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Does Schmidt-Cassegrain telescopes need corrector or flattener?

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#1 gelolac

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Posted 23 May 2020 - 07:39 AM

Hi, newbie here.

 

I have a refractor but planning to upgrade to an SCT in the future mainly because of their focal length (Im more interested in Galaxies). I've seen that there's a F6.3 focal reducer/corrector but haven't seen(or maybe I haven't looked enough) a corrector that does not reduce the focal length? Is there one such? And does SCTs need a flattener?

 

 



#2 SilverLitz

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Posted 23 May 2020 - 08:59 AM

Newbie, here as well, but I believe SCTs need coma correctors, instead of FF.  The EdgeHDs have the coma corrector (lenses) embedded inside the tube, so Edges do not need an additional corrector.  The 0.63x FR/corrector for "normal" Celestron SCTs corrects for coma in addition to being focal reducers, nothing else needed.



#3 james7ca

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Posted 23 May 2020 - 09:15 AM

Probably the only long-focus SCT that is in common use that doesn't require some type of flattener or reducer/corrector for moderate to wide-field DSO imaging in the EdgeHD series from Celestron. Having said that any scope can be used without a flattener but that just means that there is some limited field that can be covered without issues. So, it depends upon the size of your sensor. I'm not sure how wide of a field a standard SCT can cover, I'd guess maybe something between 5 and 10mm on the diagonal and probably not much different than an uncorrected refractor.

 

Meade also offers an Advanced Coma-Free SCT that should work well for DSO work, but it probably needs a flattener or reducer to equal the field that is offered by the EdgeHD series.

 

As for the f/6.3 or 0.6X to 0.7X reducers, I'd say those will cover something below APS-C size, certainly not full-frame. If you want to do DSO imaging with an SCT you probably want one of these reducers.



#4 SoDaKAstroNut

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Posted 23 May 2020 - 04:37 PM

In addition to focal reduction (thereby widening the field of view) reducers have a few pros.

 

They widen the Field of View (FOV) and shorten exposure times. For example, my EdgeHD at native focal length (2032mm) is f10 (means longer exposures and more accurate guiding is required.) Longer FL/narrower FOV means you need better guiding which means a better mount, guider/OAG, etc. More gear = more weight = bigger mount to keep everything stable...it's a ripple effect.

 

With the 0.7 reducer the FL shortens (1422mm) and f7 means slightly shorter/faster exposures. The shorter FL means a wider FOV. It also means slightly improved guiding since the wider FOV adds more stars to the OAG's FOV.

 

The cost, besides $$ for the reducer, is that you lose some resolution when shooting planets and galaxies. It also shortens your back focus which could make spacers, focusers, OAG, filter wheels, etc. more complicated. You may have to get a longer plate/dovetail to push your OTA, and all that stuff hanging off the back of it, further over/forward on your mount.

 

For my setup (OTA, mount, etc.) and the targets I wanted to shoot (narrowband DSOs and selected galaxies), the 0.7 reducer had more pros than cons.


Edited by SoDaKAstroNut, 23 May 2020 - 04:41 PM.


#5 Michael Covington

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Posted 23 May 2020 - 05:02 PM

Hi, newbie here.

 

I have a refractor but planning to upgrade to an SCT in the future mainly because of their focal length (Im more interested in Galaxies). I've seen that there's a F6.3 focal reducer/corrector but haven't seen(or maybe I haven't looked enough) a corrector that does not reduce the focal length? Is there one such? And does SCTs need a flattener?

SCTs definitely do not have a flat field.  That is why Celestron developed the EdgeHD, with field-flattening correction built in.  I am not aware of a corrector that is an add-on device that does not change the focal length.




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