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f/6.3 Focal Reducer/Corrector and UWA Eyepiece Question

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#1 PJBilotta

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Posted 07 April 2020 - 02:17 PM

Hi, all.

 

I've recently upgraded most of my eyepiece collection for my 8" SCT to ES and Meade 68's and 82's (also a couple of Hyperions). The 82-degree eyepieces and Hyperions naturally soften a bit in the outer 25% due to the normal spherical curvature of an SCT system. I'm contemplating adding a Celestron or Meade f/6.3 Reducer/Corrector to flatten the field and sharpen the edges a bit. However, since SWA and UWA eyepieces (particularly the 82's and Hyperions) tend to have edge softening and exhibit greater coma in faster scopes, will I gain anything?

 

Does an R/C improve edge performance even though the system is now f/6.3, or does the faster f/6.3 system essentially negate any improvement from the correction/field flattening the R/C provides? I am primarily a visual observer, so I'm not concerned with AP issues.

 

Thanks for any guidance you can provide.



#2 AhBok

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Posted 09 April 2020 - 07:19 AM

Edge performance is not the biggest problem with combining a C8, .63 reducer and 82 degree eyepiece. The main problem is vignetting. You will lose both effective aperture and field of view. The best use of a .63 r/c is for expanding the field of view for 32mm and below 1.25" eyepieces and/or increasing and flattening the field for imaging with a chip APS size or less.


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#3 SeattleScott

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Posted 09 April 2020 - 11:52 AM

Vignetting would only occur with max FOV eyepieces like a 30/82. You could use 1.25” ultrawide just fine. Not sure how the edge correction will be, but in general people say the reducer cleans up the edge.

A barlow does not reduce coma in a F5 newt. I suspect in the same manner, a reducer does not increase coma in a SCT. But I don’t have a SCT so I don’t know that for a fact. I just haven’t heard people complain about a reducer adding coma. I don’t have an issue with my Mak using a reducer, but Maks are better corrected for coma.

Scott

#4 jag767

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Posted 10 April 2020 - 04:17 AM

Edge performance is not the biggest problem with combining a C8, .63 reducer and 82 degree eyepiece. The main problem is vignetting. You will lose both effective aperture and field of view. The best use of a .63 r/c is for expanding the field of view for 32mm and below 1.25" eyepieces and/or increasing and flattening the field for imaging with a chip APS size or less.


This is misleading. The OP will not lose field of view, quite the opposite. He will get the maximum field of view that is possible, dependent on the aperture of the port in the rear cell. Even if there is vignetting, he is still getting more than he has now.

As far as vignetting, there are setups where I've read a plethora of people jumping on the vignette bandwagon, where it is not correct (or detectible to the eye). For example, I can use my Maxvision 40mm 68° ep on my 127 mak and there is no detectable vignetting. That's a 46mm field stop, and a 31mm rear port. I've had others look through it and they too concur, if it's there, it is not detectable. My point is for visual use, there's what should be, and what is. Also, this is without a .63 reducer/corrector. Once I introduce one to the system, different results. Still, the fov is greater with one present, vignetting aside.

#5 AhBok

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Posted 10 April 2020 - 08:19 AM

You are correct that the FOV is actually larger with the reducer. I have used C8's and the .63 reducer for about 30 years and understand that. I should have stated that the unvignetted FOV is larger with an eyepiece with a 46mm field stop (your example) and no reducer than with a reducer. Keep in mind that some people are more sensitive to vignetting than others. There is no bandwagon effect,  but rather a real effect for many of us who immediately see the effect of vignetting. 

 

I get that your mileage may vary, but your experience is not universal nor is mine.


Edited by AhBok, 10 April 2020 - 08:20 AM.


#6 jallbery

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Posted 11 April 2020 - 12:03 PM

This is misleading. The OP will not lose field of view, quite the opposite. He will get the maximum field of view that is possible, dependent on the aperture of the port in the rear cell. Even if there is vignetting, he is still getting more than he has now.

