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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 14 June 2019 - 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 14 June 2019 - 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 14 June 2019 - 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 14 June 2019 - 05:25 PM

John which do you use on your ranger?



#11 John Huntley

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Posted 15 June 2019 - 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, 15 June 2019 - 11:20 AM.

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#12 25585

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Posted 15 June 2019 - 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.  



#13 Eddgie

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Posted 16 June 2019 - 10:38 AM

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

 

A 1.25" diagonal will vignette the field slightly in an f/7.5 but it would be hard to see visually. 

 

The light path through a 1.25" diagonal from the end of the nose to the top of the eyepiece holder is about 90mm.  This means that you are passing the light cone though a hole 27mm in diameter down a 90mm long tube and for every 7.5mm the light travels, the 27mm circle is reduced in size by 1mm.  This means that over 90mm, the fully illuminated circle (which is what we are really talking about here when it comes to vignetting) is reduced in size to 15mm.  Everything outside of this circle is vignetted.

 

Now, the amount of vignetting and the application are usually what is important.  For visual use, the small amount of off axis rays that you are missing is not going to make a meaningful difference in the quality of the view.  Because the restriction is so far away from the focal plane, the vignetting will be very soft.  

 

This is similar to using an SCT.  Most Smaller SCTs only fully illuminate a field that is between 6mm and 10mm in diameter.  The baffle in the SCT, which is what causes the vignetting in that kind of system, is so far away that the vignetting is very soft and cannot be seen visually (only about a 20% falloff).

 

As to the placement, if you put the filter wheel in front of the diagonal, you now move the restriction much further forward (about 15mm) and you actually make it smaller (the clear aperture of a typical 1.25" filter is 24mm or so, sometimes less).  So now you have a 24mm opening that is 105mm in front of the focal plane.  At f/7.5, over 115mm, the 24mm fully illuminated circle is reduced to about 9mm.  This might be enough to be seen as visible vignetting under brighter skies but it would be a softer vignetting.

 

If you put the filter behind the diagonal though, here is the way it works out.   The light path though the diagonal is 90mm and the light path though the filter wheel is usually 25mm, and the eyepiece holder is another 25mm.  Now you have a light path of 140mm, but the restriction is 27mm so while the front restriction is further out, the fully illuminated field is still going to be about 9mm. 

 

The benefit of putting the filter wheel in  the diagonal eyepiece holder though is that it is an easier configuration to both manage and use.  Ergonomically the filter wheel is more easily accessed and  it is easy to remove for storage and such.  


Edited by Eddgie, 16 June 2019 - 10:40 AM.

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#14 Eddgie

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Posted 16 June 2019 - 11:01 AM

And here is how you test for and estimate field illumination falloff. 

 

Use longer focal length eyepiece with a known field stop size.

 

Place a bright star at the center of the field and defocus the field slightly. Enough so that maybe five or six diffraction rings are visible.

 

Now, drift the pattern towards the edge of the field.  At some point, you should observe a smooth, large arc intrude into the edge of the pattern.  The point at where this starts, as measured from the center of the pattern, is the point where the field is being vignetted.   Let's say you are using an eyepiece with a 27mm field stop (24mm Panoptic) and this occurred when you observed the center of the pattern to be about 1/3rd of the way to the edge of the field: since 1/3rd of the field size of an eyepiece with a 27mm field stop is 9mm, then that would be the size of the fully illuminated circle. 

 

Lenstarg_lin_720.png

 

Now for severity... Continue drifting the pattern to the edge of the field.   As it nears the field stop, more and more of the pattern will be covered by the vignetting source.  Let's say that by the time the pattern is at the edge of the field, 35% of it (by area) is obscured.  This would mean that you have lost about 35% of the illumination at the edge of the field to vignetting.   

 

Now  when the instrument is in focus, this small amount of dimming will usually be hard to see. The star test shows it, but it does not cause enough limiting magnitude loss to affect all but the very faintest stars, nebula, or galaxies near the edge of the field of view.

 

Most visual observers tend to choose eyepieces that follow a 1/3rd framing (the target occupies only the central third of the field) and unless their sky is unusually bright, they will not visually detect the sky darkening. 

 

And observer in the city though will see that the outside area of the field does appear darker, and gradually lightens until the edge of the fully illuminated field is reached. From the city, more than about 30% soft vignetting is unpleasant to me personally, but many are oblivious to it.

 

For nebula, where the nebula often extends far past the edge of the field stop, 35% illumination falloff can cause some of this nebula to simply be missed.    For my own use (image intensified), I try hard to keep the entire field 100% illuminated.   

 

For normal visual use though, a 10mm fully illuminated circle is pretty standard.  Most fast reflectors designed for visual use and most SCTs will be in this ball park.

 

Most refractors with 2" diagonal will do between a 20mm and 30mm fully illuminated field.  Most  Imaging refractors and reflectors will almost always do 30mm or more. 

 

Bottom line though is that a 10mm fully illuminated circle is pretty common, and most people never see the effects of vignetting from sources far from the focal plane because the vignetting tends to be very soft (gradual). 


Edited by Eddgie, 16 June 2019 - 11:06 AM.

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#15 helpwanted

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Posted 16 June 2019 - 10:11 PM

thank you Eddgie, excellent info, I'll think I'll just skip the wheel!



#16 Eddgie

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Posted 16 June 2019 - 11:20 PM

thank you Eddgie, excellent info, I'll think I'll just skip the wheel!

I would not do that.  The amount of vignetting is not at all serious and having a filter wheel is a great convenience.  I even have a Barlow mounted in my filter wheel!  At f/7.5, unless your skies are urban, you will not even notice the vignetting.  As I said, most SCTs and reflectors designed for visual use only have a 10mm fully illuminated field and you never hear these people complaining.

 

Having vignetting that you don't even notice is a fine compromise to getting the convenience of a filter wheel if you use a lot of filters.



#17 helpwanted

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Posted 17 June 2019 - 09:13 AM

good point




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