Newbie question : is it worth to process ?


Feb 18, 2017
Lajoux (JU) - Switzerland

I'm quite new with PixInsight and know the basics.
I have imaged NGC7331 and I think it could be worth do develop my frames BUT there is a reflexion that comes from I don't really know where (dome aperture ?).

See one of the raw frames only streched to better show the problem :

I have tried to process my images the usual way and hope to get rid of this reflection at the end but the result is not the one I was hoping for.
My question: is it worth to fight with such frames or is it hopeless ?

Thank you for your ideas and answers.


Well-known member
The image looks very good, apart from the reflection. In all fairness, I think you should get this sorted out. Otherwise you will be battling this in every image. The image at hand may be improved in processing, but it will be difficult to remove the band completely.

Hope this helps,


Well-known member
Nov 26, 2013
England, UK
Reflections are a pain for sure, not being an expert here but it doesn't look to me like a reflection caused through your imaging train to the camera itself.

You don't say if your using a refractor or a reflector, that can make a difference as far as what is causing this problem.

I use a Newtonian and have problems where close lamp post light creeps in around my focuser where the draw tube goes into the body of the focuser.

Solution: I purchased some darkroom blackout material from a photographic shop (which you would use to line a dark room for processing images under red light), once my focuser is out at the focus position I wrap this material around the focusers tube which totally blocks out any light from entering this way.

If your using a Newtonian this could be not a reflection as such but more light leaking in at the mirror cell end (most cheaper Newtonians will have a fully open mirror cell where you can see the back of the mirror) - I had a huge problem with this and had to build a cover (which would still allow airflow for the cooling fans) to give me 100% light block at the bottom end.

My best advice to you is what I had to do:

In daylight, maybe in your living room, set up your scope, even on the floor, with the lens cap in place and the camera installed as though you were doing an imaging session. Take what would be the equivalent of a 3 min Dark. Look at the image when it comes and look for anything not perfectly dark (except for hot pixels) which might indicate light leaking in somewhere. If you see this anywhere on the image, get some kind of dark/black material and wrap it tight around your focuser tube for example, take the image again and see if the Dark is improved, do this all over your scope until you work out where light is leaking in. (Be careful with lens covers, sometimes they can allow 'some' light in through the plastic material itself, get a sheet of tin foil you would use for baking, wrap it over the end of your scope tube and secure with an elastic band or tape to ensure the lens cover is leaking no light whatsoever).

If the above trials prove that no light whatsoever is affecting a 3 min Dark frame then the reflection has to be external light in your environment, get your innards flocked, put up a screen if there is a light source close by, try the above tests while you scope is in it's imaging position outside.

Persevere and you will get there !

As to if it's possible to remove the problem in you image ?, the only thing that comes to mind is the new feature in 'Image Integration' called 'Larges Scale Pixel Rejection', whether it could take it out really is a question only Juan could answer but it's potentially possible based on what this new feature is aimed at, it's certainly a large scale artefact for sure.

Good Luck


Well-known member
May 17, 2012
Normaly you should remove this single sub. If it is a master and you intend to save it by all means, there is a way, with some losses of quality, but enough, to enjoy a pretty picture.

Make a clone with previews, which include the reflections, except the galaxy.
Execute MMT (MultiscaleMedianTransform), 8 Layers, 8,7 and perhaps 6 unchecked to the preview image. The galaxy will become rather weird.
Execute Script SubstituteWithPreview to the orignal image.
Correct the remaining borders with Clonestamp; Continue with DBE and the other tools

Perhaps some borders of the galaxy will become a little grubby, stars and galaxys within the reflection a little bit dimmer....
The method works better with artifacts that don?t touch the object.

Enjoy your NGC 7731

Regards  Gerhard



Feb 18, 2017
Lajoux (JU) - Switzerland
Thank you so much for your replies.

So it seems that I should not throw my frames away but give them a second chance.

As my goal was not to discuss the "why" these reflections, I did not give any information about my scope.
It is a 14" RC reflector. The weird thing is that I have usually no reflection at all. But sometimes, when my scope aims the zenith it can happen... That is why I suspect my dome aperture not being aligned but I am not sure !



Well-known member
Jul 12, 2015
Apologies for bumping an ancient thread but I have a similar issue three very faint arcs across the image that only show up in the red frames around SH2-82. I suspect this is from an out of field star but I'm not sure why they would only appear in red. Anyways this method is ingenious and works wonders.