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# Vignetting Phobia - How to calculate?

4 replies to this topic

### #1 Simcal

Simcal

Ranger 4

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Posted Yesterday, 10:16 AM

Hi everyone,

As I work through my various problems and sort out solutions for adapters, thread sizes, back focus distances, and not to mention eventual imaging cameras, sizes etc.. I keep banging up against the concern for vignetting... specifically being that I don't want to introduce an unintended baffle in my image/observation train, which will rob me of photons.

So my first question, given any refractor, how do I figure out light cone diameter, for any particular point along the back focus continuum?

And then the second question being, does every eyepiece which does not match field stop to light cone, essentially stop down the light gathering? (edit: thinking about this while I'm waiting for the knowledge to come in, I'm feeling increasingly stupid LOL).

Edited by Simcal, Yesterday, 10:32 AM.

### #2 calypsob

calypsob

Fly Me to the Moon

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• Joined: 20 Apr 2013

Posted Yesterday, 10:25 AM

The light cone question has always puzzled me.  I asked this to someone in the past and could  not get a straight answer so Im curious to hear from others.

I do wonder how a 100mm F5 can cover a full frame sensor while another 120mm is only suitable for aps-c

### #3 AstroBrett

AstroBrett

Vostok 1

• Posts: 149
• Joined: 26 Jan 2009

Posted Yesterday, 10:49 AM

Sim,

The easy way to do that is to construct a scale drawing with the diameter of the objective indicated at one end, and a perpendicular line drawn form it's midpoint out to one side. Using the same scale, mark the focal length of your system and draw lines from the focal point to the two edges of your objective. That will show you the basic light cone, and it's diameter at any point back from the objective.

Now, you will also need to take into account the size of your sensor, so draw it to scale with the focal point as its midpoint. Then, similar to above, draw another pair if lines to the outer edge of the objective. This will show you the expanded light cone that will result in vignetting if any of your components impinge on it.

Calypso,

The answer to your question is not related as much to the light cone as it is to field curvature at the focal plane.  A scope with a highly curved focal plane will not cover as large a sensor as a similar scope with a relatively flat field.  The use of field flatteners helps expand the diameter of the acceptable image circle.

I hope that helps.

Brett

### #4 2ndRecon

2ndRecon

Explorer 1

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Posted Yesterday, 11:05 AM

https://astronomy.tools/

### #5 Simcal

Simcal

Ranger 4

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• Posts: 395
• Joined: 01 Oct 2018
• Loc: Caledon, Ontario

Posted Yesterday, 11:37 AM

I think it's going to be more complicated for my FSQ / Petzval.  With the multi-element design the rays could be spread or tightened in the last elements, making the cone angle steeper or flatter.  I think there must be some equation incorporating image size and back focus of some sort?

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