A couple months or so ago, I posted an image from a flat field Oiii sub and how I was getting a weird pattern. I was using a light panel that I made roughly 10 years ago. I had used a incandescent rope light that was controlled by a rotary dimmer switch. All my new flats looked great until I got to the Oiii filter. During the course of that thread, William B suggested making a Sky Flat. When I did that, I got a normal flat. So I then started out to find the source of the problem. I then shut off the light panel and used a clip on(incandescent) light shinning on the front screen. the results of that was that I still had the original weird pattern but only about 20% as apparent. I removed the clip on light & then used a LED light shinning on the screen and that got me a normal FF image. I then placed a single LED stick light behind the panel screen ( knowing that the screen was not going to be evenly illuminated) and got the normal FF image with out the distortion. In the end, I rebuilt my Light Panel using using LED lights and as of right now, everything appears to be good. Sample FF subs attached below.
The results are rather counterintuitive Carl, you would normally expect incandescent lamps to produce a continuous spectrum with no "tooth comb" gaps in the light output, the exception being some halogen lamps that are coated with strong UV block filters, or halogen lamps that are coating filtered to produce warm pinks or creams for indoor lighting. A simple tungsten filament lamp should be almost full spectrum. I wonder if your "Rope Lamp" is actually a LED rope, it is unusual to have tungsten filament lamps in a rope because they produce a lot of heat and will melt the rope if set on full brightness. Thinking about the diffuser for your flats panel that may also have a selective filtering effect depending whether it was made from a mixture of plastics or glass and opaque powder to create a diffused flat-white appearance or whether it was made as a clear plastic or glass sheet and then surface treated to produce the diffusing effect. The OIII flat produced with your new LED source looks just as you would expect, the OIII flat produced with the incandescent source is not at all normal but your sky flats were also ok, which makes me question again if that incandescent source really did contain filament lamps, or perhaps the material for the rope itself (PVC?) is filtering the light around the OIII region? There are some rope lamps you can buy now that contain electroluminescent strips and they do produce a very "toothy" output but I thought they were a relatively recent invention and not something available ten years ago when you made the first light box. If you ever do take the camera filters out for cleaning it would be really interesting to look at the sensor in a strong light with the shutter open to see if there are any water marks around the outside of the sensor that correspond with the outermost "circular" artefact. Good to hear back from you that the new LED source is working well for all your filters and one for Doug's "Museum of Mysteries", I'm sure he has one! William.
In front of the light source, I have two layers of fluorescent light (made from the drop ceiling) defusers with a front screen of opaque white. 1): That is a good point, I can't say for sure as it was a long time ago when I bought it. 2): I got that same pattern with a clip on light just not as intense. 3): That was my first thought, but I know nothing of the light spectrum. 4): I have done that a couple of times. I have rotated the filter roughly 120° two different times. Still had the same distorted image. Yes this was/ is a strange one for sure. Thanks for your insight William.
