Strange Gradient - Need Help Please!

Dave - if dew/frost are not the issue then it might be either vignetting or an internal reflection.

I really suspect vignetting. This looks exactly what I get with the Proline. I get it more so when there are clouds about It certainly isn't an internal reflection as far as I can tell. Though mine is slightly offset from center.

For now, flats do the job (if I can be bothered to use them).

HTH

Dave

Silly question but did you take flats DG?

What way do you store the camera, filter and field flattener? in the observatory or in the house?

Guys I'm pretty sure it aint vignetting. I used to get vignetting when I used 1.25" gear. Vignetting looks far more gradual that that. In any case with 2" fittings and only a 2/3inch diagonal CCD vignetting is not a problem. The strange thing is this just started last night and is evident on the ATIk 16HR and the ATIK16HRC

Deebee: This is just a single uncorrected frame.

Michael: The cameras are stored in the house. I know what your thinking, dew on the optical window. I do get this sometimes but I checked for that and in any case this normally looks like the inverse of what you see there, i.e. darker in the middle in an oval pattern. SO it aint that.

Anyway I did some tests this evening by taking some flats through the CLS, R, G, and B filters. The central brightening is only evident in the CLS and Blue channels.

Kieran: You might be onto something there alright, although no dew was evident on the outside, there might have been on the inside. However There was a kendrick tape active around the scope so this should have kept the whole scope clear.

I think Phil has it there; I need to strip out each of the variables to see which one it is.

More tests needed methinks.

Thansk for all the feedback.

Kieran: You might be onto something there alright, although no dew was evident on the outside, there might have been on the inside. However There was a kendrick tape active around the scope so this should have kept the whole scope clear.

I had the dew heater running aswell when it happened to me. I think the dew cleared on the outside of the lens quicker due to air movement and I then zapped it with a hair dryer.

Yeah Kieran, I suspect you are right. I had a look at a few more subs from a while back and there is no evidence of this. Also the dew could possibly have been inside the flattener.

I had a go at flat fielding these frames and the flats seemed to overcorrect the central brightening (introducing a central dimming.)However by adding 15,000 to the ADU of the flat fields it pretty much got rid of it. So at least I should be able to produce something from last nights set!