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Mirror Shadow or What?

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#1 DonBoy

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Posted 12 October 2016 - 03:36 PM

I just bought a modded T3i and the first few nights I used it with my 80ED f/7.5 with a Celestron 6.3 FR yielding a f/5.25.   All my frames had a darker area at the bottom of the image that ran the full length and had a uniform straight edge like a dark transparent banding.  I've included a jpg of a RAW file as an example from the 80mm lights.  When I used it with my C8 and the same 6.3 FR with the same spacing, which by the way yielded less focal reduction, I ended up with f7.65, but there wasn't any shadow or banding.

 

Flats will take care of the banding but I want to make sure that the modding of the camera was properly executed, so if need be I can send it back to have it adjusted if that's what it takes.

 

Does anyone have any idea what is going on?  Are the angle of entry of the optical rays hitting the top of the mirror edge on the 80mm and not the C8?   

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#2 DonBoy

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Posted 12 October 2016 - 04:23 PM

Here's another image to show bottom dark banding with the 80ED.     The circular vignetting is actually cut off at the bottom by this shadow or what ever it is.

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  • IC5070-120s x 11-240s x 4.jpg

Edited by DonBoy, 12 October 2016 - 04:29 PM.


#3 nofxrx

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Posted 12 October 2016 - 04:23 PM

That looks like a mirror shadow to me (assuming this image is not rotated or anything. The mirror is obviously on the top side of the camera but remember the picture is actually inverted when read by the sensor and the camera flips it).

Why it is happening here and not with the C8? I do not know..maybe it was but was less pronounced, or perhaps it is proportional to the focal range, not sure.

I seriously doubt it was the mod itself. Unless you had exposures using the same setup pre-mod that did not show this issue I could not see how the mod would do this..

Are you using mirror lock (actual in camera or virtual with software) or anything like that?

I am not entirely sure what others use to fix this issue, if any thing would be possible.

fortunately I have been lucky and have never had this issue on any of my cameras, so not much help there, sorry.



#4 DonBoy

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Posted 12 October 2016 - 04:28 PM

 

Are you using mirror lock (actual in camera or virtual with software) or anything like that?

Brent, I was using Mirror Lockup with both scopes with a few seconds delay before the shutter activated in BYEOS.   



#5 ngc7319_20

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Posted 12 October 2016 - 05:13 PM

The light cone of the ED80 + 0.63x FR will be fatter than the light cone of the C8 + 0.63X FR near the mirror, so that might explain why you don't see the effect with the C8.  Its something like F/5.25 vs. F/7.65 as you said.  So if the camera mirror obscuration is 1 inch from the sensor, the light cones are 1/5.25 and 1/7.65 inches in diameter at the mirror, respectively.



#6 sharkmelley

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Posted 12 October 2016 - 05:25 PM

Yes, this is quite normal.  The horizontal dark band of vignetting at the bottom of the image is caused by the parked mirror cutting into the light cone for the pixels at the bottom on the image (which correspond to the pixels at the top of the sensor).  The faster the F-ratio of the scope, the worse the problem becomes because the light cone widens.  It doesn't generally happen with lenses because lens optics are designed to prevent this.

 

Some people have been known to remove the mirror entirely if the camera is only going to be used on a scope:  http://www.markshell...mirrorless.html

 

Mark




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