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Author Topic: Elongated stars at the bottom left of image  (Read 1023 times)

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Offline konstantinos

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Elongated stars at the bottom left of image
« on: 09:01:50, 25 April, 2016 »
Hi

I send you this 5 minutes image of Castor I have captured with my setup.

https://www.dropbox.com/s/ytxhkswloufx3rq/castor-5min.jpg?dl=0

As you can see at the bottom left the stars are not round, but in the other parts of the image the stars are round.

My thought is that this is not a collimation issue because if that was the case I would expect elongated stars all over the image.

Any ideas what makes the elongated stars at the bottom left will be appreciated.

My setup:

Vixen VC200L f/6.4, sbig st2000xm CFW9

Imaging and guiding with sbig st2000xm

Thank you

Konstantinos

Offline Einari

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Re: Elongated stars at the bottom left of image
« Reply #1 on: 09:17:27, 25 April, 2016 »
Most likely tilt somewhere: mirror, focuser, ccd.
Don't know if you can tilt your focuser or ccd but there are tilting adapters available.
Tapio

Celestron C8, C9.25" & Hyperstar, iOptron CEM60-EC, TS 80 triplet,  Coronado PST, SXV-694,  ASI120MM/ASI174MC Cool

Online chris.bailey

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Re: Elongated stars at the bottom left of image
« Reply #2 on: 09:39:19, 25 April, 2016 »
Hi Konstantinos

The eye is not a particularly good judge of critical roundness as we perceive things as round even though they are not. Your stars are slightly off all over the image but only in the bottom corner are they off enough to be perceived as  really out of true.

Not ideal with a single .jpg image but CCD Inspector suggests quite a tilt from top right to bottom left with the bottom left being worst affected. Tracking it down is not going to be easy but check focuser droop and that all elements in the imaging train are tightened fully. The st2000xm and filter wheel is quite a weight to be hanging off the back. One way to check is to run a few images on a star field up at zenith. If the stars are round then it tends to confirm there is a weight related droop. If they are still elongated then the tilt is locked in somewhere, maybe collimation.

Chris
LX200|ZS70|FSQ85|FLT110|Altair DF250RC|EQ6 Pro(Rowan Belt Mod)|ParamountMX
ATIK383L+/EFW2/OAG|Lodestar|Baader 36mm LRGBHaSIIOIII
Starlight Express SXVRH16/ONAG/FW|Lodestar X2|Baader 2" Filters
Starlight Express SXVRH814/ONAG/FW|LodestarX2|Baader 2" Filters
Lunt LS60PTBF1200|DMK41|Quark Chromo
Samyang 135mm f1.8

Offline konstantinos

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Re: Elongated stars at the bottom left of image
« Reply #3 on: 07:22:46, 26 April, 2016 »
Hello
 
Thank you all for your reply to my issue. I have loaded the 5 minutes FIT image of Castor to the CCDInspector.
The result shows that my field, when the CCD camera is attached to the focal reducer, is not flat and has a 18.7% curvature.
 
Also the CCD camera image plane is not square to the optical plane and there’s a 14% total tilt at the bottom left direction.
 
The CCD camera with the filter wheel are attached to the focal reducer using this Baader adapter.
Baader Adapter from M68 and M60 to 2" -  BA2458196
But Vixen suggests the following adapter
 
Use the optional Vixen Direct Wide Photo Adapter 60 and the proper T-Ring or the Vixen Camera Adapter 43 and proper T-Ring if you'd like to mount your camera to the Focal Reducer.
 
But I don't know if the Vixen Direct Wide Photo Adapter 60 is suitable to connect the SBIG ST2000XM to the Focal Reducer.
 
So it looks it’s the spacing distance between the focal reducer and the CCD camera sensor which produces the field curvature in my case. And this together with the little tilt I have produces elongated stars.
Konstantinos

Online chris.bailey

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Re: Elongated stars at the bottom left of image
« Reply #4 on: 07:48:05, 26 April, 2016 »
I would not judge field curvature on a single frame. Load a few into CCDI and measure them all. CCDI is quite sensitive so best with an average.

Your field is pretty flat so if you can get rid of the tilt, I think you would be happy.

Chris

LX200|ZS70|FSQ85|FLT110|Altair DF250RC|EQ6 Pro(Rowan Belt Mod)|ParamountMX
ATIK383L+/EFW2/OAG|Lodestar|Baader 36mm LRGBHaSIIOIII
Starlight Express SXVRH16/ONAG/FW|Lodestar X2|Baader 2" Filters
Starlight Express SXVRH814/ONAG/FW|LodestarX2|Baader 2" Filters
Lunt LS60PTBF1200|DMK41|Quark Chromo
Samyang 135mm f1.8

 

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