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Is this poor collimation? Or something else?

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#1 Mike Wiles

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Posted 19 January 2013 - 01:06 AM

Hi all,

Prepare to be amazed at my lack of skill when it comes to diagnosing optical problems. I'm struggling with an imaging issue and I wonder if it's not collimation of my refractor...see the image below. All of my images are coming out with stars shaped like those shown. Here's what else I know from trying to diagnose it myself:

  • I'm sure it's not poor tracking. The mount is an AP900, less than a year old and well polar aligned with 2" of periodic error.
  • The image below is a 10 second image a few arc minutes from M1 when it was near the zenith.
  • I see the same "flare" on stars when I put in an eyepiece and observe visually. If I go to the other side of focus - the rings look concentric to me - or very close - but the out of focus star is flattened along the opposite end. Using the image as an example...the bottom of the out of focus stars is flattened if I go to the other side of focus.
  • The entire frame is this way, all in the same direction and all about the same amount of oblong shape. This doesn't change if I move to a different part of the sky.
  • If I rotate the camera, the oblong stars will rotate as well - so I believe this rules out the camera, and confirms that it's the optical train itself.
  • I don't believe it's astigmatism - as the "football" shape doesn't change its axis from one side of focus to the other.

Thanks for any help or assistance in diagnosing this issue.

Mike

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#2 oo_void

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Posted 19 January 2013 - 02:24 AM

Focuser, or time to see how good that warranty really is?

#3 Erik Bakker

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Posted 19 January 2013 - 05:03 AM

Hi Mike,

Looks like the camera is not exactly square to the optical axis of the scope. Can be a focuser alignment thing. Or the objective cell not being properly aligned with the tube/focuser. I am sure the astrophotography guys will chime in here to help you out.

#4 Jared

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Posted 19 January 2013 - 11:38 AM

It is hard to be certain from your description of out-of-focus stars, but I am pretty sure from looking at your sample image that you have fairly severe coma. If you are seeing the same distortion in about the same amount across the entire image plane, it is almost certainly coma not astigmatism.

Unfortunately, coma can be difficult to address in an air-spaced triplet. Yours is a triplet lens, yes? The problem is caused by one or more elements becoming decentered. For triplets, I wouldn't recommend trying to fix this yourself. It should probably go back to the manufacturer for servicing.

You mentioned that you see this visually as well. I assume that this is without the flattener in place, yes? Just want to eliminate that as a potential source of the coma...

Refractor designs are generally intended to be aplanatic, so there shouldn't be any coma. Refractors do suffer from some astigmatism, but for this to show up visually (as opposed to in the corners of an image), the collimation has to be off by a lot. Even if that were the cause of your problem, I wouldn't expect to see the same error across your entire image. I'm pretty sure you have a problem with a decentered lens element.

Focuser tilt is not likely to be the cause, either. Like lens cell collimation, this would show more variation across the image than you are describing.

#5 Jon Isaacs

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Posted 19 January 2013 - 12:39 PM

Mike:

What happens if you rotate the tube in the rings? One would assume that the aberration would rotate but it might be worth a try just to see if it might be some thermal issue.

What do you see visually when you crank up the magnification? It does seem like a collimation problem, it should be more apparent at high magnifications.

Jon

#6 Mike Wiles

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Posted 19 January 2013 - 12:40 PM

If you are seeing the same distortion in about the same amount across the entire image plane, it is almost certainly coma not astigmatism.....

....Unfortunately, coma can be difficult to address in an air-spaced triplet. Yours is a triplet lens, yes?....

....You mentioned that you see this visually as well. I assume that this is without the flattener in place, yes? ....


Jared,

Thanks for the detailed answer. As is typical with one of your responses...I learned a few things that I didn't know beforehand. Thanks for that. To answer to some of your comments...quoted above.

  • It is indeed an air spaced triplet.
  • Visual observations were made without the flattener in place. Stars have the oblong "tail" on them and the tail is discolored. They look very much like comets as you would see with coma, but the tail isn't nearly as fan shaped as I'm used to seeing in my experience with Newtonian designs.
  • The aberration is consistent across the entire image plan. Below is the same image as before showing all 4 corners and the center on a KAF-8300 chip. Only processing in this 10s frame of M1 has been bias subtraction and basic stretch.

Thanks for all the responses. They've been informative.

Mike

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#7 dedo

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Posted 19 January 2013 - 03:03 PM

So aberration direction is the same across the entire field, and rotates with the camera and, of course, with the focuser visually? If so then you have a severe miscollimation. Try with a laser in the focuser and rotate this one to see if the beam stays centered on the front element.

#8 Jared

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Posted 20 January 2013 - 12:55 AM

Miscollimation in a refractor shows as astigmatism, not coma. Also, miscollimation would not yield the same (or nearly the same) aberration in all four corners of the image--the 8300 chip is big enough that I would expect to see more variation in the elongation if this were simple miscollimation.

It's certainly worth double checking with a laser to make sure the collimation isn't terrible, but I suspect the issue is decentering of one of the elements based on the posted image and Mike's additional comments.

#9 edl

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Posted 20 January 2013 - 01:16 AM

Correct me if I'm wrong, but I believe de-centering of one of the objective elements could cause coma. I've seen it before in a triplet.

Best,
Ed L.

#10 Jeff B

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Posted 20 January 2013 - 06:50 PM

Ed is correct. A decentering of the center element for example will result in coma.

Jeff

#11 tomcody

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Posted 21 January 2013 - 12:39 AM

1. How cold was it when you had this problem?
2. Did this problem just start? Did you have normal looking stars before and this happened or has the scope always looked like this?
3. Some Tak FSQ,s develop something like this in very cold weather, from pinched optics caused by a cold lens cell.
Rex






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