With the exact same chroma set and QHY600M camera, my flats also look god awful. With my imaging set up it ends up being nearly a 75s flat for SII. I even flipped the filters despite there being an AR coating. it helped a tiny amount
It’s the same issue with the NII filter. I spoke to Optec (Alnitak flatpanel) and they said it’s the sensitivity of their filters to the wavelength of the FL panel. I’m tempted to drop my sub exposure for the Flats on those filters down to sub 20s sacrificing 1/3ADU to get something that doesnt cause more issues with my light files.
I’d love to get to the bottom of this issue as well…
The difference in lenghts for the flats are totally normal.
Could there be some kind of light coming in from somewhere else that affects those long flats that you can’t see the effect of in short flats?
I’m using a ASI6200 which has the same sensor and a Artesky flat panel which is much much brighter allowing sub 1s flats for LRGB, they look really great! This setup also use Chroma filters.
For L it would be possible with shorter than 0.1s, but SGP rounds up to 0.1s which is yet another small unfixed issue with SGP
One thing to note about the Chroma filters is that they are 50mm instead of 50.8mm, this causes a problem for some filter wheels so light can leak past the edges.
Atik’s filter wheel has this problem and the fix was to 3D print some masks that blocks the edges of the filters.
Flats without and with the mask, note the 0.3s exposures (F5 scope and ASI6200 at high gain)
There’s still some uneveness with the mask, but it’s also there in the lights so it calibrates out, with the setup at a remote site 3000km away i don’t see the point in paying lots of money in fixing an issue that doesn’t cause problems.
LRGB + OIII flats are perfect, uneveness is just for Ha 3nm + SII 3nm.
I have some masks in the post since before Xmas, should arrive in a few days although I’m sure that’s not the issue.
SGP shows 3s subs for OIII but when I looked at the actual flats they were 30s. Not sure what happened, possibly user error, I’m hoping to do some more testing tomorrow.
I don’t understand why the brightness of the light panel could be an issue. Dimmer panel = longer exp but should be the same result. Sounds like wavelength could be the issue. Are your Ha and SII 3nm flats ok?
More testing to do to be sure but I reversed orientation of the SII filter and the flats for this now look ok. Still need to check other filters when my masks arrive.
UPDATE I have now also reversed the Ha filter and flats now look normal. It doesn’t seem to matter which way around the OIII is for flats but I get reflections in one direction but not in the other.
The exposures are quite different to my last NB filters with Ha 32s, OIII, 3s and SII 60s. I haven’t tried using the flats yet so fingers crossed.
