Internal Reflection on Bright Stars?

[Gib007]

Please have a look at the following two images. The first is the Garnet Star in Luminance (1x1, 420s) and the second is the Garnet Star in Red (2x2, 315s). Imaging was done through my Skywatcher Explorer 150PDS Newtonian Reflector and ATIK 383L+ Monochrome CCD camera. The imaging train was the CCD camera, a couple of spacers, Skywatcher's 2" Coma Corrector, the Baader filter (Luminance and Red, respectively) and then the Hutech IDAS LPS 2" filter in front of it all.

Luminance (1x1, 420s):

Red (2x2, 315s):

What I am wondering about is what causes those reflective rings around bright stars? I have the same issue with the three Orion's Belt stars, for example. I've seen images where people don't have these and am wondering how that can be. At one point I even noticed the rings were literally a reflection of my telescope mirrors (with detail, including the three primary mirror clips!). Would flocking the inside essentially stop this happening? Thanks for your help, in advance!

[Jessun]

I'm guessing it is down to the Hutech - that's why mine is in a drawer these days, unused. Others here may think differently though.

/Jesper

[Gib007]

I'm guessing it is down to the Hutech - that's why mine is in a drawer these days, unused. Others here may think differently though.

/Jesper

Hmmm, when you mentioned this, I remembered several months back when I had read about this in several places so yeah, it does sound like it's internal reflections from the LPS filter. I've been reading that if the LPS filter doesn't do that, it's not doing its LPS job properly, so this is expected. Hutech even mention this on their webpage for the filter but state that it should simply be reflected back out of the telescope if the telescope mirrors are multi-coated. As far as I'm aware, Skywatcher Explorer telescopes are have fully multi-coated optics and since I place the LPS filter in front of everything in the optical train, I would expect this not to be an issue. Clearly it's a bit of an issue, though...

[FraserClarke]

They are caused by internal reflections off any (and all) optical components in the path. It won't just be the LPS. The very best anti-reflection coatings you get are still only 99% efficient (and that would be a very good coating!). That means at least 1% of the light is bounced back by each surface, and then 1% of that bounced forward by the surface before (if you see what I mean?). So even with only two very well coated surfaces in front of the CCD, you'll get 1% * 1% = 0.01% of the light reflected into ghost images. When you have such a bright star, even 0.01% of the light is noticable.

The worst offender is often the CCD itself. Those often have reflections of >10%, and you also have the window infront of the CCD. So as an absolute bare minimum, you have two surfaces (CCD+window) producing ghost reflections.

There is not a lot you can do. Minimise the number of surfaces. Make sure you have good coatings. Avoid stupidly bright stars

Flocking the tube will help a bit for bright stars outside the field of view.