As far as vignetting, there are setups where I've read a plethora of people jumping on the vignette bandwagon, where it is not correct (or detectible to the eye). For example, I can use my Maxvision 40mm 68° ep on my 127 mak and there is no detectable vignetting. That's a 46mm field stop, and a 31mm rear port. I've had others look through it and they too concur, if it's there, it is not detectable. My point is for visual use, there's what should be, and what is. Also, this is without a .63 reducer/corrector. Once I introduce one to the system, different results. Still, the fov is greater with one present, vignetting aside.

 

I've used 2" eyepieces and/or a 0.63x reducer with wider 1.25" eyepieces with both smaller SCTs (e.g. C5) and Maks (e.g., 127mm).    I absolutely agree that despite the small baffle tubes, eyepieces with effective fieldstop diameters significantly larger than the baffle tube diameters can absolutely be used enjoyably and effectively.   I've been doing it for years.  I've been doing it with 8" SCTs for decades.

 

The view can be useful and pleasing.  And the human eye (and brain) is rather good at ignoring vignetting.   Certainly, the vignetting may not be obvious.  And I'm all for squeezing as much useful FOV out of your scope as you can.  

 

But there is a big difference-- at least to me-- between non-obvious or non-apparent and non-detectable.  The off-axis vignetting of the typical SCT is both there AND detectable.   Find a star that that you can just barely discern in the middle of the field.  Then slew the scope to position the star at the edge of the field.   Somewhere on that path it will disappear.


Edited by jallbery, 11 April 2020 - 05:53 PM.


#7 jallbery

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Posted 11 April 2020 - 06:18 PM

Hi, all.

 

I've recently upgraded most of my eyepiece collection for my 8" SCT to ES and Meade 68's and 82's (also a couple of Hyperions). The 82-degree eyepieces and Hyperions naturally soften a bit in the outer 25% due to the normal spherical curvature of an SCT system. I'm contemplating adding a Celestron or Meade f/6.3 Reducer/Corrector to flatten the field and sharpen the edges a bit. However, since SWA and UWA eyepieces (particularly the 82's and Hyperions) tend to have edge softening and exhibit greater coma in faster scopes, will I gain anything?

 

Does an R/C improve edge performance even though the system is now f/6.3, or does the faster f/6.3 system essentially negate any improvement from the correction/field flattening the R/C provides? I am primarily a visual observer, so I'm not concerned with AP issues.

 

Thanks for any guidance you can provide.

 

A few things...

  • The Celestron F/6.3 Reducer / Corrector is first and foremost a reducer.  Its field-flattening value has been debated here and elsewhere.   I'm not sure the Meade variant is even advertised as field flattener. It your primary reason for buying the R/C is because you are hoping to reduce field curvature, my guess is that you will be disappointed.
  • For visual astronomy, the primary value of the R/C is that it allows you to achieve similar views with 1.25" eyepieces that would otherwise require 2" eyepieces.  This can save you money and the weight savings can be significant, as well.   Well-corrected, max (or near max) field 2" eyepieces are big and heavy.   A 24mm 68-degree 1.25" eyepiece plus the R/C gives you almost exactly the same field you'd get  with the 40-41mm 68-degree two incher. 
  • The poor off-axis performance of many simpler eyepiece designs at faster focal ratios is a type of astigmatism; it is not coma.  ES82 eyepieces should be well-corrected at F/6.3.  Hyperions, too (except may be the 25mm, which does not have an built-in barlow element).


#8 SeattleScott

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Posted 11 April 2020 - 06:20 PM

Another good test is the Moon as the decrease on brightness is readily visible as it moves into the vignetted portion of the field.

I currently have a reducer on my Mak until the planets return. With my 42LVW the vignetting is obvious so I use my 24/82, which still vignettes but less distracting. I estimate 1.8-1.9 degrees FOV. Not bad for a Mak. An 8” SCT is likely similar, having similar FL and almost identical size baffle tube.

Scott


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