Newbie question/problem

[Sorarse]

I know I am trying to run before I can walk, having only taken up astronomy in the last couple of weeks, but I thought I would try stacking a series of images.

I used the standard lens on my 40D and took 60x4 second exposures of Orion which I subsequently stacked using the freely downloadable Deepsky Stacker.

Now I know that I should have used longer exposures if I wanted to enhance the colours to be found in Orion, which is all part of the learning process, but what I can't fathom is what seems to be some sort of 'streaking' horizontally across the entire image. As far as I can tell, the streaking doesn't appear in the individual subs, so I can only assume that it has come about as a result of the stacking process.

Without knowing what it is or where it has come from, I am at a loss as to think how I might avoid it in the future. Does anyone have any ideas or suggestions?

http://www.88qv.com/net/Orion.jpg

[Fidelio]

I don't know much about astro, but could it be banding from using too high an ISO? The highest I'll use on night-sky stuff is 200, and even then I get really ugly noise from my XTi. Just a thought.

[R Hardman]

It is the noise and hot pixles that was removed during the stacking process. The sky moves in relation to the cameras ccd but the noise will always be in the same spot in each frame. When stacking, your moving the individual frames to keep the stars aligned for each frame. So, the noise appears to move in each frame in realtion to the stars. Since a stack is an overlay process, the noise for each frame shows through the next frame thus making it look like a streak. Adding more exposures will reduce the streaking since your expanding the signal to noise ratio.

[Nighthound]

To best remedy this you need to dither your exposures.

The repetitive noise pattern can be greatly reduced or eliminated by simply moving the telescope with the mount hand controller just a bit about every 5 exposures or so. You don't need to move it much only a few pixels in any direction. Set the mount slew speed to the slowest setting and you'll just need to nudge it. The slight shift will be enough to stagger the noise pattern so that this effect won't be produced in your stacked image.

[Sorarse]

Thanks for the replies guys, and your suggestion for eliminating the problem Steve. This set were actually taken on a normal photography tripod, but I will definitely give your suggestion a go with the camera mounted on the scope mount.