M109 shot - why the gradient/ noisy sky?

[Atacamallama]

Hi

I'm wondering why I seem to have such a gradient/noisy back ground in this picture. It has been stacked and given minimal PS treatment (no curve stretch) for illustration purposes.

I have a remote set up in Nerpio, using a 102 mm f7 triplet apo, Atik 314L colour CCD, paramount mount and decided it to leave it running on this target through the night.

I collected 36 ten minute frames and took 25 flats. I first stacked all of them, but then I took out about 12 frames which looked a bit lighter than the rest for this version (the first dozen, although the object was already very high to begin with, which is why I decided to go for it...).

Usually after about 4 hours I get a perfect inky black back ground, but not in this case. (see http://www.flickr.co.../in/photostream for example).

this is not a question about how to process this out, but about why it should have happened?

I am wondering if maybe in this case it is because it was nearly all one side of the meridian and so there is a wider range of sky darkness? In other words if I did three hours either side of transit (which is what I tend to do) this would not happen.

As far as I could tell last night was very clear (this was taken last night) so sky colour.

The only other thing I can think of is that I set the cooler to -15 instead of -20.

TIA for any input.

[themos]

Hello, I think you should try and get some numbers on the situation. Let DSS stack 10 10min frames from this night and another night. Then import the result to Iris or Maxim and select a small region of background and let it compute statistics. Do the same for a single sub. Keep the image linear as you do this, no stretching at all.

[Atacamallama]

thanks Themos, just to clarify these are all from one night, this is not the issue I mentioned to you about stacking frames from several nights...

[themos]

Ok, but you want to know what's different on this one night. It seems to me we need to compare the sky background statistics of this night with some other night that you have comparable data for.