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Diagonal Clear Aperture & Eyepiece Field Stop...

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#1 Dave Bush

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Posted 22 February 2019 - 01:34 AM

My 2" diagonal has a 38mm clear aperture.

 

Does this imply that any eyepiece with a field stop of greater than that will begin to vignette or suffer light loss?



#2 SomebodyElseEntirely

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Posted 22 February 2019 - 02:55 AM

Yes.

 

But if it's only a few mm more, the vignetting may be gentle (it might not appear as a distinct shadow) and may thus not be noticeable in practice.


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#3 ngc7319_20

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Posted 22 February 2019 - 03:45 AM

Yes, if the there is some obstruction smaller than the eyepiece field stop, you will have vignetting at the field edge.  It may not be very noticeable.  But also keep in mind that, depending on the F-ratio of your scope, there may be vignetting at the field edge even if the diagonal aperture is larger than the eyepiece field stop.  This is because the light cone gets fatter as you move backwards from the focal point to the objective. So its more a question of how much vignetting can you tolerate.  Usually 20% or 30% light loss at the field edge is not very noticeable, provided the roll off is fairly smooth.


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#4 junomike

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Posted 22 February 2019 - 07:23 AM

IME it's dependent on the location of the Field Stop as a larger FS  (42mm -46mm) that is closest to the Diagonal might clip into the light cone, but a FS further away (up inside the eyepiece) is less prone to showing any issues as the light cone is smaller going through the Diagonal.

I've only seen Vignetting with a 46mm eyepiece in a Generic Diagonal (Orion, WO, Etc) which have the large "Stop Ring" located inside the Diagonal.


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#5 TOMDEY

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Posted 22 February 2019 - 08:49 AM

Yep, it does vignette, and exactly how that manifests gets somewhat complicated... and how much loss one is willing to put up with. Here is an excerpt from measurements I did a few years back, on 15 commercial star diagonals, (p13 of a 33-page paper/report). I also found that Cary Chleborad, at Optic Wave Laboratories, had investigated and analyzed this. Aperture, internal length, F#, and entire geometry determine vignetting.  >>>    Tom

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  • 45 star diagonal vignetting.jpg

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#6 Starman1

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Posted 22 February 2019 - 06:04 PM

Dave, I wouldn't worry about the diagonal.

The clear aperture on the back of the scope vignettes the light cone significantly before it even enters the diagonal.

If you're concerned, there are 2" diagonals with larger clear apertures all the way through.

Ideally, though, the opening on the scope should be larger, but it isn't going to be on an 8" SCT.


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#7 Dave Bush

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Posted 22 February 2019 - 06:16 PM

Dave, I wouldn't worry about the diagonal.

The clear aperture on the back of the scope vignettes the light cone significantly before it even enters the diagonal.

If you're concerned, there are 2" diagonals with larger clear apertures all the way through.

Ideally, though, the opening on the scope should be larger, but it isn't going to be on an 8" SCT.

Got it.  Say, you wouldn't happen to know the field stop diameter of the APM XWA 20mm would you?  I can't find it anywhere. 



#8 Starman1

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Posted 22 February 2019 - 06:31 PM

Dave,

A calculation shows 34.9mm.

That could be off by up to 3mm, but in the absence of published data, that will do.

I assume the field is 100°, which could also be in error.


Edited by Starman1, 22 February 2019 - 06:32 PM.

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