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Help! Arcing artifact?

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#1 Funkonaut

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Posted 18 April 2021 - 12:23 PM

Well here's a new one.  I have a QHY183C and I've been using Gain 1, 180s subs.  After going down the Gain and Offset rabbit hole I decided to make some changes.  Last night I set Gain to the unity gain, 11, Offset to 30, and 90s subs.  I'm in a Bortle 8 zone.

 

So imagine my disappointment after 2 hours of imaging when I stack M101 and see two weird arcing artifacts on the left side.  Hoping it wasn't in all of the images, I manually stretched the individual subs and found that the artifacts are present in every image:

 

sadness.jpg

 

What could that be??  Thanks in advance for your help and advice.  This community is incredible!


Edited by Funkonaut, 18 April 2021 - 12:28 PM.


#2 rj144

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Posted 18 April 2021 - 12:30 PM

It kind looks like part of two lens flares.



#3 Funkonaut

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Posted 18 April 2021 - 12:47 PM

It kind looks like part of two lens flares.

Agreed.  Would that mean there was some local light source hitting the front lenses (I have a triplet APO)?  I image from my backyard and haven't had this happen before.  I did move from my usual location last night so I could clear a tree to get a view of M101.  Just checked for what light sources could have hit the lens and the only thing that's close is a downcast street light level with where I had the scope but mostly blocked by two large bushes.  Scope was pointing NE and the moon was NW the whole night.

 

And two separate lens flares?  Would that be from two light sources?

 

Anything else that could've caused this in your experience?



#4 Tapio

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Posted 18 April 2021 - 01:11 PM

Could be some shiny surface in optical train.

You could try taking camera off the imaging train, shine a bright light down the tube and spot visually if you can see it.

Reducer/flattener/filters are usual suspects.


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#5 Funkonaut

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Posted 18 April 2021 - 02:45 PM

Could be some shiny surface in optical train.

You could try taking camera off the imaging train, shine a bright light down the tube and spot visually if you can see it.

Reducer/flattener/filters are usual suspects.

Hmm, I'm not seeing anything.  No streaks.  Saw just a few tiny specs of dust that I could remove with my hand-powered blower.

  1. Could an indirect light source be the issue?
  2. What about a dew heater cable outside the image sensor?  The power cable from the heater strip I use for the flattener going down the outside back focus extension tubes and then curving by the image sensor... (doubt that was it)

A Google search for astrophotography lens flare isn't turning up much.


Edited by Funkonaut, 18 April 2021 - 02:45 PM.


#6 Tapio

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Posted 18 April 2021 - 03:05 PM

Something similar to this :
https://www.cloudyni...act-light-leak/
Do you have flattener/reducer in your imaging train?
Try without it.

#7 Funkonaut

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Posted 18 April 2021 - 03:17 PM

Something similar to this :
https://www.cloudyni...act-light-leak/
Do you have flattener/reducer in your imaging train?
Try without it.

The subs I took of the Sombrero Galaxy last week didn't have this issue, and I used the exact same setup (FF, extension tubes to get 55mm back focus).  Here's a super stretched view of one of those subs (there is a slight arc in the top-left corner but nowhere near as pronounced as in last night's subs):

 

last-week.jpg

 

Would slight bending/angling of the imaging train from the focuser's compression ring cause the issue?



#8 sharkmelley

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Posted 18 April 2021 - 04:14 PM

The arcs are caused by individual bright stars outside the field of view reflecting off internal curved surfaces in the optical train.  It may take a lot of detective work to determine exactly what component in the optical train is doing this.  If you can place a torch at 100 metres from the scope you should be able to reproduce the arcs and then the detective work can begin.

 

Here's an example of doing that:

 

Astronomik_L2_arcs.jpg

 

In my case this was caused by my Astronomik L2 filter, which had a reflective edge.  I replaced it with a Baader IR/UV filter that didn't have a reflective edge and the problem disappeared.

 

Mark


Edited by sharkmelley, 18 April 2021 - 05:09 PM.

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#9 Funkonaut

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Posted 18 April 2021 - 06:10 PM

The arcs are caused by individual bright stars outside the field of view reflecting off internal curved surfaces in the optical train.  It may take a lot of detective work to determine exactly what component in the optical train is doing this.  If you can place a torch at 100 metres from the scope you should be able to reproduce the arcs and then the detective work can begin.

Ugh. That sounds like a nightmare to troubleshoot. I will take some test shots before I execute my light plan to see if I can frame the scene better and report back here with an update. 



#10 Funkonaut

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Posted 19 April 2021 - 01:04 AM

The arcs are caused by individual bright stars outside the field of view reflecting off internal curved surfaces in the optical train.  It may take a lot of detective work to determine exactly what component in the optical train is doing this.  If you can place a torch at 100 metres from the scope you should be able to reproduce the arcs and then the detective work can begin.

 

Here's an example of doing that:

 

attachicon.gifAstronomik_L2_arcs.jpg

 

In my case this was caused by my Astronomik L2 filter, which had a reflective edge.  I replaced it with a Baader IR/UV filter that didn't have a reflective edge and the problem disappeared.

 

Mark

That was exactly it!  Tonight I moved the FOV up and to the left, enough to bring in the additional stars (from the big dipper) into the frame, took a 90s sub, and lo and behold the arcs are gone!  Now I have to figure out which component is causing this.  Thanks so much for the diagnosis, Mark!!


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