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Focal Reducer - reduce vignetting?

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11 replies to this topic

#1 mtc

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Posted 13 November 2018 - 04:50 PM

I finally got a chance to image with my F6.3 Focal Reducer directly attached to my 127mm Mak!

I like it - but I'd like to reduce the vignetting...

 

My optical train is:

127mm Mak (the "older" model with the 1" opening at the rear of the scope)

SCT Schmidt Thread Adapter

Celectron F6.3 Focal Reducer

Celestron 1.25" Visual Back

1.25" barrel adapter

Camera to t-thread adapter

Canon T5i

 

Is my vignetting caused by:

the 1.25" visual back? (should I instead have a 2" path from the FR all the way to the camera?)

or

because my camera is 'too far back' and I need to move it closer?

 

Any thoughts on this?

Thanks!

 

ImageInfo: 12 90s frames, ISO 800, Bortle 6, Seeing: Poor

 

 

Attached Thumbnails

  • M42-12frames17M-mini.jpg


#2 luxo II

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Posted 13 November 2018 - 05:16 PM

You could use a really big file to ream the backend out to a full 2” clear aperture and rip the baffles out, and see how that goes. That’ll fix it.

Might devalue your scope though.

Using a small f/6 APO would be a better approach however.

Edited by luxo II, 13 November 2018 - 06:19 PM.


#3 Ishtim

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Posted 13 November 2018 - 06:52 PM

Have you tried using flat frame reduction to even the illumination? idea.gif


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#4 orangeusa

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Posted 13 November 2018 - 06:56 PM

You could use a really big file to ream the backend out to a full 2” clear aperture and rip the baffles out, and see how that goes. That’ll fix it.

Might devalue your scope though.

Using a small f/6 APO would be a better approach however.

Yikes that made me laugh !!!



#5 Ed D

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Posted 21 November 2018 - 10:04 AM

I had a 127mm Mak and it was great for planets and the moon with small sensor planetary cams, not so good for larger DSOs.  It does appear your camera is too far back, but I'm not sure that efforts to place it further forward will eliminate all of the vignetting.

 

You may want to try processing out the vignetting.  I downloaded your image and tweaked the Gamma to even the background sky.  Color adjustment would probably improve it further.

 

post-8956-0-89723400-1542145159.jpg

 

Have you tried using one of your camera lenses for wide field DSO imaging?  I attach a Vixen-style dovetail plate to my Canon so I can mount it to my GEM.  My 18-55mm lens gives good results.  If you have one, the 75-300mm lens would be even better.

 

Ed D


Edited by Ed D, 21 November 2018 - 10:16 AM.

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#6 happylimpet

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Posted 21 November 2018 - 10:14 AM

Flat -fielding will take that out. Doing an image stretch on an image that hasnt been flat fielded always looks like theres a horrific issue. I wouldnt be surprised if its just a 10-30% reduction in illumination that flat fields will laugh at.



#7 Tapio

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Posted 21 November 2018 - 10:18 AM

Sooner or later you have to use flat frames so why not now?

#8 mtc

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Posted 21 November 2018 - 03:57 PM

I have a spot on my mount for my camera and the 75-300 lens. I will try that out for wide field.
Thanks for the post processing tips. I think I'll try to move the camera closer to the FR to reduce the vignetting when using it at prime focus.
Thanks

#9 jallbery

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Posted 21 November 2018 - 11:07 PM

To avoid vignetting, you need to fully illuminate a sensor with a 27mm diagonal.  If using a 0.63x reducer, that means you need a scope capable of evenly illuminating a 43mm diagonal without a reducer.  A 127mm Mak with a 26mm rear baffle diameter isn't going to do that.   Reducing the spacing between the reducer and the sensor will help reduce vignetting, though, because it will reduce the reduction factor.


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#10 Eddgie

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Posted 22 November 2018 - 04:21 PM

Jallbery has nailed it.

 

These scopes out of the box probably by design only fully illuminate a circle only 5mm to 7mm in diameter and past this point, the illumination will fall off to about 75% out to a diameter a bit smaller than the size of the baffle.

 

So, lets say your baffle is 27mm (or whatever, I am just using this as an example).  If your field is only illuminated to 75% at the edge of a 27mm image circle and you use a .63x reducer, the size of the circle that is fully illuminated becomes (let's say it starts as fully illuminated over 6mm) a 4mm fully illuminated circle, and from the 4mm circle, the area that is 75% illuminated will now be found to be (.63 x 27mm) 17.01mm, and outside of this size circle, illumination becomes much more visible and sever.

 

This is what reducers do.. They reduce everything including the size of the fully illuminated circle at the focal plane. 


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#11 Nicole Sharp

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Posted 22 November 2018 - 07:14 PM

I finally got a chance to image with my F6.3 Focal Reducer directly attached to my 127mm Mak!

I like it - but I'd like to reduce the vignetting...

 

My optical train is:

127mm Mak (the "older" model with the 1" opening at the rear of the scope)

SCT Schmidt Thread Adapter

Celectron F6.3 Focal Reducer

Celestron 1.25" Visual Back

1.25" barrel adapter

Camera to t-thread adapter

Canon T5i

 

Is my vignetting caused by:

the 1.25" visual back? (should I instead have a 2" path from the FR all the way to the camera?)

or

because my camera is 'too far back' and I need to move it closer?

 

Any thoughts on this?

Thanks!

 

ImageInfo: 12 90s frames, ISO 800, Bortle 6, Seeing: Poor

 

What brand is the Mak?  The Celestron 127-mm Maksutov-Cassegrain OTA is listed as having a baffle tube diameter of 27 mm, so with a 0.63X reducer and a Canon APS-C camera sensor (27 mm), the field of view will be vignetted by 0.63X.  You need either a smaller camera sensor (17 mm or smaller), a Cassegrainian OTA with a larger baffle tube (43 mm or larger), or a non-Cassegrainian OTA such as a Newtonian astrograph or a refractor.


Edited by Nicole Sharp, 22 November 2018 - 07:21 PM.


#12 mtc

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Posted 23 November 2018 - 08:46 AM

It is the Orion 127mm Mak


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