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Help! Elongated Stars

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#1 matt_astro_tx

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Posted 18 June 2021 - 11:27 AM

I spent some time shooting M31 the other morning.  I got 16 good frames before my mount turned off (another story, since solved).  I stacked those in DSS and here is a link to the uncropped result.  It's not much, but it's a start.  One thing I noticed right away is the stars on the right side of the image are elongated whereas on the left they are pretty much spot on.

 

My PA was dead accurate.

 

I am using the AT60ED with a 0.8x flattener/reducer and the ASI183MC Pro.  The mount is a star adventurer 2i.

 

Exposure info: 16 x 30", gain 111 (unity), temp -10c

 

So I'm convinced its not a PA error, because star trailing would be present everywhere if it were.  I'm confused as to why I'm seeing elongation at all because I've been told with the small sensor size on the 183MC I don't need the flattener at all.

 

So is this a tilt issue?  If so, how do I fix that?  I don't have a tilt adjustment on the 183.

 

gallery_357716_16874_15961405.png



#2 bazookaman

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Posted 18 June 2021 - 11:48 AM

I'm about to post similar situation so I'll be watching your thread as well.



#3 06AwzIyI

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Posted 18 June 2021 - 12:14 PM

Assuming this is your 8" newtonian, I suspect tilt. I had issues with mine and I narrowed it down to the focuser, possibly the compression ring introducing a very slight tilt. I was able to work around it by using the flange of my paracorr as a flat, perpendicular surface to seat the coma corrector/imaging train. What focuser are you using? Have you tried rotating the camera and verifying that the elongation occurs on the same side of the image?

 

I suspect tilt. Have you tried rotating the camera and verifying that the elongation occurs on the same side of the image?

 

Edit: nevermind about the newtonian stuff, I see this is with your AT60ED


Edited by 06AwzIyI, 18 June 2021 - 12:18 PM.


#4 matt_astro_tx

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Posted 18 June 2021 - 12:40 PM

Assuming this is your 8" newtonian, I suspect tilt. I had issues with mine and I narrowed it down to the focuser, possibly the compression ring introducing a very slight tilt. I was able to work around it by using the flange of my paracorr as a flat, perpendicular surface to seat the coma corrector/imaging train. What focuser are you using? Have you tried rotating the camera and verifying that the elongation occurs on the same side of the image?

 

I suspect tilt. Have you tried rotating the camera and verifying that the elongation occurs on the same side of the image?

 

Edit: nevermind about the newtonian stuff, I see this is with your AT60ED

Haha.  Too many times I've written a perfect response, only to reread the OP and realize I was completely wrong!

 

I will give that a try tonight.  I haven't done that.  Make sense it would diagnose it.

 

What's maddening is that I'm totally "threaded in."  I'm not using any compression rings; everything is threaded together.  It goes:

 

AT60ED > Focuser > R/F > Spacers > Camera

 

I do have a filter threaded within the camera aperture, but it doesn't interfere or interface with the extension ring that's threaded into the camera.

 

So let's say it is tilt.  What then?  That would mean that the sensor itself is likely tilted within the housing?  And I assume my only recourse is to buy a tilter for it.


Edited by matt_astro_tx, 18 June 2021 - 12:44 PM.

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#5 james7ca

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Posted 18 June 2021 - 12:49 PM

Could be collimation (in the scope or reducer) or tilt in the focus plane. Also, you'll definitely need a flattener or reducer/corrector with the AT60ED and the ASI183MC. Field curvature goes by the focal length and the AT60ED has a short focal length (meaning more field curvature).


Edited by james7ca, 18 June 2021 - 02:04 PM.


#6 Tapio

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Posted 18 June 2021 - 12:55 PM

Could be tilt even if everything is threaded - focuser sagging, bad threading, lens or camera sensor.

There are tilt adapters available if it's tilt.

Try imaging in different parts of sky (altitude) if it varies.

And FF is not a bad idea.



#7 matt_astro_tx

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Posted 18 June 2021 - 01:19 PM

Ok, thanks everybody.  I'll look at it this weekend.  Clear skies as far out as the weatherman can see!  (After two months of clouds)



#8 06AwzIyI

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Posted 19 June 2021 - 09:36 AM

So let's say it is tilt.  What then?


If so, you'll likely need a tilt adjuster like this one:
https://astronomy-im...t/new-t2-tilter

#9 matt_astro_tx

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Posted 19 June 2021 - 03:14 PM

Yeah I was looking at that. It happened again last night. I didn’t rotate because I just wanted to get down to imaging. But I’ll make it a point to try tonight.


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