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100% Illuminated Zone Size and Results in Imaging

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#1 hukhound

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Posted 14 July 2019 - 08:45 PM

Greetings,

 

I'm on a steep learning curve lately, related to modifying my 8" f6 Newtonian OTA for imaging purposes, and could use some advise from those who understand how optics setup effects imaging results.

 

It all started when I noticed the shape of out-of-focus stars on my camera live view screen, near the edge of the field, were clearly showing that my camera sensor could not "see" the edge of my primary mirror except at the very center of the screen.  This caused the out-of-focus star to look like cat's-eye shape with the secondary shadow way off to one side (nearly half the primary mirror was not visible at the edge of the screen!!).  What I later discovered, as you'll see below, was that my 100% illumination zone at the sensor was tiny. 

 

First, some details about my setup and goals:

  • Goal: astrophotography of DSO's, no visual, minor interest in lunar/planetary imaging (have a Maksutov for that)
  • 8"f6 Newtonian on GEM
  • 1.82" secondary mirror minor axis (modify to 2", 23% obstruction increases to 24%)
  • Camera is Canon M50 mirror-less, sensor size 0.88" x 0.59"
  • Based on analysis using Newt for the Web, my current OTA setup yields the following (values in parenthesis is where I think I will be after modifications):
    • 100% illuminated diameter = 0.185" (0.87")
    • 75% illuminated diameter = 1.13" (1.49")
    • Focuser min height = 2.8" (0.675" - custom super low profile camera mount, minimal focus range)
    • Primary mirror face distance to focuser hole = 37.5" (39.95")
    • Now and with the modifications, there are no issues according to Newt with vignetting or admitting 100% of the rays by the secondary mirror.

So the bottom line is my 100% illuminated zone is tiny (0.185") and I want to expand it to fill my camera sensor as much as possible (0.87" 100% zone vs 0.88" sensor width).  I plan to pull my primary mirror back 2" in the tube, and push the camera about 2" closer to the secondary mirror by building an ultra low profile camera mount.

 

Besides the obvious issue that I'm loosing nearly all of my adjustment in focus position, am I heading in the right direction with this modification?  Are there any serious flaws you can see?

 

One thing I'm unsure about is whether or not the image coma will get better or worse, since the camera sensor will see the entire primary mirror image.

 

Anyway, sorry about the detailed read.  I appreciate any help or comments!


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#2 hukhound

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Posted 14 July 2019 - 09:01 PM

OK, let me first scold myself for not searching on this topic first shameonyou.gif .  I now see this topic came up in this thread.  Still, being dully ashamed smirk.gif , if there's any comments specific to imaging (not covered much in the other thread) or the effect of my proposed mods on coma in images, please forgive me and enlighten via this new thread. 



#3 hukhound

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Posted 22 July 2019 - 11:21 PM

Well I think this post might be suffering from too much information out of the gate confused1.gif , given the absence of replies.  My apologies for that.

Let me see if I can summarize my question.

 

Since discovering that my scope's 100% illumination zone on my sensor is tiny, 0.185" diameter, I've been brainstorming how to get the camera much closer to the secondary, and trying to keep the secondary as small as possible.  So I think my question is two-part: 1) how important is it to keep a small secondary for IMAGING, or put the opposite way is it better to have a large secondary for imaging, and 2) how important is the 100% illumination size on my sensor?  

 

Thanks again!!



#4 TxStars

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Posted 22 July 2019 - 11:49 PM

The 100% zone only makes a difference if you are trying to do scientific studies.

For making nice image it is not a big issue as you are going to use flats do balance the drop off.

If you want to reduce the illumination drop off then either a larger secondary and or a shorter and larger focuser can help and or fix this.

The focus travel only need needs to be enough to account for any filters you may use so a total of 0.500" travel would work.

 

I have an imaging lens with only 5mm of total focus travel.


Edited by TxStars, 22 July 2019 - 11:59 PM.


#5 hukhound

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Posted 23 July 2019 - 12:06 AM

Thanks TxStars for your comments.  For sure I'm not doing any science, just looking to get the best images I can with this scope.

The very few references I've seen regarding 100% zone when doing imaging suggested that it was important, but didn't say much about how the size of the zone effects the image results.

It does make sense that there would be less illumination outside the zone, and I understand you point about using flats which I do.

At this point, I think it's safe to say 0.185" 100% zone is a bad setup, and should be improved.  So while I'm at it, I'm pushing for full coverage, but would like to understand the effort/reward trade-offs.

The more I read, the more it seems that I shouldn't be too afraid to throw a large secondary into the setup, so it would seem that it doesn't have a large negative impact in the image.


Edited by hukhound, 23 July 2019 - 12:07 AM.


#6 TxStars

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Posted 23 July 2019 - 01:00 AM

A lot of the information on sizing a secondary for 100% illumination is from "Film" imaging.

There was no easy fix for light fall off so the larger the 100% zone the better.

 

Correct for the overall image a larger secondary is not a huge problem..

But remember this, a larger secondary is heavier and can require a more robust spider to hold it in place.

 

*When using the different secondary sizing programs make sure you put in the inside diameter and wall thickness of your tube..

 



#7 tommm

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Posted 23 July 2019 - 10:23 AM

You would get more response if you posted your question here.



#8 hukhound

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Posted 23 July 2019 - 01:43 PM

Right.  At first it seemed to be more of an optics/DYI question, but there's definitely an emphasis on imaging. 

Happy to see it moved over there if the Moderator thinks appropriate.




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