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Backspacing issue with focal reducer and 1600mm

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#1 CrossAerialPhotography

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Posted 14 September 2020 - 09:29 PM

Hey guys!

 

gear: 

 

WO ZennithStar ED80ii

eq6r-pro

asi1600mm pro 

 

So I have a Tele Vue TRF 2008 focal reducer that should have a back spacing of 56mm. My current imaging train is the standard one on ZWO's website with the 16.5mm extender, 11mm ring, T2-T2 2mm adapter, EFW, then the camera for a distance of 56mm +/- 1mm for a filter. 

 

However, I seem to be getting some Coma in my images. Is this because my distance is too much, or too little? My thinking is that I am too far back by around 1 or 2mm. Below is an example from a test shot I did. 

 

get.jpg?insecure

 

Bubble_Coma_30.png

 

 

Before I just blindly start adjusting, I figured someone would be able to tell me if I need more or less distance. 


Edited by CrossAerialPhotography, 14 September 2020 - 10:01 PM.


#2 AZ Maverick

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Posted 14 September 2020 - 09:50 PM

Hard to tell from that image.

It's always a chore to figure out star shape problems, tilt, back focus, field rotation, etc...

Here is the diagnosis chart usually referred to when it comes to back focus.

 

 

Attached Thumbnails

  • BackFocusDiag.jpg


#3 bjulihn

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Posted 14 September 2020 - 09:58 PM

Hi There;

 

A suggestion: It might be helpful if you listed your equipment in your signature lines. 

 

Assuming you are using the correct reducer for your scope, it does look like a spacing problem to me. The stars increasingly trail away from the center the farther you move out from the center. If they all trailed in the same direction it would point toward a tracking issue. If the trailing were different in different quadrants I would suspect a tilt in the imaging train. I can only quess whether it should be moved in or out. Do you have an adjustable T-2 extension you can play with? There are also thin Delrin spacers available which could be used to add small increments to the distance. 


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#4 bjulihn

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Posted 14 September 2020 - 09:59 PM

If that chart is correct, you are too close by my eye.



#5 CrossAerialPhotography

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Posted 15 September 2020 - 11:24 AM

The reducer isn’t specifically for my scope, but it should work according to what I’ve seen.

#6 Deesk06

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Posted 15 September 2020 - 12:50 PM

Funny enough, I just got the same reducer. I'm using a 183 and I need to increase my backspacing. It's hard to tell from the image quality, but it looks like you need to as well. Do the stars all point to the center at the corners? If so then increase. I have increased mine to 58mm yesterday but packed up before going further. It was getting late and I had work. Going to increase to 59 and 60 tonight.

The backfocus is listed as 56mm +- 4mm

Edited by Deesk06, 15 September 2020 - 12:51 PM.


#7 CrossAerialPhotography

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Posted 15 September 2020 - 03:25 PM

Assuming I understand correctly, the smallest point for my Star is towards the middle, then it widens as you get farther out. I’m at 56mm +1mm for a filter. Maybe 58 is better

#8 Rovert9988

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Posted 15 September 2020 - 03:48 PM

Hard to tell from that image.

It's always a chore to figure out star shape problems, tilt, back focus, field rotation, etc...

Here is the diagnosis chart usually referred to when it comes to back focus.

That chart is how I sorted out my spacing with my 1600 and SV102 with 0.8x reducer. I started at the ~55mm and took some test shots, then adjusted accordingly. I would really scrutinize the corners where the issues were the most obvious. Took a bit, but now I just have that set of spacers together all the time so I don't have any variation.



#9 CrossAerialPhotography

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Posted 15 September 2020 - 07:28 PM

That chart is how I sorted out my spacing with my 1600 and SV102 with 0.8x reducer. I started at the ~55mm and took some test shots, then adjusted accordingly. I would really scrutinize the corners where the issues were the most obvious. Took a bit, but now I just have that set of spacers together all the time so I don't have any variation.

Well if the chart is correct, it looks like mine is too close. This should be a larger version of my picture above 

Bubble Coma (Custom).PNG




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