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Need help understanding my telescope's star test results


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This is more of a follow up for this topic: http://stargazerslounge.com/topic/247817-mirrorcollimation-seems-to-be-messed-up-after-cleaning-and-reassembling-the-telescope/

I found that after reassembling my scope after cleaning, it seemed to have astigmatism. I'm a beginner with telescopes, so I'm not sure if the problem existed before I disassembled my scope. Today, I opened up my telescope, and aligned the primary and secondary mirrors from scratch (the telescope is a Celestron Astromaster 130 EQ). I collimated it, and verified the collimation visually using a collimation cap and a laser. The collimation appears to be near perfect. In the evening, I took out the telescope to do a star test. I'm not sure how to embed images in this post, so I'm posting links to Google drive where I've uploaded the photos of the star test.

Defocus toward the inside:

https://drive.google.com/file/d/0B_zQinhtzyDZYTJJU0RmTUFxMWM/view?usp=sharing

Defocus toward the outside:

https://drive.google.com/file/d/0B_zQinhtzyDZdEtIQWlVbjd1TDg/view?usp=sharing

This looks very wrong to me. Firstly, the circles are not really circles, but ellipses. I think they indicate astigmatism. I've made sure that the mirror is in no way deformed. The mirror holders are loose enough that the mirror can rotate freely in the mirror cell, but it doesn't shake. Also, You can see irregularly shaped flares. The 5-inch telescope was cooled for about an hour before the test, and there was little/no atmospheric turbulence. The irregular flares don't move around, and are fixed. Also, I noticed that the shape of the circle changes whenever I move my eyes around even by a few millimeters. I'm using a Baader Hyperion Zoom Mark 3 EP at 8mm for the test. At some positions, the circle almost appeared as if it was pinched at the middle, forming a long ellipse. The inside defocus image in the photo is not very clear. I had very sharp concentric circles (ellipses) with a bright point in the center, which are not clear in the photo. The outside defocus image is pretty much as it looked through the EP. Some irregularly shaped flaring that didn't move around. I don't think there was much atmospheric turbulence today. Properly focused, at the center of the field of view, I had one of the best views of Saturn I've ever had, and there was almost a hint of the Cassini division visible. 

So, I don't think this has anything to do with collimation issues, as I've tested that the collimation is extremely good. Also, the mirror is not stressed in any way, and is free to rotate in its cell without shaking. So, I'm unable to understand what's causing this kind of results with a star test. There's a small hint of astigmatism, and something that looks like spherical aberration, but I'm not sure. I'd love to hear some opinions and if this can be fixed without having to replace my mirror/scope.

P.S. The photos were taken by holding my DSLR with a 50mm lens at the eyepiece, so the quality isn't all that great, but I think it gives you some idea of how the star test looked. :)

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Hello, 

Unlike me, there are some real experts on this forum and I hope they respond to this thread :smiley: . 
 

I will however, throw in my 2 cents in the meantime.
I could be wrong, but the massive difference in the airy disk inside and outside of focus is normally a pretty good indication of spherical aberration.

In this case youve got very strong rings inside of focus and for want of a better word; mush, outside of focus. This surely indicates Undercorrected Spherical Aberration. 

Dan

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If you turn the primary mirror will the ellipse move with it?

I haven't tried that.. I'm not sure how to try that either! :D Do i have to remove the mirror cell from the bottom of the scope, rotate it, and reattach? I'll try it the next time I have clear skes and check. 

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I'm not sure how your mirror cell is... if you can rotate it in the cell or not, be careful that it doesn't get scratched if you try it. Also is your secondary "round" under the focuser?

Even if this scope turns out to "not be perfect" it will still give nice views, Cassini is a good test. How much mag will this scope support as is?

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I'm not sure how your mirror cell is... if you can rotate it in the cell or not, be careful that it doesn't get scratched if you try it. Also is your secondary "round" under the focuser?

Even if this scope turns out to "not be perfect" it will still give nice views, Cassini is a good test. How much mag will this scope support as is?

I cannot rotate the mirror without disassembling the whole thing. It'll probably require a few trips in and out of the house to disassemble, rotate and reassemble for star tests. it's a 5 inch mirror, so I guess it should theoretically support around 250x, but I've found that around 130-150x is the limit under really good seeing ( saturn and the likes). Also, I've never been able to see Cassini as nicely as I see in some astrophotos. The best I've seen are  a slight hint of the division in the rings under exceptionally good seeing and great collimation, and that too only when I've been observing it for a few minutes. Otherwise Saturn is usually just a small ball surrounded by a plain ring.

Also, I'm sure that even though the mirror is not perfect, it'll give realy nice views - under very rare circumstances, I've been able to see great views of some nebulae like Orion, lagoon, trifid, Ring, etc, but living in the middle of a city, and not having a dark sky area in almost a 100-mile radius pretty much limits my everyday observing to lunar and planetary! :( 

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If you have mutually perpendicular ellipses either side of focus, that indicates astigmatism. The very different airy disks also suggest spherical aberration, as does the very broad outer ring, though it could be a turned up edge.

These are just my guesses of course, I'm waiting for someone who knows more than me to come along.

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I think I see astigmatism and spherical aberration in your images, but I'm not sure by any means.

There is a program that will simulate all sorts of aberrations: Aberrator by Cor Berrevoets (the guy who also wrote Registax). Try it to find out which aberrations might cause the effects you see.

It's free.

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I agree with previous posters here. Astigmatism ( relatively slight ) and spherical abberation are present.

However, this telescope is at the budget end of telescopes and is mass produced. In production only the occasional mirror might be tested and that test will not be a vigorous one. You might get lucky and change it for a better one but it is just as likely the next one will be no better or even worse.

As you are limited to planetary why not try masking the telescope down a bit to, say, 110mm and see if that gives you any benefits. Just cut a 110mm dia. hole in a bit of card and tape it to the open end of the tube. That will remove any turned edge you might have.

Nigel

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Thanks for the suggestion, Nigel!! That's what I'm thinking too. I'll try it out the next time I have clear skies and let you guys know how it goes. I really appreciate all your feedback!

Cheers!

Sent from my A0001 using Tapatalk

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