Astrophotography: Optical Alignment

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mark.m
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Astrophotography: Optical Alignment

Unread post by mark.m »

One of the things that I'm struggling with is residual coma in my images. (I'm using a Starizona Focal Reducer and Coma Corrector for all my images.) For example, notice the top left corner of this image:
Typical Image (coma in top left corner)
Typical Image (coma in top left corner)
TypicalImage.jpg (171.71 KiB) Viewed 696 times
If I take that image, and draw straight lines through some of the stars that show that elongation, I get the following:
Vectors showing coma origin
Vectors showing coma origin
ComaVectors.jpg (133.81 KiB) Viewed 696 times
It looks like the "coma-related optical center" is down near the "bottom" of my frame, and nowhere near the center of the camera's sensor.

What can cause this?
  1. Can this be caused by sensor tilt?
  2. Can this be caused by a tilted main mirror?
  3. Can this be caused by the imaging equipment "drooping" from its attachment point to the rear of the scope's back?
  4. Can this be caused by collimation error?
What do you think? What experiments can I try to understand this better? (And, by the way in case it helps, my vignetting pattern is not symmetric about the center, either.)
- Mark M
Mark M, AJ1B
Portsmouth, RI
Celestron 14" and Meade 10" SCTs
QHY268M + SBIG ST-9
GM2000 (10Micron)
American Association of Variable Star Observers (AAVSO) observer code: MMU
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Pete
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Re: Astrophotography: Optical Alignment

Unread post by Pete »

The vignetting pattern sounds like a huge clue Mark. Off axis chip? Cocked primary? Would spinning the camera 180° provide another clue?
Pete P.
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menardre
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Re: Astrophotography: Optical Alignment

Unread post by menardre »

Mark

Have you tried different backfocus distances.

One night I spent most of the night trying different backfocus distance for each of my scope configurations. I took a sample 2 minute image and then evaluated the stars across the field. I started with no extra spacers, then added 5mm spacers until I found the combination that resulted in pin-point stars across the field. I then made individual spacer combinations for each configuration so that when I now want to use a configuration, all I have to do is grab the assigned spacer combination.

Your image looks 'bloated' in mostly the top left, so there may also be some sensor tilt.

Roger
Roger M.
Celestron CPC1100 EDGE, Stellarvue 130T refractor dual mounted on iOptron CEM120 on permanent pier mounted in Observatory. Imaging camera ZWO ASI2600 OSC, guide camera Lodestar or ZWO ASI290MM.
Bruce D
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Re: Astrophotography: Optical Alignment

Unread post by Bruce D »

Could be mis-alignment at the eyepiece tube? There always seems to be some play and especially if the EP holder has only one thumb screw to hold the eyepiece (or camera) instead of three, that may skew the axis of the camera? Often I will center an object in the fov visually, replace the EP with the camera and the camera isn't seeing the object and I have to hunt for it. The EP and the camera were not centered on the same point on the primary mirror, and who knows which was pointed closest to the center of the primary mirror?
Bruce D
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bluemax
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Re: Astrophotography: Optical Alignment

Unread post by bluemax »

Mark, I agree with Roger on back focus. I wrestled with this as well a few years ago and tweeking the shims, got a better result.
Frank N

Stellarvue 80mm APO, Skyguider Pro, Celestron AVX

"I'm a seeker too. But my dreams aren't like yours. I can't help thinking that somewhere in the universe there has to be something better than man. Has to be"

Taylor in "Planet of the Apes" 1968
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Pete
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Re: Astrophotography: Optical Alignment

Unread post by Pete »

Most cameras do have a tilt adjustment, but beware as if you rotate the camera you may need to redo the tilt.
Pete P.
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mark.m
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Re: Astrophotography: Optical Alignment

Unread post by mark.m »

Ah, thank you all.

I rotated the camera 180 degrees. (Actually, I rotated the entire imaging chain from the focal reducer back to the camera.) It made a big difference in the image:
  • The image center shifted by a little more than 4 arcmin,
  • The "coma center" moved almost exactly to the center of the camera (just what I wanted!).
But what I realized is that the connection between the focal reducer and the focuser is definitely a problem area for me. The connection is not rigid, relying on just a 1/2" compression ring to hold the entire package of focal reducer/spacers/filter wheel/camera. And so I have no real confidence that the connection will remain motionless and aligned as the scope moves around throughout the night.

Plus, I have some spacing issues (focal reducer to camera). Plus, there's a bit of vignetting that doesn't always "come clean" after flat field correction. That poor Starizona focal reducer seems to be getting more than its fair share of blame in all this.

- Mark M
Mark M, AJ1B
Portsmouth, RI
Celestron 14" and Meade 10" SCTs
QHY268M + SBIG ST-9
GM2000 (10Micron)
American Association of Variable Star Observers (AAVSO) observer code: MMU
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Pete
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Re: Astrophotography: Optical Alignment

Unread post by Pete »

Glad the rotation was informative Mark. I'm out imaging right now and can't give you full details but I've some thoughts regarding a rigid optical train.
Pete P.
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mark.m
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Re: Astrophotography: Optical Alignment

Unread post by mark.m »

Okay, time for an update.
  1. I replaced the simple "visual back with compression ring" with a Baader ClickLock adapter (from Agena Astro -- great place for adapters and accessories), and it made a big difference. The Baader device provides a much stronger grip to the camera/reducer/filterwheel assembly, which is now held more rigidly and more square to the focuser.
  2. I increased the distance between the focal reducer and the image plane by 9mm. The distance is now 9mm more than Starizona recommends for the reducer/flattener/corrector, but it corrects residual coma much better
By making those two changes, two things happened: the optical center and the mechanical center align more closely, and image quality is much less sensitive to those misalignments. I'm much happier with image quality now. There's still a collimation error -- I'll get to that during my next tweaking session.
Image after recentering and adjusting focal reducer spacing
Image after recentering and adjusting focal reducer spacing
image514.gif (1.36 MiB) Viewed 592 times
Mark M, AJ1B
Portsmouth, RI
Celestron 14" and Meade 10" SCTs
QHY268M + SBIG ST-9
GM2000 (10Micron)
American Association of Variable Star Observers (AAVSO) observer code: MMU
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Pete
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Re: Astrophotography: Optical Alignment

Unread post by Pete »

Lookin' good.
Pete P.
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