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SCT Focal Reducer, TFOV, Brightness, etc. for DSO visual only

Eyepieces Beginner SCT
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#1 jjdbhb


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Posted 10 May 2021 - 11:11 PM


Looking for some eyepiece advice for DSO visual observing (no photography/imaging) as I'm starting to suffer from analysis paralysis.


I have a Meade LX10 (203mm aperture, 2000 focal length) and trying to figure out if the various directions I can take with eyepieces make any difference in terms of:

  • DSO image brightness
  • Amount of detail one can see viewing a DSO (hoping to increase magnification without impacting brightness - which I understand as not really being possible)
  • Overall viewing pleasure (what might get me a better "Wow" factor)


There appears to be at least 3 directions:

1) Wider FOV 1.25" FOV eyepieces

2) 6.3 focal reducer with less wide FOV 1.25" eyepieces

3) Convert to 2" eyepieces


The following link and image shows options 1 & 2 with 4 eyepieces using M81 as an example:


Screenshot 2021-05-10 224131.png


Those 4 each give about 0.56 TFOV.  My brain is thinking that each of them will give the same viewing experience of M81 as the # of photons hitting my eye is about the same for each.  However I can't help but see that the 12mm with focal reducer option gives less magnification than the others.


Will I see less detail of M81 with that 12mm with focal reducer option?

All other things being equal, what viewing difference does one notice viewing with a focal reducer that provides the same TFOV as a wide FOV eyepiece?

Is converting to all 2" (with no focal reducer) a real game changer for viewing in this scenario?


Thanks much!

#2 ravenhawk82


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Posted 11 May 2021 - 12:06 AM

Think of it this way: You are collecting an 8" diameter circle's worth of light and spreading that light across your field of view. The detail you see with any of these eyepieces will be the same for all intents and purposes. The image will appear slightly dimmer in the higher magnification, higher AFOV eyepieces however because (despite showing essentially the same field of view) the light from your target will be spread over a larger area in the eyepiece. What this image doesn't show is the fact that the 70* AFOV eyepiece will provide a larger image circle than the 60* AFOV. 

So the higher mag, larger AFOV eyepieces will make your target look larger.

For visual use, the focal reducer is unnecessary. The main purpose of a reducer is to speed up an optical system for photographic use but has little purpose in a visual setup. The eyepieces dictate magnification so choose your eyepieces accordingly. Also, the only advantage of a 2" eyepiece over a 1.25" is that it can (when designed to do so) provide a larger field of view at a given focal length. You can't make an 82* AFOV 30mm 1.25" eyepiece, the tube just isn't big enough. 

I hope this helps clarify some things!

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#3 wrvond



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Posted 11 May 2021 - 01:28 AM

Both the Celestron and Meade 0.63 focal reducers are also correctors. These reducers improve visual image sharpness at the edge of the field by correcting for coma and field curvature. They don’t work well with 2” eyepieces though.

Edited by wrvond, 11 May 2021 - 06:54 AM.

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#4 Redbetter


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Posted 11 May 2021 - 02:16 AM

The reason to use a focal reducer/corrector or 2" diagonal with an 8" f/10 SCT is primarily to allow wider true field of view than can be achieved in native uncorrected 1.25" mode.  You aren't doing that with a 12mm eyepiece...producing roughly 0.56 deg FOV.  Actual max TFOV in 1.25" is roughly 57.3*27/2032 = 0.76 deg.  This latter is possible with a 40mm or 32mm Plossl, or a 24 Panoptic.  


TFOV is determined by the field stop diameter of the eyepiece and effective focal length of the scope, not the apparent field of view which is an approximation since "apparent" includes distortion.

Before anyone heads down the rabbit hole, the first thing to note is that contrary to myth, an 8" SCT should be able to handle the widest full field stop of a 2" eyepiece in 2" visual mode without a reducer/corrector.  The baffle diameter is less than this, but the visual vignetting is not really noticeable at night.  That is the way I have used my 8" SCT since the early days. 


There are some advantages and disadvantages of using a 2" diagonal vs. 1.25" diagonal with reducer/corrector.  

  • The longer overall length of a 2" diagonal set up might not be compatible with some mount arrangements.  
  • The longer optical length of a 2" diagonal arrangement changes the effective focal length of the scope (because of the moving primary focus system).  For this and the reason above, an SCT threaded diagonal is preferable to minimize the extra distance--rather than a 2" visual back and 2" refractor diagonal.  (However, my 25+ year old system on my 8" SCT is the latter with 2" visual back/refractor diagonal and it works well.)  I do use this
  • The 2" diagonal is more secure/less likely to rotate suddenly than the OEM 1.25".  
  • There are some very good 2" eyepieces for low/mid power views, so from a viewing comfort perspective I tend to favor them.  
  • The negative is that well corrected 2" eyepieces and nebula filters are pricey.  
  • 2" systems with heavy eyepieces can require some upgrades to balance.  I use an adjustable dovetail weight on my OTA.
  • The big advantage of reducer/corrector is that it corrects the coma and field curvature inherent to the basic SCT design.  I have never used one so I can't speak from experience, I want to try one sometime--I just haven't gotten around to it.
  • Using the reducer/corrector could make it more problematic to reach high magnification.  Generally folks don't like to have to repeatedly remove and reattach this accessory while observing, because it means removing the diagonal and the SCT threads can be troublesome in the field--especially fumbling with them in dark and cold conditions.  

At a given true field of view, the magnification level (and therefore apparent field) that is best usually depends on the target.  At the low power/max true field end, more magnification generally provides more detail.  However in dark sky with nebula filters, diffuse nebulosity is often better seen with lower magnification because the exit pupil is larger and the image therefore brighter (the Horsehead is perhaps the best example of this.)  Also in dark sky some intermediate exit pupils work better to show extent of low surface brightness objects such as galaxy envelopes, rather than smaller exit pupils at higher magnification.  


So the 1.25" cases of the max true field of view 40, 32, and 24mm eyepiece will produce 4, 3.2, and 2.4mm exit pupils and 0.76 TFOV at the native f/10.  A .63 reducer will change that to 6.34, 5.1mm, and 3.8mm with roughly 1.2 deg TFOV.  By comparison a 2" setup with a 55 Ploss or 41 Pan will produce ~5.5mm and 4.1mm exit pupil with about the same field of view--calculation says almost 1.3 deg, but the actual focal length of the scope makes it a little over 1.2 true based on drift timing I have done (an SCT diagonal will make it a little larger still.)


40 Plossls are a pain to use...the narrow apparent field of view is combined with very long eye relief, making eye placement more difficult.  Consider that even in pristine sky, one will want to cup a hand around the eyepiece and face to reduce ambient glare, particularly because of the excessive eye relief.  


The 55 Plossl (and 41 Pan) have long eye relief, but their adjustable eyepiece cups help in this regard.  The 55 Plossl has a standard Plossl apparent FOV, unlike the 1.25" 40mm Plossl that is restricted considerably by the barrel.

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