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Vignetting

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#1 nyx

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Posted 05 March 2021 - 10:00 AM

Something I noticed recently while going through some recent flat frames: when a scope claims it can cover a full frame sensor and I shoot with an APS-C one, why do I see vignetting e.g. in the flat frames? Is it normal? Is vignetting always there regardless of sensor size?

 

Clear skies! :)


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#2 Tapio

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Posted 05 March 2021 - 10:20 AM

Smaller sensors might be less affected but yes - so that's one reason for taking flat frames.
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#3 Midnight Dan

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Posted 05 March 2021 - 10:21 AM

"Covering" a sensor is not the same as "Fully illuminating" a sensor.  And even "Fully illuminating" is usually within some threshold.  For example, a manufacturer may consider the fully illuminated image circle to be the point where the light drops off to 80%.  But there is no standardization as to what these terms mean.

 

Then there's the "well corrected" circle, which is more or less free of aberrations such as coma.  Point is, the area covered by the image circle will be different from the fully illuminated circle, which will be different from the well corrected circle.  And manufacturers rarely specify all three.  

 

But to your question, yes there is pretty much always some off-axis dimming with any scope, but it is more prevalent in scopes where the primary optic (mirror or objective lens) is far back from the front of the tube (including the dew shield on refractors). 

 

Think of it like this.  All stars are far enough away such that the rays striking the various points across the objective can be considered a parallel column of light.  If a star is in the exact center of the field, then that column of light will strike the entire surface of the objective, regardless of any tube in front of it.  But, if a star is a bit to one side of center, then the column of parallel light rays is entering the scope at an angle.  Because of that angle, any sort of tube in front of the objective will shadow one edge of the light column and therefore reduce the amount of light hitting the objective.

 

A dew shield on a refractor is larger than the objective so it causes little off axis dimming until you get fairly far from the center. But on a reflector, the mirror is in the back of a fairly long tube, which is only a little larger than mirror, so that will cause more off axis dimming.  Scopes like SCT's have baffle tubes which also makes it worse.

 

-Dan


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#4 nyx

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Posted 05 March 2021 - 10:26 AM

Thank you for the prompt replies, you guys are awesome!


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#5 PirateMike

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Posted 05 March 2021 - 10:29 AM

That's a nice easy to understand explanation Midnight Dan. waytogo.gif

 

 

Miguel   8-)

 

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Edited by PirateMike, 05 March 2021 - 10:29 AM.

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#6 robbieg147

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Posted 05 March 2021 - 11:11 AM

I think it is extremely poor if a manufacturer claims a scope covers a full frame sensor and the scope vignettes over say 30%.

 

When I purchased my scope the spec for the reducer said something like "covers 95% of a full frame after which minimal vignetting starts to appear"

 

So this is quite clear, but I will be extra careful in the future thanks to the info above.



#7 imtl

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Posted 05 March 2021 - 12:49 PM

In order to "fully illuminate" a full frame, you really need at least a 3" opening focuser in matters of vignetting. All these claims by mass produced manufacturers are really misleading.


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#8 Midnight Dan

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Posted 05 March 2021 - 01:00 PM

In order to "fully illuminate" a full frame, you really need at least a 3" opening focuser in matters of vignetting. All these claims by mass produced manufacturers are really misleading.

Good point!  I was just explaining the innate off-axis dimming of the scope.  But of course, there's all the stuff on the back of the scope that further limits the ability to fully illuminate a sensor.  The focuser is one, as you point out.  But filters, extension tubes, T-rings, etc. also restrict the light. 

 

If you're using standard T2 extension tubes or a T-ring (42mm), they are definitely not large enough for a full frame sensor.  48mm accessories are barely large enough, but will still incur a little vignetting depending on where they are located and what your f/ratio is.

 

-Dan


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#9 imtl

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Posted 05 March 2021 - 01:11 PM

I think 99% of serious FF imagers use at least 50mm square filters in the imaging train.

 

I think vignetting is important but I think having an aberattion free frame is even more so. You can "fix" vignetting with flats. You cannot fix aberattions in post processing. (well that's not true, some stuff you definitely can).

 

When serious manufacturers give specs, they don't just write "fully illuminate". They actually give proper analysis of curvature and other aberrations. "fully illuminate" is just a vague general term.


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#10 Complexmystery

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Posted 05 March 2021 - 02:25 PM

Good point!  I was just explaining the innate off-axis dimming of the scope.  But of course, there's all the stuff on the back of the scope that further limits the ability to fully illuminate a sensor.  The focuser is one, as you point out.  But filters, extension tubes, T-rings, etc. also restrict the light. 

 

If you're using standard T2 extension tubes or a T-ring (42mm), they are definitely not large enough for a full frame sensor.  48mm accessories are barely large enough, but will still incur a little vignetting depending on where they are located and what your f/ratio is.

 

-Dan

I think this is the main concern. Generally all scopes that I know of can support at least APSC. Focusers and extension tubes are the real killer especially on fast optics. For full frame, a 2.5” focuser is practically a minimum but then again you have to think of things like flatteners and their clear apertures. To the OP........A lot goes into vignetting, it’s not a matter of “can the scope cover it”. If you do some reading on various forums you’ll realize where vignetting is likely to occur and how to prevent it. Calculators being a great tool. 


Edited by Complexmystery, 05 March 2021 - 02:26 PM.


#11 Der_Pit

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Posted 06 March 2021 - 05:03 PM

Something I noticed recently while going through some recent flat frames: when a scope claims it can cover a full frame sensor and I shoot with an APS-C one, why do I see vignetting e.g. in the flat frames? Is it normal? Is vignetting always there regardless of sensor size?

Yes, there will always be a cosine-shaped falloff:  Light that hits the sensor close to the edges also enters the aperture of the telescope at an angle, and therefore 'sees' a smaller aperture in one direction (i.e., an ellipse).   That effect gets more pronounced for shorter focal length (and thus larger view angle).

 

The other point is of course the 'how strong'.  If you just look at a flat and see dark corners that alone doesn't tell too much, as usually the image will be stretched between min and max for display.   Even a few percent darkening can look dramatic when stretched.  So look at the numbers wink.gif




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