Ghosting reflection

[Nadnoraa]

Hello,

I've had this ghosting problem for a while and though it may be some internal refection caused by a unpainted nut or something else shinning in the ota but I've eliminated all that by flocking over everything that could possibly reflect any light.  Now since this refection is blue and it's shape is rectangular it makes me think that it   it could be the sensor of my canon 1200D reflecting back onto the corrector plate of my schmitt newtonian.  I've gone through lots of pains collimating this scope so I don't think that is the problem but now I am out of ideas.  Has anyone ever seen anything like this?  What is the solution(other than getting another scope)?

Dan

 

[Clarkey]

Not something I have seen before, or at least, not to this extent. Have you tried rotating the camera? Might at least give you a clue whether it is an internal (or external) reflection. If it changes orientation, it is probably image train related - if not a reflection. I assume this is just near bright stars? The rectangular shape does suggest a sensor related issue though. Is there any bright metal areas around the sensor?

[Nadnoraa]

There is no metal around the sensor but there is a small border on the face of the sensor that could be the culprit, but I'm guessing nothing can be done about that.  I did the full spectrum mod on this myself and  roughed up the sensor bezel getting all the adhesive that was holding the ir cut filters in place so if anything that should be more light absorbent.  I've attached more photos showing this ghosting  in relation to the position of the star and was able to get the reflection to superimpose on the star itself in the last photo.  I'm not sure what to do next other than photoshop them out (not sure how to do that yet).  I'm wondering if I were to misalign the primary mirror if it would go away.... more experimenting tonight....

Edited February 18 by Nadnoraa

[Clarkey]

 

I think if you try rotating the camera, it will give you a better idea.

You say you have done a full spectrum mod, so I assume you are using some sort of filter for you images? For most imaging you will still want a UV / IR cut filter.

[DougM43]

have you covered the viewfinder? my camera has a little rubber cover that fits over it to prevent any stray light getting through.

Just a thought. 

[Clarkey]

2 hours ago, DougM43 said:

have you covered the viewfinder? my camera has a little rubber cover that fits over it to prevent any stray light getting through.

Just a thought. 

Good shout. I forgot about this. It's been a few years since I imaged with a DSLR.

[Nadnoraa]

Ahh ....I will try this tonight thanks for the great advice.  I was just sitting here brainstorming what to do next and I thought about taking the camera apart (again) and flocking the edges of the sensor or using a 1.25" prime focus adapter instead of the 2" one I'm using now to maybe reduce the diameter of the light cone, at the price of more vignetting, but the covering of the viewfinder is a no brainer.  I will be back with the results....  Here is what I got last night and my feeble attempt to remove these reflections in PS before and after....this is about 750 x 20 sec exposures with a optolong cls-ccd filter and the moon not far from M45 with San Antonio's bortel scale at class 8.

[Nadnoraa]

Well here is my update.  Covering the viewfinder did not help.  Using a smaller prime focus adapter did not help.  Switching cameras did not help but gave insight to the problem.  I switched from my full spectrum modded canon T5 to a Meade LPI-G camera that I use for autoguiding and the refection was still there however it's geometry changed.  The sensor in the LPI is more square and the one in the canon is more rectangular and the reflections show that.  I'm kinda surprised that no one else ,with the sn-8, has had this problem and posted it.  So I guess there is nothing that can be done to defeat this nussance.  I also collected about 2.6 more hours on M45 and stacked it with the previous nights data for a total of 5.3 hours of exposure....but I am still learning to process all this with Siril and PS.  So thanks for the advice guys and let me know if you have any other insight based on these pictures.

 

Dan   

[Clarkey]

Is there anything else in the imaging train? Filters or something?

[Clarkey]

Found this that references the problem as a known issue:

Schmidt Newtonian for astrophotography? - IceInSpace

Maybe a non-reflective filter would help prevent the ghosting?

Edited February 20 by Clarkey

[Nadnoraa]

can you suggest one?  The only thing in the imaging train is a optolong cls-ccd filter but I get the same reflection without it. 

[Clarkey]

I was just wondering whether a good quality uv/ir cut filter with an anti reflection coating might help. Something like an Astonomik L1 or similar. Bit of a long shot but might help.

Another option maybe to introduce a small amount of tilt into the scope. Just enough to stop a direct reflection back to the corrector plate. Not ideal, but slightly imperfect stars maybe better than the reflection?

[Nadnoraa]

I found this on the iceinspace.com and cloudy nights message boards so there may be hope for me yet....

the Achilles heal of any Schmidt is internal reflections. Basically the light that bounces off the CCD (or at least a portion of it) will be collimated by the primary, returning up the tube assembly until it hits the Schmidt plate.
A portion of that light will be reflected by the front and back surfaces of the corrector back down to the primary which refocuses this light to form ghost images on the CCD.

and this on cloudy nights

Hello. About a year ago, I posted a question regarding weird blue internal reflections (Schmidt ghost)in images including bright stars, taken by my Schmidt Newt (Meade SN-10) and SCT (Celestron C8). At the time, the only solutions suggested were (1)image processing, (2)*very* careful collimation, and (3) just live with it.

Recently, I purchased one of the Baader's fringe-killer filters. This filter seems to greatly reduce the schmidt ghost with a minimal loss of blue light in the image. I have done some comparison tests and would like to report the results here.

The white balance frames used for the shots had the exactly same exposure time. I also tried Veil nebula and Altanik & Horsehead and the results were the same (no blue, "UFO-shaped" ghost).

Because the fringe-killer filter is inexpensive (less than $90 for a 2" filter), this seems to be a must have item for SN and SCT owners who want to image moon/planets/DSOs incl bright stars. 

 

[Clarkey]

The first quote was in the link I sent above.

With regards to the fringe killer, it will help as it takes out the blue wavelengths - normally to reduce chromatic aberration. However, this is really an observing filter so will mess up the white balance. Also, you need to be sure it is only reflecting in the blue channel. If it in the other channels too, then the fringe killer may not work. If you take one of the images above and separate the RGB channels using and standard editing software, you will be able to see if the blue removal works. It is worth noting that if it is only on one channel, it is possible to remove the reflection by editing one channel only by copying a section of one of the other channels over the reflection. When you recombine it should give a reflection free image.

Edit: I have just looked at the first image and the reflection seems to be in R,G and B so the fringe killer probably will not work.

Edited February 21 by Clarkey

[Nadnoraa]

One thing going for me is that I'm not autoguiding so PE dithers the images and the ghost moves with repect to the stars and gets reduced by averaging, basically treated like noise.  The ghost in this stack of 950 x 20 sec is in the lower right corner.  It is pretty amazing that this level of detail can be pulled from the sky using a cheap scope in a class 8 bortle zone.