Internal Reflection

[John_Gill]

Hi,

I reimaged the Horse and Flame nebula and the bright star Alnitak has caused a reflection on the image.  I have tried numerous ways to remove this artifact, but without success.  Please refer to the attached cropped image.  Any suggestions or solutions will be most welcome.

Look up without reflections 
John

[bulrichl]

Hi John,

I think it is nearly impossible to eliminate the reflection from this image by processing because the greenish blue reflection has a complex shape and is situated in the red emission nebula. So my suggestion is: try to avoid the reflection.

The reflection is not a halo around the bright star, and it is not round. So I guess it will not be caused by a filter between focuser and camera. Are you using a filter in front of the objective?

You showed only a very small crop of the image. It remains unclear how large the field of view is. Thus the question:
- Is Alnitak in the field of view?
If the answer is yes:
Presumably the reflection is centrosymmetrical (e. g. is the center of the image in the middle of Alnitak and the reflection). Then you can figure out where the reflection would be situated if you oriented the camera in a different way. Orient the camera so that the reflection is in a part of the image that is not so important.

If the answer is no:
In this case you have a good chance to avoid the reflection. Detach the camera (at daytime), shine with a flashlight into the objectiveslightly at a slant and carefully look from the other side into your scope. Are there any reflecting parts visible in the light path (extension rings, adapters)? These must be either exchanged or painted with anti-reflex paint or pasted with velour.

Sometimes it is difficult to determine the cause of a reflection in your imaging set-up, but if you achieve it and remove the cause, it will be worth it.

Bernd

[John_Gill]

Hey Bernd,

Thanks for your suggestions and help.  I will have to find the source of the problem.  I have attached a full image, I might just process it, flaws and all, and if someone asks, I will tell them it's a "Shapeless Nebula"

Look up
John

[pscammp]

I was getting nasty reflections like this from bright stars due to my IDAS P2 LP filter screwed on the front of my camera, same colour too.

Tried everything to get rid of it but to no avail 

With Juan and the team regularly working miracles in PI it would be so nice if they could come up with a tool to attack artefacts like this,
after all, the reflection in your image defiantly qualifies as a large scale structure of sorts.

I wonder if it would be possible to isolate the reflection in one of the layers produced by 'ExtractWaveletLayers' if you ran the script on split
colour channels   

My reflection only shows up in the green and blue channels, nothing in the red what so ever, can something be done in this situation maybe 

Only the big men on campus will know !

Paul

[pfile]

is this an SCT? the shape is reminiscent of a "schmidt ghost"

rob

[John_Gill]

Hi Rob,

Yup SCT.  A Celestron 8"EdgeHD on CGX mount and Canon600d

[bulrichl]

> I might just process it, flaws and all, and if someone asks, I will tell them it's a "Shapeless Nebula"

Yeah, :-)

No, really, I didn't mean to hold someone off to suggest specific image processing steps.

Yet I'm curious: what kind of scope are you using? Are there any filters, a reducer, flattener or whatever accessories in the light path?

The greenish blue color of the reflection suggests that it is produced by a coated lense surface, but I'm intrigued by its shape that strongly deviates from round or oval. In the meantime while I wrote this, Rob suggested this might be a "Schmidt ghost". I think he's right.

You can do an experiment in order to find out whether the reflection is centrosymmetric (in your image: to the middle of Alnitak and the reflection): take an image of the region (center exactly identical) with the camera rotated by about 120° clockwise. Then the reflection should be outside of the image or all at the edge.

Bernd

[John_Gill]

I forgot to mention that I was also using a 0.7 focal reducer (that had the mildew problem, mentioned in a previous post) but no filters.  I have not had this issue with other deep-sky images, but none had a great big bright star close by.  The camera was also rotated from my "norm" so that I could try and fit the Flame and Horsehead into the image.

mmmh, I think I will tell the wife that I now need a new scope .... and ccd camera!

Look up
John

[bulrichl]

Yes, I think the reflection stems from the corrector plate, and in this discussion there are some images that show how the reflection might move when the orientation of the scope is slightly changed:

https://www.cloudynights.com/topic/133271-angel-in-m45/#entry1758759

Maybe that helps to plan an orientation of scope and camera so that the desired object is in the field of view and the reflection isn't.

Regarding the purchase of a new scope: different types of scopes will have different issues... Please be sure to tell me, if you found the perfect one!

Bernd

[Geoff]

Have you tried using RangeSelection to make a mask of the reflection? You could the apply Curves to tone it down.

[RickS]

Have you tried using RangeSelection to make a mask of the reflection? You could the apply Curves to tone it down.

A cyan/blue ColorMask would be worth trying too.

[John_Gill]

Hi,

I tried a RangeSelection and the ColorMask and then Curves/Histogram/MMT which toned the color/brightness down a little ... but it still looks bad. 

I will just have to re-image on the next clear night ....

Thanks to all for their input.
John