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Bad weather = Optimization time... Fighting insanity

collimation accessories beginner
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#1 astroPaulG

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Posted 18 March 2019 - 10:47 AM

(TL;DR see below)

 

Hello everybody!

 

We have bad weather (yeah, I know, big news). So what to do with a scope, that I barely used but eager to test on some nice DSOs? Learn to use it properly and optimize the **** out of it. At least as far as a beginner like me can do. I only have made some pictures while the moon was up, because the only clear nights in last months had at least 60-70% moon. So I photographed the moon and star fields. But first comes first. The equipment I use is:

 

  • EQ6-R (with the v5 handset, which is great because it has a USB port, so no "expensive" cables needed)
  • 8" f/5 Newtonian (which is round about 1.000 mm focal length).
  • GPU coma corrector (4-lens design, threaded to the focuser)
  • M48/EOS adapter threaded to the corrector
  • Canon EOS 6D Mark II (yeah, I know, not the best for astrophotography but I am a "daylight photography child")

Before buying the newtonian to get some serious astrophotography done ;), I only used a small refractor to learn the basics (Skywatcher EvoStar 72) and in between also used an SCT 9.25" (owned by my father in law). The EvoStar is now my guidescope.

 

So since I got the scope I tried to learn collimation and do it as best as I can, which I think i succeded. Because I had a lot of cloudy nights to train those steps. The attached images where made yesterday evening. Before this attempt I didn't improve the collimation, just the a quick check on a defocused star and saw a slight miscollimation but left it, because my main topic was to check the distance between camera sensor and coma corrector. And this is the point, where I am unsure about.

 

Some additional information: The GPU coma correcter is a 2" corrector and used in a 2" focuser. So the vignetting is expected and not the point, that is bothering me. Keep in mind: I use a full frame sensor (36 mm width).

 

The montage of the corner stars is from a image made yesterday, which has very little stars visible, because I made a very short exposure to elliminate effects of guiding or poor polar alignment. The background is bright because of the moon. 

The stars are a bit distorted. Through my tests in the last weeks and month I also had way worth corner stars than this. One problem was a distance ring, that was not perfectly machined and introduced a slight tilt. 

After the results got better and better, I downloaded the test version of CCDinspector, which opened the door to insanity for me. As you can see from the attached images, there is still tilt (which I think, I can ignore and maybe introduced by the M48/EOS adapter from Skywatcher). 
The field seems to be okayish centered. But there is a lot of curvature (at least 34% sounds like a lot for me). 

 

TL;DR

 

Is this amount of curvature to be expected with that setup or is this an issue of imperfect spacing betweend corrector and sensor? And if so, should I increase or lower the distance? I am not totally sure from the stars (not as I have been by star images of my refractor with bad spacing).

 

Thank you very much and clear skies for all of you!

 

- Paul

Attached Thumbnails

  • curvature.jpg
  • curvature-3d.jpg
  • corner-stars.jpg

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#2 OldManSky

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Posted 18 March 2019 - 04:18 PM

A good exercise for cloudy weather, Paul!

 

As a contribution, here's an analysis of my little WO ZenithStar 61 from CCDInspector.

First, without the field flattener (and I should note that the apparent collimation problem was actually a bit of sag from the 2"-nosepiece t-adapter I used to connect the camera to the scope):

 

Attached Thumbnails

  • WOz61_noflat.jpg
  • WOz61_noflat_3d.jpg

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#3 OldManSky

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Posted 18 March 2019 - 04:22 PM

(continued)

Then the same analysis with the AstroTech Field Flattener (sold for the AT60ED, which optically and almost mechanically is the same scope as my WO), this time using all-threaded connections for the camera:

 

I'd say the flattener is doing its job very nicely!

 

 

Attached Thumbnails

  • WOz61_withflat.jpg
  • WOz61_wuthflat_3d.jpg


#4 astroPaulG

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Posted 18 March 2019 - 04:29 PM

Thank you for your contribution! Interesting to see some other analysis. Looks a good amount flatter and not such strong raise in the corners. I will see what I can get with further spacing fiddeling.
First problem I identified 30 Minutes ago with some more test images of real stars (yes, I have some clear sky today): Indeed the M48 adapter is a bit of a problem. Because the inner part, which you can loosen with 3 screws to rotate it, does not exactly fit with the plane of the outer ring. It sticks out slightly. Will file it clean and see what I get.

Edit:
Saw your second post after answering :). This looks like the analysis I am dreaming of. But I wont get it that perfect with the big sensor. That is okay for me as long as I can use 80-90% of the frame without ugly stars ;).
Now I know where the benchmark is :D

Edited by astroPaulG, 18 March 2019 - 04:32 PM.


#5 OldManSky

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Posted 18 March 2019 - 04:31 PM

It's always the adapters, isn't it?  

Good luck with your testing/tweaking.  I'll look for the results later!



#6 astroPaulG

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Posted 18 March 2019 - 05:08 PM

Ok, I made it worth bawling.gif

But this one is maybe more an collimation issue than the other problems. I really should have collimated before grin.gif

 

Maybe the next weekend will give me some clear skies for further testing lol.gif

Attached Thumbnails

  • Unbenannt-1.jpg


#7 astroPaulG

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Posted 18 March 2019 - 05:09 PM

Those double spikes also need some attention. My todo list is just getting longer grin.gif

Attached Thumbnails

  • doublespikes.jpg


#8 OldManSky

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Posted 19 March 2019 - 09:59 AM

Interesting.

What's causing the double spikes -- one side of the spider vanes not "straight" with respect to the other side, i.e. twisted spider?



#9 astroPaulG

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Posted 19 March 2019 - 01:01 PM

Interesting.
What's causing the double spikes -- one side of the spider vanes not "straight" with respect to the other side, i.e. twisted spider?


I think so. At least this is what I found so far here on Cloudy Nights as explanation. Will make a test setup do align the cam exactly on the optical axis and midpoint of the scope and take some pictures from the front. Should show quickly if there is some misalignment.

I hope it is something I can fix. Maybe I screwed in the spider vanes too harsh. Will get it sorted out this evening to be ready for the weekend to test again. Chancen are good that Friday night will be clear. *fingers crossed*

#10 astroPaulG

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Posted 19 March 2019 - 04:13 PM

I think I could fix the star spike issue. At least the attached image shows, that those vanes where bent (and also a bit of twist). I fixed that. Will see, if it has the desired effect on the next clear night.

Further fixes will be coming ;).

Attached Thumbnails

  • front.jpg

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#11 astroPaulG

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Posted 11 April 2019 - 03:00 PM

Jepp, the star spikes are fixed now smile.gif

 

IMG_7107.jpg

 

For those, who are encounter similar problems, here is what to check and to fix at your secondary mirror spider:

  • Bending (individual or continuous)
  • Twist
  • Vertical and horizontal parts of the spider not perfectly in line

Everything must be aligned as good as possible. It definitly helps to align the  optical axis of your camera with the axis of your scope and photograph it. Use a lens with 100 mm focal length or more to avoid distortion in your photograph and paint lines in the image, to see if there are imperfections. At least that helped me.

 

 

Clear skies!

Paul


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