Orion Optics Newtonian design question.

[Moonshane]

REALLY interesting Alexis. I might have a play with some figures. might be worth me extending the tube slightly at the cell end as I like the idea of having a larger illuminated field as I get the impression that the field does get a little darker out toward the edge. or even trying to replace the secondary with something smaller as you say may be possible.

I love that Dobs are so 'meddlable'.

[Moonshane]

ps

I am looking forward to reading the Kriege and Berry book that's off my girls for Christmas! (I clearly NEED to read it too).

[SiriusB]

There's a useful little freeware program called NEWT that lets you

play around with newtonian design parameters.

How bad has vignetting got to be before its noticable visually anyone? i'm assuming

something you'd notice in imaging might slip by the humble eyeball??

[sixela]

Yes. It's actually hard to detect when 30% of the light is gone (except by careful measuring; my guess is that you can manage to see 10% if you're an experienced variable star observer). 40% and you can start to see it.

[michael.h.f.wilkinson]

Yes. It's actually hard to detect when 30% of the light is gone (except by careful measuring; my guess is that you can manage to see 10% if you're an experienced variable star observer). 40% and you can start to see it.

10% is doable for a variable star observer only by comparison to stars of known magnitude, if they are in close proximity in the FOV. You might just be able to see 10% if you move the object quickly from one side to the other in the FOV, but that would be difficult. You might notice it best if the objects are close to the limiting magnitude.

[sixela]

Exactly - I was trying to position 10% as the limit that could in the very best of circumstances still be seen.

For observing faint objects, 10% can make a difference between detecting something and not detecting it, but only if you're talking about contrast between object plus background vs. background. It will make much less of a difference when it's just a difference in surface brightness, because the contrast threshold won't move that much by vignetting, as both the object and the background are darkened by vignetting.