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strange star deformation

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#1 CvBadengoth

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Posted 21 March 2019 - 04:20 AM

Hi folks,

 

I observe a strange star deformation I have no explanation for. The star deformation I know already is coma, star deformation caused by differential flexure or star deformation by field curvature.

 

Correct me if I am wrong but coma or star deformation due to field curvature seems to be circular centric around the middle of the frame, so you see the star deformation only in the corners.

Differential flexure affects all stars in the frame.

 

So now here comes my problem: I only see star deformation along the diagonal, so from upper left corner over the middle to the lower right corner. Upper right and lower left is okay.

 

Examples:

 

Upper left, lower right with deformation:

Regim-2019-03-21-10-07-09.png
Regim-2019-03-21-10-11-37.png

 

Center with deformation:

Regim-2019-03-21-10-13-06.png

 

upper right and lower left WITHOUT deformation:

Regim-2019-03-21-10-14-39.png
Regim-2019-03-21-10-15-01.png

 

This pattern appeared after changing from Ha to OIII Filter in my filter wheel. I could see it in the last frames of Ha maybe also a little, but it came up much more present in the OIII images.

The first images I took in Ha were absolutely flawless, circular sharp stars over the whole frame.

So the whole problem has a time aspect and also the shift to OIII made it much more present...

 

I used a ESPRIT100, 0.79 flatner (TSRED379), ASI1600MM, imaging train is screwed together, filterwheel is a starlightXpress 5x2"Filterwheel, Filters used: Baader Planetarium 7nmHa, 8.5nm OIII. Mount: EQ6R, Guidescope: 60/240 with ASI290MINI

 

Havent seen anything like that, do you experienced folks know what might cause that?

 

Any advice is highly appreciated.

 

Clear Skies

 

Chris....


Edited by CvBadengoth, 21 March 2019 - 04:38 AM.


#2 einarin

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Posted 21 March 2019 - 04:35 AM

That is most likely a tilt somewhere in the optical path.

You could try removing your gear piece by piece to see if it's one of them (flattener, fw).


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#3 CvBadengoth

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Posted 21 March 2019 - 04:39 AM

That is most likely a tilt somewhere in the optical path.

You could try removing your gear piece by piece to see if it's one of them (flattener, fw).

....thank you for your reply. Imaging train is screwed together, is tilting possible then? Also why were the images okay at the start of the session? Strange....


Edited by CvBadengoth, 21 March 2019 - 04:40 AM.


#4 einarin

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Posted 21 March 2019 - 04:55 AM

Then it could be focuser sagging.


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#5 CvBadengoth

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Posted 21 March 2019 - 06:01 AM

Then it could be focuser sagging.

....the 3" Focuser is built like tank, and its only 2.23cm moved out, the focuser brake was also activated....

 

Lightroom3330-5.jpg
Lightroom3330-12.jpg



#6 jd200

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Posted 21 March 2019 - 07:42 AM

I see a similar behavior - I have a William Optics GTF81 (integrated field flattener), Moonlite 2" focuser, Orion TOAG, ZWO EFW, and ASI1600MM-cool. There are a couple of extension tubes ahead of the OAG.  

 

The problem is only strongly evident in the smaller stars, so I'm getting by with it, because my clear sky time is so precious right now.  

 

I'm very interested in hearing what you discover about your rig and the source of the problem.  Thanks for posting!



#7 CvBadengoth

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Posted 23 March 2019 - 04:34 PM

.....found the problem: its the OIII filter, seems to be tilted, or it otherwise affects the star shape in the edges.

 

Made the following run: one Ha frame, switch to OIII and takeing one image, switching back to Ha and taking an image.

 

The effect is that the Ha frames look okayish, the OIII not

 

CARCOSA-2019-03-23-22-30-15.png
CARCOSA-2019-03-23-22-20-17.png
CARCOSA-2019-03-23-22-26-36.png




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