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Vignetting at Focuser 75% Ray

Started by markp, 11/16/2012 05:38PM
Posted 11/16/2012 05:38PM | Edited 11/17/2012 06:24AM Opening Post
Looking for some feedback on this...

I have a 12" dob (F/4.9) with a 2.75" secondary and a 2" focuser. The Newt program indicates there is vignetting at the focuser of the 75% ray. The diameter of the 75% illuminated area is 1.8304 inches.

If my lowest power eyepiece (Nagler 26mm) has a field stop of 35mm (1.3774 inches) which is half an inch smaller than the 75% zone. So is the vignetting really an issue? It seems like the field would be illuminated by the 100% and 75% rays and the vignetting is occuring outside the field.

Lastly, if I change the secondary to 2.6" the vignetting goes away but the 100% zone gets smaller. I'm not sure if the tradesoff are worth it. The central obstruction only changes from 23% to 22% and I am not sure I would see a difference.




Posted 11/17/2012 05:48AM #1
I'm interested in this, since I'm currently working on a f/5.9 12.5" mirror. I was wondering what the focal length is for your 12" mirror. That might affect what kind of comments others want to make.

I'm no great expert on the topic, but from all my reading, I've also picked up the idea that a lot depends on what kind of viewing you want to do with the scope. So that info might also be important.


In lumine tuo videbimus lumen.

8O Home-made 10” Dob / Home-made 4” refractor

EPs: Konig 32mm (1.25") / Zhumell WF 30mm (2") / Nagler 13mm T1 / Orion Sirius Plossls 25 & 10mm / Zhumell Plossl 9 mm / Meade MA 9mm
Posted 11/19/2012 05:04AM #2

As you have correctly deduced, the Vignetting of the 75% ray is a non-issue since the 75% ray is outside the maximum possible field of view. In general this warning in Newtwin is just an indication that you have a large fully illuminated field of view. If the 100% ray is vignetted by the focuser, it simply means you have a 100% illuminated field of view..

Another way to get this red herring to go away is to increase the diameter of the focuser.