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C6 with reducer and 2" eyepiece

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#1 Adun

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Posted 10 January 2019 - 11:36 PM

Warnings:

  • SCTs are not Rich Field Telescopes
  • A C6 / Evo 6 / NexStar 6SE has a narrow back opening (27mm) that limits the maximum TFoV.
  • With an F6.3 reducer, the max true field of view of the C6 can be had with a 1.25" eyepiece (32 Plössl, 24ES68, or similar) using the Celestron prism diagonal.
  • Using a focal reducer with a C6 SCT might mean you are giving up a couple millimeters of aperture.
  • Focal reducers shift the focal plane towards the objective. Add the longer light path of 2" diagonals, and you might not even reach focus.
  • Even if you reach focus with a big 2" eyepiece in a C6 SCT, it'll probably be vignetted, and the larger the field stop, the more severe vignetting will be.

So, with that said, I just want to share the following:
 
I just got a GSO 2" 99% dielectric SCT diagonal, for my C6. When I use it with my Meade 4000 F6.3 (China) Focal reducer, plus my Meade 82° UWA 20mm eyepiece, the following happens:

  • It does reach focus
  • The field stop (black edge) is still visible, and clearly defined (sharp)
  • I don't notice any evident vignetting. There might be a small illumination fall-off but I didn't see it.
  • The view is much nicer than with a 32mm Plössl (same TFoV but more majestic in the 82° vs the 52° Plössl)
  • I like the ergonomics of the 20mm UWA much better than the 32mm Plössl (I'm fine with short eye relief and don't wear glasses)

But beware:
With my 34mm Meade SWA, the field stop gets blurry, vignetting is evident and the view just feels wrong, it's is nowhere as nice as what the same eyepiece provides in my 10" dob.
 
I'm posting this in case anybody is wondering about possibilities of mixing 2" eyepieces with focal reducers. I already had the 20mm UWA and am quite happy to use it instead of a 32mm Plössl, and I didn't feel like getting a 24ES68 (a third eyepiece with a ~27mm field stop felt wasteful).


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#2 SeattleScott

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Posted 11 January 2019 - 12:18 AM

When you factor in the longer FL using a 2” diagonal, I suspect the 24mm or 32mm with 1.25” diagonal would yield close to 10% wider FOV than the 20mm 82 deg with 2” diagonal. Plus you would be operating closer to full aperture with less spherical abberation (probably more of an issue as you increase magnification) and lower effective CO%.

Given you already have the 2” diagonal and 20mm 82 deg, and given you do not have a 24mm 68 deg, it makes sense for you. 32mm is kind of a long FL eyepiece to use at F6.3. So it works for you, but if someone is just starting down this path an hasn’t already invested in equipment, a FR and 24mm 68 deg makes a lot of sense.

Scott
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#3 Adun

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Posted 11 January 2019 - 08:07 AM

When you factor in the longer FL using a 2” diagonal, I suspect the 24mm or 32mm with 1.25” diagonal would yield close to 10% wider FOV than the 20mm 82 deg with 2” diagonal.

 
The longer light path is happening after the reducer, so it's not extending focal length. On the contrary, a longer light path between reducer and eyepiece increases reduction, producing a slightly lower magnification and thus a bit wider TFoV (although with more off axis aberrations). Maybe I should platesolve a couple snapshots to measure this for certain.
  
I agree this only made sense for me because I already had the 20mm UWA. Even then it was a tough decision, because a 24ES68 would have worked in my smaller telescopes. For someone just starting the ES68 or maybe a 25mm Paradigm makes more sense.



#4 George Methvin

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Posted 11 January 2019 - 10:20 AM

I keep a Meade FR on my C-6 at all time but have never tried using 2 inch eyepieces with it. I may just try using my Meade 20mm 82 deg two inch eyepiece and see if I get the same result just for fun that's if we ever see clear skies again lol.  



#5 AxelB

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Posted 11 January 2019 - 11:19 AM

I already made the test and calculations for a C8 with a Celestron f6.3 reducer. To get proper spacing and avoid increasing the focal lenght (almost up to f10.5), you can’t use a 2" diagonal. I did the test using a Baader t2 Zeiss prism and appropriate attachements and was able to use an ES82 18mm with excellent results. The ES82 24mm was too much and showed evident vignetting.

The baffle tube on a C8 is wider than on a C6 so I’m pretty sure a careful examination would show vignetting with the ES82 18mm on a C6 at f6.3.

Edited by AxelB, 11 January 2019 - 11:23 AM.


#6 Adun

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Posted 12 January 2019 - 06:58 PM

I already made the test and calculations for a C8 with a Celestron f6.3 reducer. To get proper spacing and avoid increasing the focal lenght (almost up to f10.5), you can’t use a 2" diagonal. I did the test using a Baader t2 Zeiss prism and appropriate attachements and was able to use an ES82 18mm with excellent results. The ES82 24mm was too much and showed evident vignetting.

The baffle tube on a C8 is wider than on a C6 so I’m pretty sure a careful examination would show vignetting with the ES82 18mm on a C6 at f6.3.

 
Yes, and for the 20mm 82° there must definitely be vignetting too, since it's impossible to fully illuminate a 27mm filed stop from the C6's 27 mm back baffle through a focal reducer.
 
So all eyepieces with 27mm field stops, be them 32mm Plössls, 24ES68, or 20UWA, all of them will show vignetting in a C6 with a reducer. Still, lots of people use the 24ES68 on their C6 with reducers and enjoy the view despite the vignetting, and that's because, like Jon Isaac's said:
 

However, the human eye is relatively insensitive to vignetting so it is rather difficult to detect even if you take a bright star from the center of the field and move it towards to the edge.

 
So whether the vignetting is immediately evident (or requires "careful examination") will depend on how close the (fully illuminated) image circle is to the size of the eyepiece's field stop, and individual sensitivity (plus personal preference) of the observer will determine whether the view is enjoyable or not.
 
I liked the view through the 20UWA but someone else might not like it. My point is more about the 20mm being able to reach focus (a big uncertainty for me despite much research). All the other eyepiece caveats still apply: illumination fall-off, more of the SCT's field curvature and coma becomes visible, etc.


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