Cant get a decent flat image from my lightbox

[blinky]

My setup is a ZWO ASI1600 mono with the ZWO filters and a ZS73 scope. I made a light box with some CCFL tunes in a box with translucent plastic to diffuse the light. To my eye it all looks nice and even, I also put a couple of layers of white pillowcase over the scope as well to further diffuse it. The problem I have is shown below in one of the sample flat images, I always seem to get a donut in the centre of the screen, no matter what filter I use. Any ideas, workarounds?

[blinky]

Just set the filter to clear and still have the same issue - I don't think its the camera as I don't see this problem in subs just flats.  Its also not related to short exposures as even with the Ha filter and 3 min subs to get a decent signal I still see it.

[blinky]

This was taken without the flat box at all, just the pillowcase and aiming it upwards st the illuminated roof

[Skipper Billy]

Ice on objective or sensor?

Variable thickness of the cloth?

I don't know - just chucking these ideas up in the air.

I hope you find the answer - that would drive me nuts!

 

[blinky]

Doubt it was ice, never know of course but I don't think it was, never had it before when imaging so cant see how I would have ice when taking flats, tried without the cloth and its the same.

[michael8554]

Has the focuser slipped, or the imaging train slipped in the focuser ?

Michael

[blinky]

Nah, its all on an electronic focuser so not moved at all

[spillage]

what if you put a half a dozen sheets of plain paper on the end of the scope and point it skywards in daylight.

[blinky]

Not tried, working 9-5 and its dark when I get home but from memory that works fine

[Starflyer]

Does the FITs header tell you the gain and offset Craig? If so compare these with your lights.

I know you were using SGPs flats wizard, have you tried shooting a flat as a light? Just thinking the flats wizard might be not shooting at the same gain and offset, or somehow seeing something else up.

[blinky]

That thought occurred to me - I think the wizard is using the default I set to use for focus which is 300 gain and I wonder if reducing it will resolve the issue.  Will try that

[Jokehoba]

I have a similar setup at the moment - ASI1600 + filter wheel + ZS71. I imaged on Sunday night and took flats on Monday morning around 10am (still about freezing). My flats looked just like yours with a big dark circle in the middle. However my issue was condensation - either on the ASI1600 window or the rear of the Ha filter. I got a hairdryer to warm up the filter wheel/top of the camera, set the camera to expose for 1s continuously and watched the dark area disappear! Got perfect flats after I let it all stabilise for a few minutes.

What was the temperature when you were taking your flats?

John

[blinky]

-20 - the camera was up and running for around 20mins, I did think that but could not see anything and why would I not see that when imaging?  Its a possibility though, especially as you had the same issue.