbleBeeArray

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Posted 26 May 2024 - 02:05 AM

If you position the Hyperstar corrector at the correct distance it shouldn't matter that there's a flat in between. The Hyperstar is not so much a reducer, just a corrector. But there may not be enough backfocus for a filter wheel, no matter where you put it. These distances can't be changed. But the bigger problem is the size diagonal mirror needed. Most of these primaries are in the neighborhood of f/2. That means a minor axis larger, and maybe MUCH larger than 50% obstruction. Probably a deal breaker.

I did run some "back of the envelope" calculations and as you indicated such a change would have some significant performance consequences (central obstruction probably increased from 32 to 46 %) but not outside the range of some hyperbolic primary Newtonians. Since inserting any reasonable sized elliptical would decrease the diameter of the light cone at the Hyperstar, I thinking to use one sized for a smaller scope (e.g., the 6-inch) with a consequent cost savings.

 

The backfocus is another matter and could very well break the deal for the Hyperstars but perhaps not all flattener-reducers at the probable cost of also decreasing the hoped for performance improvement. No free lunches, I'm afraid.

 

Another crazy approach is to operate off-axis, as many major observatories do, but from what I've read this seems definitely outside the realm of practical.



#5 BumbleBeeArray

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Posted 26 May 2024 - 02:09 AM

Thanks for the link. I never knew, back in the day, that Parks and Cave also offered hybrid variants. Perhaps improvements in optical design and performance will provide a reason for a more practical return.




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