How to eliminate streaks of light from star reflection

[bonz0]

Hello,

First post here.

I just got some data on M108 and the Owl Nebula recently and all of my subs after the pier flip look okay:

But all my subs before the pier flip have weird streaks of light on one side. Here's an example:

Most of the subs I have are from before the pier flip and because of this, when I integrate my subs, these streaks really stand out:

I talked to a few friends about this and we came to the conclusion that these were streaks of light were caused by some internal reflection from the light of Merak (just out of my FOV).

So my question is, other than DBE/ABE (already tried out), is there something I can do to get rid of/minimize the effect these streaks? I tried performing Local Normalization as part of my integration process, and that helped a little but, but not much.
Any ideas?

 

[pfile]

i think you are correct about the source of these artifacts. the only 'right' way to fix it is to change the pointing until the bright star causing it is outside the FOV of the telescope. but that might be at odds with the framing that you want.

when you did LN, what did you use as a reference?

DBE might be able to put a dent in this if you put a zillion samples over the rays, but it will be tedious.

rob

 

[ngc1535]

Since you have some frames that do not have the issue- you have opened the door to a number of possibilities. LN (with the average of the artifact-free frames as reference) might help some. In addition, you can selectively choose the area to reject wholesale for the images that have the artifact where small structures still show at image integration time. LN will not take care of this. Please see the synopsis of the technique I shared in a previous thread (https://pixinsight.com/forum/index.php?topic=11609.msg71757#msg71757) . This will mean you will have the S/N average equivalent of the number frames you choose to include in that area. But the justification is a good one! You really only have the sky (background) and bright stars. The S/N hit will not affect the stars in a visually meaningful way (they are high S/N) and the background "noise" discrepancy will likely be difficult to discern - especially after noise reduction.

I really dislike imaging with GEMs! But you have found one of the few benefits of a flip- the change in optical geometry can lead to a useful image difference.

So LN + Selective Rejection + DBE (to clean up the smooth large scale light variation) really should get you pretty close. The likelihood of success is strongly determined by the number of images you have have (especially on the "good" side).

-adam

 

[bulrichl]

Hi bonz0,

the only 'right' way to fix it is to change the pointing until the bright star causing it is outside the FOV of the telescope.

I disagree. I had similar streaks caused by a bright star outside the field of view (target: M78, stars: Alnitak (Zet Ori) and Alnilam (Eps Ori)) when I adapted my new camera to my refractor. The streaks were barely visible on the subframes, but after integration it was clear that the image was ruined. In my case I could not detect the cause of the diffraction artifacts when viewing (camera removed) through the light path and illuminating from the other side, it was really discouraging. Then I experimented with changing the distance of the IR/UV cut filter to the sensor and found out that the streaks vanished completely when this distance was decreased by some mm.

My conclusion is, that you will be able to modify the light path in a way that no reflections will occur. This will be some trial and error, but it's worthwhile. And this is the only 'right' way to fix it (my personal view).

Bernd

 

[John_Gill]

Hi Bulrichl

When I image close to a big bright star "Alnitak" or "Acrux" I get some weird reflection manifesting itself on the stacked light frames.  I am using a Celestron 8"EdgeHD with an unmodified DSLR camera.  This is reflection is a pain in the butt and I cannot hide it when processing the image.  Are you saying that I should adjust the distance between the camera and the scope?

Look up
John

 

[bulrichl]

Hi John,

When I image close to a big bright star "Alnitak" or "Acrux" I get some weird reflection manifesting itself on the stacked light frames.  I am using a Celestron 8"EdgeHD with an unmodified DSLR camera.  This is reflection is a pain in the butt and I cannot hide it when processing the image.  Are you saying that I should adjust the distance between the camera and the scope?

No, you shouldn't. In my case (Takahashi FSQ 106N without reducer) this is indeed an option because the corrector is situated in a fixed distance from the objective, but this is a rare case. In almost all cases the distance between camera and scope is critical and is determined by the flattener / reducer in order that no aberrations are produced in the image. You can change the distance between a filter and the camera, but at the same time you have to make sure that the distance between camera and scope remains constant.

However, there are different types of reflections. I am only referring to reflections caused by bright stars outside the field of view.

Arc-shaped reflection (see attached image; target: Rho Oph, Antares (Alp Sco) outside the field of view) generally indicate a large reflecting area. In this case a visual inspection as I described above will easily show the cause (here: a shining extension ring). If you detect reflectig areas in the light path you can either replace shining parts (e. g. extension rings), or use anti-reflex paint or self-adhesive velours.

The streaks that bonz0 (and I in an image of M78 https://www.dropbox.com/s/5ks4jrz90ggsqpj/M78_ASI.jpg?dl=0) observed, are diffraction artifacts. They must be caused by a reflection at a sharp edge. Apparently this type is not easily detected by a visual inspection of the light path. I wanted to change the distance between the filter and the camera for some other reason and by chance discovered this fine side effect. I have no patent medicine for this case. However, these artifacts are generated by reflections in the light path. I am sure that one can find out - maybe by trial and error - which measure is suitable to avoid such reflections.

Bernd

 

[rbotero]

I've had the same issue with a different set up.  See here: https://pixinsight.com/forum/index.php?topic=10645.msg66628#msg66628

These are reflections caused by Merak (Beta Ursa Majoris) which sits just outside the frame.  I only managed to solve this by recentring my shots and taking them again!  The final shot is here:  https://www.astrobin.com/281716/B/?image_list_page=2&nc=&nce=



Roberto