Anyone who engages in the hobby of astrophotography is familiar with the need to create “flat frames.” These are frames shot of a uniformly lit light source and are used to remote vignetting and mitigate the effects of dust on camera cover glass and filters. However, it can be challenging to take good quality flat frames. It’s the “flat” part that is challenging. Ideally the light source needs to be uniform to within 1% to produce good flats. In practice that can be challenging to achieve.
Many people have had success with LED tracing panels. They are inexpensive and can be operated from any USB source but my own experience has been less than ideal. The first panel I bought was visibly non-uniform and the second, while not obvious, suffered from uniformity issues. The second one could be mitigated by rotating the panel every few frames but that quickly becomes tedious and error prone.
I have seen posts around the net by others who have had success with electroluminescent panels. These are thin films of material that light up with a very uniform light when current is applied. They are also relatively inexpensive, at least at smaller sizes. For my 80mm refractor a five inch panel would be large enough to have some leeway in positioning the panel on the telescope and still avoid getting too close to the edge of the panel. I don’t know whether the panel is less uniform toward the edge but it seemed like having a reasonable overshoot on each edge was a way to ensure that super precise positioning wouldn’t be necessary.
There was one commercial electroluminescent panel on the market but it was quite expensive and had the reputation of being fragile. I didn’t want something I was going to have to handle with extreme care. Instead I opted the for the 5 inch version listed here. It was priced at a point where I felt I could take a chance. It came with a battery operated power supply that would let me move it around easily.
I knew I wanted to put it in some sort of frame. The panel itself is not particularly stiff. Keeping it flat on top of the telescope would require something to keep it flat. I opted to pick up an 8×10″ photo frame at a local store. I also wanted something to diffuse the panel. This would help cut down on the light and even out any non-uniformity. I decided to order a piece of acrylic in Sign White 9% from Tap Plastics. I ordered a ¼” sheet but it proved too thick for my frame requiring me to use binder clips to hold it together. Eventually, I’m going to need a thicker frame. If you are ordering something for a new project then ⅛” thick is probably fine. I ended up using binder clips to to hold everything together giving this a definite home brew appearance.
If you find that the panel is too bright for you to get the exposure length you want, you can add neutral density filters inside the frame between the EL panel and the acrylic diffuser sheet. One or more sheets of neutral density filter (available at any camera shop. I bought a sheet of neutral density film from B&H Photo to use if needed. In my case I was fine but those with especially fast optics might find it useful.
I did a measurement by taking 50 flat frames with the panel in one orientation on top of the telescope and then 50 more after rotating the frame 90 degrees. I built master flats from each set in PixInsight and then used Pixel Math with the expression h / v / 2 (h was one flat and v was the other). I then examined the diagonal from one corner to the other and found no differences larger than 1%. It looked like I had a winner!
The only downside is the panel is visibly blue and really lacks in red. The exposure lengths from the L, R, B and G frames ranged from 0.12s to 1s. If you want to match the lengths of your flats it ought to be possible to use color filter sheets to balance the output but I haven’t figured a way to calculate that and didn’t want to go through a trial and error process. For mono cameras with filters that’s not really an issue but for one shot colors it would help. However, the same issue occurs with LED tracing panels as they are quite blue also.
I have no relationship with the vendors mentioned in this article other than as a customer. I bought the products and didn’t receive any compensation for mentioning them here. I had no reason to use their tech support so I can’t speak to how good or bad it is. I suspect, but don’t know for sure that all these EL panels are essentially the same but, again, I’m no expert on them.
My total cost for components was about $60.
Here are some photos of the components and the final result. I hope this helps in your quest for flat frames!