What could be causing this banding?

[BrendanC]

I'm acquiring images with a modded Canon EOS1000D, with Skywatcher coma corrector, via a Skywatcher 130PDS on an NEQ6 mount, no guiding. The other night I captured part of the North American Nebula and noticed some curious streaking in the image, but didn't know for sure whether it should be there. See below - it's a fairly ok image but you can see persistent streaks going up from bottom right to top left, almost as if it were a pencil shading (this was taken portrait).

 

Last night I captured around 250 30-second subs of M13. As with the above image, I stacked in DSS and applied darks, dark flats, flats and bias frames and noticed the same streaking. Given that this and the previous shot are the first I've taken with my new calibration files library, I wondered what would happen if I removed all the calibration files to see if it was the subs themselves. And it was, see below - stretched image to show the streaks, although now they're going from bottom to top because this was taken landscape.

One thing I'm extremely aware of is that this is not the best time to take photos in the UK because we don't reach astronomical darkness until the end of July.

So, could this just be an effect of taking shots when it's not really dark enough?

Or could there be something wrong with the coma corrector (ie check there isn't any smearing), or the sensor? I doubt it's the mirror.

Or is it something to do with Deep Sky Stacker? I've been through all the options and set them as per recommendations for StarTools (which I use for post-processing, and which I used for the nebula shot).

These were all 30 second subs taken at ISO800. I've never seen it before.

Any ideas?

Edited June 23, 2020 by BrendanC

[dobblob]

The first image is a classic example of "walking noise" caused by the sensor repeatedly recording the image at the same position.

I experienced this often with my D 600.

The usual answer for this is "dithering" whereby the image on the sensor is shifted by a few pixels each frame.

This is usually accomplished by guiding software but it looks like you have tracking only.

If the scope can be offset manually between frames by a very small amount in RA and DEC then this can help. I suppose it might be possible to VERY briefly stop tracking for a few seconds but offsetting DEC will depend on whether you have DEC fine slewing. (I don't know the NEQ 6 mount).

I attempt this myself but not every frame. That would be tedious and time consuming.

The second image shows what appears to be banding from the camera . I am not sure here. It could be some interaction because of the cal frame removals but it seems an unlikely coincidence that the bands are so vertically aligned to the sensor. StarTools has a banding reduction module which seems to work well.

Short nights, no dithering and higher camera temperatures never help and considering that I have to say the image ain't bad at all! (green is a little bilious though...)

[BrendanC]

Thanks for this, plenty of food for thought. So you think they are separate issues?

I'm using APT for the acquisition, I should have mentioned, and I think it has dithering features which I'll look into. I'm still getting acquainted with the NEQ6 mount too, so lots of new stuff going on (I had an AZ Goto mount before - so inaccurate that it kind of had its own dithering built in!). New mount, new calibration library, and this is the first time I've attempted photography during summer (started last Feb).

So, dithering might fix the walking noise. I'll see what I can do about the banding.

Thanks again.

[BrendanC]

Hmmm, looks like APT does in fact dither non-guided shoots automatically. So, this won't cure it.

One thing I used to do with the AZ mount was to shoot in 'blocks' ie use platesolving to get the target in the centre, shoot say 100 subs, then repeat so that the target was back in the centre. It was the only way to cope with the mount's inaccuracy. Now, I totally get that I need to improve my polar alignment, but as an interim fix (ie so that I can make the most of tonight and tomorrow, which are rare good nights in the UK!) would this help?

[BrendanC]

Update: APT doesn't dither un-guided shoots automatically, so I've switched that on. Fingers crossed.

I'm still going to do the 'blocks' of shots anyway, the idea being that I maximise the chances of success and eliminate stuff until it stops working, rather than keep failing and adding things until it works.