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Star Not Round During Collimation

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#1 parrothead2567

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Posted 03 December 2021 - 04:19 PM

I apologize if this has already been discussed. Ive searched and haven’t found something exactly like this. When I try to collimate my EdgeHD 8”, the unfocused star isn’t round-it looks more like a cats eye-oblong and pointed on two sides. I’ve turned the collimation screws and it got better some but can’t get it round. Have I screwed up my secondary mirror?

#2 Bigzmey

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Posted 03 December 2021 - 04:31 PM

On EdgeHD SCTs the secondary mirror holder has threads and could be taking out. Make sure that it is threaded all the way in. It could be also diagonal/visual back issue, with diagonal sitting at an angle or not collimated.



#3 Borodog

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Posted 03 December 2021 - 05:17 PM

An image or sketch would help.

 

When you stand a couple of meters back and look straight down the barrel of the telescope, does everything look concentric?



#4 parrothead2567

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Posted 03 December 2021 - 05:31 PM

Thanks all. A few days ago I tightened the collimation screws because the secondary mirror seems loose. I don’t tighten all the way but it looks like that was the culprit. I verified that the central screw for the secondary mirror was not loose. I then looked through the front of the scope and it was very misaligned. I worked with the collimation screws and seems to be more in line. I’ll try collimation again tonight.

After all that, I’m still concerned that the secondary mirror is loose again. The metal cover under the collimation screws moves freely. Is there a way to keep that from happening without tightening the collimation screws?

#5 Redbetter

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Posted 03 December 2021 - 06:01 PM

First pass would be to locate a star high in the sky (or even Polaris will be sufficient...and it won't move very rapidly.)   Get it in the center of the FOV.  Then, to make sure is it not some sort of diagonal problem, remove the diagonal, put your eyepiece into the visual back.  Refocus and recenter.  Now, once centered, use your highest magnification eyepiece (what focal length eyepiece do you have for this?)   This will allow a really simple star test.  [Moving mirror focusing in SCT's will result in an operating focal length far different than nominal, with the eyepiece, but I am trying to eliminate diagonal problems as a concern in the first pass.]

 

A star like Polaris at high power should appear as a brilliant ~uniform point.  If the focal length of the eyepiece is 10mm or less you might be able to see the airy disk pattern--depends on the observer/eye and what focal length the scope itself is operating at..  At any rate, going back and forth slightly from focus should give you an idea of any non-uniformity to the shape and if that is flipping back and forth (astigmatism.)

 

If with a bit more distance from focus you see the beginning of the center doughnut offset, then it should be a collimation issue. 



#6 parrothead2567

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Posted 04 December 2021 - 05:08 AM

Thanks Redbetter. Took your advice and found a bright star (Polaris is blocked from me by trees) and was able to get an airy disk and eventually collimation successfully with my ASI 224. Now my sleepless nights will be from astrophotography-not worrying about my screwing up my scope!
  • davidc135 and Old Speckled Hen like this

#7 gnowellsct

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Posted 09 December 2021 - 08:30 AM

eventually collimation successfully with my ASI 224. Now my sleepless nights will be from astrophotography-not worrying about my screwing up my scope!


I'm not sure I agree with that. The part about your sleepless nights. If you are an astrophotographer you will be worried about every single last mind-numbing detail including such things as whether the land you are set up on is allowing a tripod leg to sink a quarter of a millimeter per 24 hours.


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