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Big Star Issue. Anyone knows whats the cause?

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#1 McErono

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Posted 18 December 2018 - 05:38 AM

Hi guys

 

It's a ZWO Asi 294mc pro with an Explore Scientific Comet Hunter. I am getting these strange big Star artifacts no matter what I try (tried gain 0-390).

My guess is, it is an optical issue but what do I know after 3 months in Astrophotography. I hope someone can point me in the right direction. Its not from processing, its in the raw files already.

 

https://imgur.com/a/1wcrvHC


Edited by McErono, 18 December 2018 - 05:39 AM.


#2 happylimpet

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Posted 18 December 2018 - 08:46 AM

These are really bight stars, so no telescope will focus 100% of the light into a tiny spot. In  fact those are good images with quite tight stars!

 

Nothing strange about it - thats what bright stars look like.



#3 McErono

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Posted 18 December 2018 - 11:24 AM

thank you happylimpet

 

I am just not used to these strange beams that are going outwards. My C8 does not have them and I can't remember seeing them in other pictures.

I collimated the heck out of my comet hunter but it is a strange thing, no matter what, the donut is offset on a star test - even if I try to collimate on a star after collimating with cheshire sight tube and laser, its impossible to bring the donut into the center with the comet hunter.

 

Or it could be bad seeing (sadly I have lots of it here) or...

 

VaophXa.jpg

 

edit, the more I look at pictures of the flaming nebula of other members, I see that alnitak has quite often the same beams ;-)


Edited by McErono, 18 December 2018 - 11:36 AM.


#4 Eddgie

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Posted 18 December 2018 - 12:19 PM

Spikes are almost always diffraction artifacts.  I don't know where they are coming from, but it is typically something in the light path between the focuser and the focal plane.  

 

Sometimes, foil spacers between the lenses will cause this kind of spike but usually not so many. 

 

Being a reflector, you might also look at the mirror clips as a cause, and look at the bevel on the edge of the mirror.  If it is not well cut, this could maybe cause issues.

 

To diagnose mirror clip, cut a sub aperture hole and place it over the corrector. Just make it maybe 10mm smaller than the mirror diameter.


Edited by Eddgie, 18 December 2018 - 12:23 PM.


#5 McErono

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Posted 18 December 2018 - 03:52 PM

Thanks Eddgie, will try that!

 

I searched for the ES Comet Hunter on Astrobin and similar/same deep sky objects. I found lots of pictures, some of them suffer even more than mine from these spikes/beams. Looks like there is nothing I can do about it with the Comet Hunter.

 

https://www.astrobin.../full/376287/0/

https://www.astrobin.com/full/25527/0/

https://www.astrobin.../full/369935/0/

https://www.astrobin.../full/220868/0/

https://www.astrobin.../full/281897/C/


Edited by McErono, 18 December 2018 - 03:53 PM.


#6 OleCuss

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Posted 19 December 2018 - 10:31 AM

I'm not sure this is specifically a Comet Hunter issue.  On bright objects getting a starburst is not at all rare in terrestrial photography and I've seen it in astrophotography as well and it's not limited to the Comet Hunter.

 

More common, of course, if you are using a camera with a camera with an iris without rounded blades and at higher focal ratios so you have a smaller aperture.

 

But I've not figured out why this is happening with the Comet Hunter/IMX294 combination.

 

Yes, they are bright stars and reducing the length of your subs might take care of some of the starburst effect.

 

But the aperture is quite generous so that should not be why you get the starburst.  No iris so that shouldn't be an issue.  No spider vanes and at least in my example of the OTA I don't have any other obstruction I can identify which should be causing that kind of diffraction.

 

Yes, there are the mirror clips but the starburst isn't following a pattern I'd expect from mirror clips.  There would be three spots of decreased light-gathering and diffraction spiking from three items I'd expect to give either a pattern of 3 or 6 spikes and the images show a lot more than 6 spikes.

 

We can get diffraction from the sensor itself.  The MN34230 sensor is rather famous for it because it has a plate over it which has no anti-reflective coating.  But that MN34230 sensor diffraction pattern is different from what is happening here.

 

I'm wondering if this is an optical/lens phenomenon but I am certainly not going to claim that I know for sure that it is.  I'm wondering if somehow the interaction of the wavelength and the refraction through the meniscus/corrector for the Comet Hunter gives this kind of pattern.  But again, I do not know enough to be at all confident that this is the case.

 

I'm hoping someone will give us the definitive answer as to why this occurs - and maybe tell us how to reduce or eliminate it.



#7 sharkmelley

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Posted 19 December 2018 - 11:34 AM

In general for a Newtonian, the multitude of diffraction spikes is caused by a rough edge on the primary mirror.  The "gaps" where there are fewer spikes are caused by the mirror clips.  Some people create a circular mask with a very sharp clean edge is mask off the mirror's rough edge.

 

Maybe it's the same for the Explore Scientific Comet Hunter.

 

Mark


Edited by sharkmelley, 19 December 2018 - 11:34 AM.


#8 PhilHoyle

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Posted 19 December 2018 - 12:50 PM

As far as collimation goes, use the laser or Cheshire to get it close, then use the donuts in the camera to finish it up.

 

Phil



#9 McErono

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Posted 19 December 2018 - 03:40 PM

thats the strange thing about the comet hunter. Its not possible to bring the donut into the center on a star test with visible poisson spot. I tried extremes. the donut is not getting there and I am not the only comet hunter owner with this issue (comet hunter thread somewhere). best bet is to use the cheshire tube and finish up with the laser. but collimation looks ok, pictures are quite sharp.

 

I think it must be the mirror edge causing the spikes. I see 6 spikes in the middle example above and I was playing with collimation between the three shots. Do I have to cover the mirror edge itself or is it enough for testing to put a cut out circle slightly bigger than the aperture on top of the corrector plate as suggested above? nevermind, I will try anyway ;-)

 

I will also get rid of the ZWO IR cut filter to exclude any issue with it.

 

And big thank you for all your inputs, I really appreciate it.


Edited by McErono, 19 December 2018 - 03:46 PM.


#10 AhBok

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Posted 19 December 2018 - 10:34 PM

I had a similar, or worse, issue with my Explore Scientific N208 newt. My mirror clip shadows were much worse. I masked off the outer 5mm of the mirror and problem solved.

#11 McErono

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Posted 20 December 2018 - 05:39 AM

just looked at the mirror. its enclosed/under a plastic circle that contains the mirror clips, its one piece. so the actual mirror edge is not visible. can it be that the plastic circle edge is not smooth enough?


Edited by McErono, 20 December 2018 - 05:40 AM.


#12 AhBok

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Posted 20 December 2018 - 05:09 PM

Probably not. How does the edge of the secondary look. If it’s beveled, is the bevel rough or nicely blackened?


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