A mystery gradient

[jezhughes]

Hello all

I'm new to CCD imaging so please excuse if this is an obvious question. I noticed after a session at the weekend that a couple of my images let out a gradient going to the top left corner. At first i thought it was the moonshine until i realised it was pretty much the same area on both images despite them being in almost opposite locations.

Can anyone explain what it might be? I've overexposed these a bit to reveal it.

[VigdisVZ]

How do the dark and bias frames look?

[jezhughes]

I haven't checked that yet - i was hoping to stay away from needing darks. A lot of people don't see the need when you're running a cam at -10.

[Ags]

I would guess that there is something not quite right with your flats.

[jezhughes]

There's no flats in this - this is just 1 x 5min light. Sorry, i should have mentioned that.

[Ags]

Ok, how about vignetting, but your CCD is a couple of mm off center?

[rfdesigner]

yup this looks like an unflattened image, and your newt looks like it needs collimating.

both simple to sort out, if you've removed the camera from your scope toen you won't get a flat for it now (you'll never get it back in exactly the right place), but you can use one of the many post-processing gradient removal tools.

never the less you've made a good start

Derek

[ollypenrice]

These are without flats?

It looks like somewhat assymetrical vignetting to me. You will certainly need flats and therefore bias with which to calibrate them. (Treat a master bias as a dark for all flats.)

Maybe you have an issue with the alignment of your focuser. I don't image with Newts so can't be much help on this.

Olly