What is this and any way to prevent it?

[StargeezerTim]

This is an image from last night. DSLR (100D) with a 12nm Ha filter. Star 71 scope. 9 x 15min subs, iso 1600, dithered. Seeing was pretty good till clouds came...

Stacked in DSS. This is sigma clipping stack, its about the best result. Average was much worse, median a bit better and this is marginally better than median. I have only converted the stack to 16 bit and used one strong curve and moved the black point to 25 from the start of the histo. 

I have done a fair few Ha stacks with similar integration time recently and most are noisy but manageable whereas this one seems full of this regular pattern noise.

Does anyone know what can cause this? or how to prevent/reduce it in future? The target was the Flying bat, my other targets have been much richer in Ha, this has a lot more 'space' in it.

 

[Anthonyexmouth]

off topic but whats your setup to get 15min subs with your heq5? just ordered one myself and looking to future purchases to guiding. 

[StargeezerTim]

9 minutes ago, Anthonyexmouth said:

off topic but whats your setup to get 15min subs with your heq5? just ordered one myself and looking to future purchases to guiding. 

I have an MGEN standalone autoguider with an orion 50mm guide scope. A computer free setup!

[Hallingskies]

1 hour ago, StargeezerTim said:

Does anyone know what can cause this? or how to prevent/reduce it in future? The target was the Flying bat, my other targets have been much richer in Ha, this has a lot more 'space' in it.

Not sure if this is related to your problem, but I’ve seen a similar noise effect when imaging with my ancient SXV-H9 in warm weather.  It has no cooling control and I put the noise down to images (and darks) being taken at a range of temperatures.

It’s much less apparent in cooler weather.

[Ibbo!]

Recently it has looked clear but I have had trouble with very thin cloud, especially noticed while attempting solar.

[wimvb]

Purely guessing: (Moiré) pattern from the stacking process. Since you're using an Ha filter on a dslr, only the red pixels get signal. Then when you stack, you put red pixels over green and blue pixels. Since you're averaging (sigma clipping still averages the used pixels, just not the clipped pixels), some will have more contribution from the green and blue (dark), and some will have more from the red (brighter). Depending on which, some pixels end up darker and some brighter.

Have you tried debayering in superpixel mode?

Btw, I assume the camera hasn't got the cfa filter removed, so it's still a colour camera and not a monochrome.