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EdgeHD 8 - Collimation

astrophotography collimation
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#1 HumblePie

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Posted 02 June 2020 - 12:06 AM

For an astro newbie, collimating your SCT can be fairly intimidating. The process doesn’t really sound that complex. But . . .

 

Last Saturday night was one of the best nights for astrophotography after weeks of clouds and rain. I was determined to not let it go to waste. Beginning the preparations just before sunset it wasn’t long until I had everything ready. PoleMaster, Starsense, focus; done, done and done. Then I starting thinking about that little nagging, less than perfect, shape I was seeing in my stars after stacking. No problem. Just take a few minutes tweaking the collimation and then move on to astro bliss. Right?    Wrong!

 

While I could spend a few paragraphs describing how my donut shapeshifted into something that looked more like a crab from Neptune than a donut I will instead spare you the horror of it all. I ended up throwing in the towel fearful I had turned my expensive telescope into an expensive ‘thing’. I was just going to tweak it. I went to bed having wasted the night.

 

Next morning I got up and began watching every Youtube video I could find about collimating a SCT. Read forums. I reviewed all those tools and procedures. Found a guy who would fix it for $475 plus shipping. Of course after it was shipped back the first thing you would want to do is collimate it. Hmm? Then I remember reading “Of course if you have an SCT you do not need any of those tools”.

 

I am the proud owner of an artificial star which was purchased for just such a task. So I stripped everything off my OTA, removed it and the mount from the pier and brought it all into the house and set it up on its original tripod. Found the furthest line-of-sight through my house and carefully placed my artificial star in view of the SCT.

 

Step 1: I turned my OTA vertical with the secondary mirror pointed straight down. I tightened all three of Bob’s knobs so the mirror was fairly level and down against its own housing.

 

Step 2: I unscrewed ONE of the knobs while counting the turns (albeit partial turns) and then divided that count by 2 and screwed it back in by the result. Repeat for the other two knobs one at a time.

 

Step 3: Find, focus and center the artificial star. Defocus forming the airy disk i.e. la donut.

 

(Note: Upon completion of steps 1 – 3 the crab was dead. I now had a donut. Far from perfect but I felt like I once again had an expensive telescope and not just an expensive thing.)

 

Step 4: Hold a pen (or stick or finger) in front of the aperture lined up with one of the adjusting screws and look through the eyepiece to discover its location. Do this with each screw and note the screw that is nearest the narrow part (wall) of the donut.

 

Step 5: Using ¼ turns (or less), turn that screw clockwise. Turn the other two screws counterclockwise the exact same amount (or as close as humanly possible)

 

Step 6: Repeat step 5 again and again while re-centering the donut each time. Re-centering is critical and must be done before each adjustment. Else the crab from Neptune may return.

 

After following this procedure I achieved a donut so symmetrical I seriously could not see the slightest deviation.

 

And of course now the clouds and rain are back in the forecast. Ya gota truely love this hobby or you will end up in a padded cell playing with your toes.

 

Hope this helps someone.  Clear skies and moonless nights.


Edited by HumblePie, 02 June 2020 - 10:07 PM.

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#2 albln

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Posted 02 June 2020 - 08:20 PM

Could you share more information about the “artificial star” you used?  I have had an SCT for some time and it is time for Collimation even I have been avoiding it....   glad the process was smooth for you !



#3 HumblePie

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Posted 02 June 2020 - 09:21 PM

Hi Sputnik, 

 

The one I have is found here:

 

Hubble 5-Star Artificial Star (about $25)

 

https://optcorp.com/...artificial-star

 

There are YouTube videos on how an artificial star is used. Basically it allows you to collimate your mirrors on cloudy nights. I suppose it is better to use a real star like Spica. The reason it has five different 'stars' is that each one is a different size and brightness which will allow for a better match for your particular scope and distance. I just pick one randomly and it works fine.

 

The one thing I proved to myself was that no matter how bad and unrecoverable it looks there is a way to get it all back to where it is suppose to be. Just stay calm and be systematic about it while making very small adjustments. Do not just start adjusting the screws hoping you will luck out. You won't. It has to be systematic. 

 

Once I applied my steps, posted, it didn't take long to achieve great results. 

 

Also see:  https://www.cloudyni...llimation-r2798


Edited by HumblePie, 02 June 2020 - 10:13 PM.


#4 RazvanUnderStars

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Posted 07 June 2020 - 10:49 PM

Hi HumblePie,

 

I have the same scope and artificial star and am curious to hear how the scope will behave with real stars. There are two difficulaties about which I found out only after getting the artificial star.

 

In "Star Testing Astronomical Telescopes" (which you may already be familiar with, it's a well known reference book on telescope optics), the author explains why artificial sources have to be placed quite far. For an 8" f/10 SCT, the recommended distance is over 24 focal lengths which gives about 48m or 160ft. I wish I was living somewhere with that much space indoors :-)

 

The second difficulty is that collimation with a horizontal OTA (if, say, the artificial star is put on something far enough) doesn't necessarily work well when the OTA is mostly pointing up due to mirror shift between these positions. This has been discussed elsewhere here on CN. This can be alleviated if the scope is collimating outdoors near a tall building and the artificial star is mounted at a high floor, so the tube is mostly looking up. I saw you had the OTA vertical at step #1 but not sure if it was kept like this and you had the star straight above.

 

These two have made me reconsider the use of an artificial star. I'm sure it helps if the scope is badly out of collimation, but there may be a difference between the collimation done with the artificial star and the ideal collimation done with a star high up. Hence my interest to hear your experience, thanks in advance.



#5 HumblePie

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Posted 09 June 2020 - 08:48 PM

High RazvanUnderStars, Thank you for the information in your post. Very interesting and useful to know. Remember that my challenge was to recover from completely corrupting the collimation of my SCT (crab from Neptune). What I achieved with the artificial star indoors was a very symmetrical starry disc (donut). Per the information you shared the distance I was able to use was no where near what it should have been. But it was good enough to return to a workable condition. Now for the rest of the story.

 

After a few cloudy nights I finally got a break and was able to use Spica for collimaiton. And absolutely in agreement with your post, when I formed the starry disc it was no longer symmetrical. Mind you it was only off by a little. one side was noticeably narrow the other side thick. So I spent a few minutes fine tuning it and once again had it looking as it should.

 

Tomorrow night promises to be one of the best nights for imaging this season so far. I am very much looking forward to imaging M83. Hope to get a least 24 five minute exposures and calibration data. But the first thing I will do after the alignment routine and focus, is check the collimation.

 

The good thing this experience has given me is complete confidence in my ability to collimate my SCT. 

 

No more crabs from Neptune or any other place in the known universe.

 

Clear Skies and New Moons!




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