Stray Light in my BK80ED Refractor

April 21^st,2024

I have found an issue with stray light getting into my astrophotos when using my BK80ED 80mm refractor.

Here is an example of the stray light. Those arcs of white light appeared immediately after I performed a Meridian flip. I tried everything to verify it was not terrestrial light and nothing changed this issue. When I finally moved the mount away from this target a little bit I noticed that these arcs of light moved as well. I concluded that the source of this stray light is starlight just outside the field of view that is bouncing off of something in the telescope.

I decided that this was caused by the retainer rings that hold the aperture lens. There are two rings…one on the front face and one on the back face.

This is a picture of the stray light when everything is removed from the focuser. I used my cellphone camera to take a picture, during the daytime, looking into the BK80ED from the focuser end. You can see stray light bouncing off the retainer rings. For this photo, I have already tried to fix the problem by painting the retainer rings with black paint. This helped a bit but what I didn’t realize is that flat black paint is highly reflective if you paint it onto a surface. The reflectivity is very low when looking at a flat black painted surface straight on. As you look at the painted surface at a very low angle the reflectivity is much higher. It was this photo that gave me the idea that a baffle would be a better solution. I thought about building a baffle. The more I thought about it…the more I didn’t want to do it…too fussy. The baffle needs to block just enough light to block these reflections so its alignment was critical.

After looking at this photo for a while I realized that I had blobbed black paint directly onto the lens at several location around the perimeter. Everywhere there was blobs of paint, the stray light was strongly attenuated. I decided that the best solution is to apply flat black paint directly to the visible circumference of the lens. I needed to apply enough paint so that it blocked the reflections from the front face retainer and the rear face retainer.

After a few touch-ups I ended up with this. I used a very thin artist’s brush. There are some blobs but I don’t want to try and remove these blobs. Too afraid of scratching the lens.

I knew that I would post about this issue so I took FLATs before and after I painted the lens. I knew I wanted to document if the FLATs were affected.

These graphs are what the red, green and blue scanlines looked like at the centre of the FLATs. The differences are very subtle.

I decided to see what the FLAT compensation will look like. The FLAT compensation is how much the pixel values will be boosted or reduced to cater for the vignetting of the optical train. To do this I used CaLIGHTs to create a calibrated FITS file where every value was the same. I set all the values to 10000. I then used this “all 10000.fts” file with a MasterFLAT created with the “before” FLATs. I repeated this using a MasterFLAT created with the “after” FLATs. The two FLAT compensated “all 10000.fts” files are display above. Values slightly below 10000 are black. Values higher than 10000 are progressively brighter. What you end up with is a picture which is brighter at the edges because of vignetting and darker where the light reaching the imaging chip is the strongest…sort of a negative image. Using CaLIGHTs I then gathered averaged values from each image at the four corners and at the centre of the images.

There isn’t a big difference between the before and after “all 10000.fts” files. These values can easily be converted into percent. A value of 11472 is equivalent to 114.72 percent. There is only 1 location where the FLAT compensation differed by more than 2 percent. This is at the lower right corner of the image. Only the red and green channels seem to be strongly affected.  Overall the FLAT compensation is less for the “After” image. I think this difference is due to the absence of reflections as a result of applying the paint. These reflections were essentially scattering light which may have caused the center of the FLATs to brighten more than at the edges. It’s worth noting that the FLAT compensation will be slightly less around the edges of the images I take from now on. Hopefully this will help because I have notices that the sky background using the BK80ED looks a little bright around the edges in some images…specifically the Pelican Nebula Top Left corner. I plan on repeating the same DSO imaging this year so that I can bring this post full circle.

Peter