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ASI1600MM pro halos with bright stars

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#1 gorang

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Posted 19 January 2019 - 06:31 AM

I have an ASI1600MM pro camera with a filter wheel for 36mm unmounted filters and a Newton TS ONTC808 8" telescope with a 3" Wynne Newtonian Coma Corrector.
When I photograph a strong star as the attached picture of IC63 shows, it becomes several very large halos throughout the picture
I use Astrodon Ha 5nm filter.
Can these halos depend on the camera or are reflections in the telescope or filter or ....

What to do?

/Goran

Attached Thumbnails

  • Master_IC63x33_ABE_web.jpg


#2 bigjy989

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Posted 19 January 2019 - 06:53 AM

The microlenses on the ASI1600 do a small checkerboard around the brightest stars.   You can reduce the amount if you focus a little to the inside (vs. outside) and be aware of temperature changes.  Also it tends to happen more with the O3 filter than others so you could use shorter exposures with that channel or be aware during stretching.   It is possible to remove the stars during processing and re-add them with less stretch later.   Startools ('heal', 'layer')

 

Now the large halo that is off center indicates a colimation/centering problem.   The star should be centered on the shadow and the halo should be balanced in width top/bottom and side/side.   If collimation was good initially then tube flexure during the exposure is causing issues.

 

The half moon halos are likely due to a reflection somewhere in the optical train.   Check for any unpainted surface, like edge of secondary holder, tube side walls, focuser, spacers in the imaging train etc.   They seem to have the same size as the central halo and in between the vanes so they are like due to reflective cylinder.

 

Hope this is helpful.


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#3 Der_Pit

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Posted 19 January 2019 - 10:10 AM

As for the huge halo, my suspicion there would be a reflection between filter and the coma corrector (curved surface), in which case the displacement would vary with the position of the star in the FOV.



#4 gorang

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Posted 19 January 2019 - 02:18 PM

An interesting thing is that I did a test picture with Baader luminance filter of 30sec and there you see no microlense effects.

/Goran



#5 Der_Pit

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Posted 19 January 2019 - 04:16 PM

It's a diffraction effect(*), that is wavelength dependent.  With an L broadband filter the pattern would be smeared a lot.  You should also see it clearly using a blue filter from an RGB set.

 

(*)only talking about the squarish inner one, not the big halos.



#6 Stelios

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Posted 19 January 2019 - 06:35 PM

I've seen the huge halo on all bright stars.

 

In Pixinsight, the Star Halo Reducer script can take care of it. See this thread



#7 AdamJ

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Posted 19 January 2019 - 07:48 PM

What is the f-ratio and exposure length please?

#8 gorang

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Posted 20 January 2019 - 07:45 AM

The f-ratio is 4 and exposure time is 300 sec.



#9 ChrisWhite

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Posted 20 January 2019 - 09:43 AM

Gamma Cas is probably one of the worst offenders for Halos and Microlens Diffraction issues. 



#10 starhopper-de

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Posted 15 February 2019 - 11:14 AM

Hello Goran,

the small pattern comes from the microlenses, as discribed.

 

The great halos results from the ASI1600 uncoated chip cover glass. This is not repairable

and all Cameras with this Chip are affected (Atik Horizon also).

 

The light is reflected partly from the cover glass of the cmos chip back in your scope an than it comes

back reflected by filters and lenses.

 

Here is a link with other examples.

 

https://bbs.astronom...php?f=21&t=7488

 

I was very disappointed because besides this the camera is great. Now I got a ASI183 -> no halos

 

Thomas




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