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1.25" diagonals and vignetting

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#1 helpwanted

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Posted 12 June 2019 - 01:55 PM

Question... I use a 2" diagonal on my SW120ed, but only 1.25 eps, with the 24pano being the longest focal lenght. I have very limited in focus, but would like to try one of the filter wheels from Orion for both planetary and DS filters. They push out the eyepiece by 20mm, meaning you need to go in 20mm, which I don't have...

 

So, if I just use a 1.25 diagonal, that pushes my focal point out, giving me the in focus I need... correct? and if so, am I vignetting the view? Also with this setup, the filter wheel could go before the diagonal.

 

(in case someone wants to ask why I only use 1.25, all my filters are 1.25, and they can be expensive to replace to 2", plus if using an f5 dob, that 24mm is big enough for me with an exit pupil... even though I know I can go larger with the refractor at f7.5)

 

thank you,

David



#2 bobito

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Posted 12 June 2019 - 02:09 PM

A 1.25" diagonal shouldn't cause any vignetting with 1.25" eyepieces.

You can also consider a Prism diagonal to shorten the path, you get over an inch more back travel with a 2" prism over a mirror (depending on models you are swapping between).


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#3 Nippon

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Posted 12 June 2019 - 02:37 PM

With refractors like your SW120 and my 4" f/7 and 4" f/7.7 I don't see any need for 2" eyepieces either. I have both 2" and 1.25" diagonals. One is a TV 1.25" Everbright and the other is a Celestron 1.25" prism. I have noticed vignetting in the Celestron with a TV 32 Plossl but not the TV Everbright. The TV is expensive though. I also had an Orion mirror diagonal that would vignette with the TV 32. I think all 1.25" diagonals have a safety stop to protect the mirror or prism from a Barlow or eyepiece that can go in to far so some do some don't. But even with the Orion and the Celestron it was slight and only with the TV 32 Plossl. 



#4 John Huntley

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Posted 13 June 2019 - 04:34 PM

Some 1.25" diagonals do vignette 1.25" eyepieces which have the largest field stop diameter that can be fitted within the eyepiece barrel format I have found. The diagonal designs that do this are those that are clones of the William Optics 1.25" ones. That design uses a thick internal ring to attach the eyepiece holder to the diagonal body and that ring intrudes slightly into the light path reducing the effective max field stop size a little.

 

In the examples I've used eyepieces such as the 24mm Panoptic and the 32mm Plossl suffer from slight edge of field light loss leasing to a blurring of the field edge. A 25mm or 30mm eyepiece with a 50 degree AFoV used in the same diagonal without such vingetting because the field stop diameter is a little smaller.

 

The photo below shows the internal ring I am referring to (arrowed in red):

 

 

 

 

Attached Thumbnails

  • d-125d-c-db_02a.jpg

Edited by John Huntley, 13 June 2019 - 04:36 PM.

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#5 plyscope

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Posted 13 June 2019 - 06:41 PM

I used to have a WO 1.25" diagonal at the same time as the TV Everbrite and the only issue I noticed was very slight vignetting using a 24mm Panoptic with the WO diagonal. This was only at f6 (TV60). It was less of an issue at f8 and longer. 

 

1.25 inch diagonals.jpg

 



#6 Jon Isaacs

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Posted 13 June 2019 - 06:52 PM

The diagonal may or may not vignette.  It's very likely that the filters will vignette .  I use 2 inch  Nebula filters with 1.25 inch eyepieces with large diameter field stops to eliminate the vignetting. 

 

As far as 1.25 inch only in a 120 ED,  that's a 1.7° maximum field of view.  I'll do 2.9°.

 

Jon



#7 John Huntley

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Posted Yesterday, 11:36 AM

I used to have a WO 1.25" diagonal at the same time as the TV Everbrite and the only issue I noticed was very slight vignetting using a 24mm Panoptic with the WO diagonal. This was only at f6 (TV60). It was less of an issue at f8 and longer. 

 

attachicon.gif 1.25 inch diagonals.jpg

When I compared the same diagonals I found that the WO definitely vingetted the field of view edge making it fuzzyly defined when the field stop of the eyepiece was at or very close to the max for the 1.25" eyepiece format. The TV Everbright 1.25" did not have this effect presumably due to it's 1-piece construction ie: no strengthening ring required to fix the eyepiece tube to the diagonal body.



#8 plyscope

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Posted Yesterday, 03:21 PM

Hi John

 

I agree with you. It did seem to me that the vignetting effect was less pronounced when the focal ratio was longer. In any case the vignetting was only noticeable with the eyepieces with the widest field stop (27mm ish). In almost all other eyepieces it was difficult to see any difference in the two diagonals.

 

More than ten years ago I upgraded from the WO 66SD to the TV60.

 

Emma & Isaac 261008 021 - Copy.jpg

 

Emma & Isaac 261008 023 (Medium).jpg

 


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#9 John Huntley

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Posted Yesterday, 05:06 PM

Hi John

 

I agree with you. It did seem to me that the vignetting effect was less pronounced when the focal ratio was longer. In any case the vignetting was only noticeable with the eyepieces with the widest field stop (27mm ish). In almost all other eyepieces it was difficult to see any difference in the two diagonals.

 

More than ten years ago I upgraded from the WO 66SD to the TV60.

 

attachicon.gif Emma & Isaac 261008 021 - Copy.jpg

 

attachicon.gif Emma & Isaac 261008 023 (Medium).jpg

I have not used the WO style 1.25" diagonal for a few years now for this reason - I did find it noticable and annoying for some reason.

 

My 1.25" diagonals are now a TV Everbright and a Baader T2 Zeiss prism.


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#10 25585

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Posted Yesterday, 05:25 PM

John which do you use on your ranger?



#11 John Huntley

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Posted Today, 11:19 AM

John which do you use on your ranger?

TV Everbright now. Prior to that a Revelation / GSO dielectric which worked well enough but I wanted to put a TV diagonal on the TV scope. Must be getting soft in my old age ....... crazy.gif


Edited by John Huntley, Today, 11:20 AM.

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#12 25585

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Posted Today, 12:34 PM

TV Everbright now. Prior to that a Revelation / GSO dielectric which worked well enough but I wanted to put a TV diagonal on the TV scope. Must be getting soft in my old age ....... crazy.gif

My 3 TV scopes all have TV 2" diagonals, but I want a 1.25 for the Pronto (same optics as a Ranger). 

 

I will get an aluminised one.  




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