Strange noise pattern in Ha images (ASI 1600)

[billyharris72]

Hi all:

After my last post (I'm suffering a weird brightening around the edge of field that is not amp glow, is unaffected by flats etc) I cleaned everythig, turned the filter around and tried again - same result. A bit annoying but some post processing gets rid of the worst of it and I reckon a new filter will sort things out.

However, on last night's run I noticed something else that's been bugging me for a while. My Ha images are typically quite noisy (hardly a unique problem) but also, the noise doesn't really look like ... well, noise.

You can see what I mean in the shot below. The sky noise to me has a definite pattern to it, like a series of striations running top left to bottom right. I know it's not hot pixels that are drifting - my setup does not have that much drift and I'm using sigma clipped stacking, which (I imagine) should eliminate that anyway.

As before, it's a ZWO ASI 1600MM.

I don't even know if this is unusual to be honest, just something I noticed and thought looked odd.

Any ideas?

Billy

 

[tooth_dr]

This looks like the noise I get with my DSLR if I don’t dither between subs. Are you dithering?

[RolandKol]

@billyharris72

Is this a stack or a single frame?
P.S. I will check mine this evening, - interesting

[souls33k3r]

That's what we call "walking noise". What software are you using to stack the image? In PI "Sigma Kappa" will eliminate that or what i used is "Average" or "SD Mask" in MaximDL. Hope this helps.

[toxic]

you will get rid of that noise if you stack using median method rather that sigma plus if your using darks i think you may need more as that can cause this hope this helps

[vlaiv]

What settings did you use for capture (sub length, gain, offset, etc ...)?

Did you do proper calibration (matching darks, flats, flat darks, ...)?

 

[billyharris72]

1 hour ago, tooth_dr said:

Are you dithering?

I've only started guiding quite recently and have not yet used dithering. I guess now is as good a time as any to start.

54 minutes ago, RolandKol said:

Is this a stack or a single frame?

25 x 300s, stacked in AstroArt using sigma clipped mean. Be interested to hear how you get on.

30 minutes ago, souls33k3r said:

That's what we call "walking noise". 

Interesting- I've noticed it before ( I seem to get it some times, not others). Any idea what causes it?

Billy

[billyharris72]

30 minutes ago, toxic said:

if your using darks i think you may need more

That's a thought. I used 15 darks but more would not hurt. I also think longer subs might help, as these took a lot of stretching (mean pixel value in the calibrated image is lower than in the dark frame).

[billyharris72]

8 minutes ago, vlaiv said:

What settings did you use for capture (sub length, gain, offset, etc ...)?

Did you do proper calibration (matching darks, flats, flat darks, ...)?

This was 25 x 300s subs, with 15 darks (matched settings, shot about a week previously). Both shot at 0C. 

I did shoot flats (and they look fine) corrected with flat darks, but these were not applied to the image - I have an issue with field brightening near the edges, probably due to reflection, and the flats seem to make this worse (I think they are correcting for vignetting that is less than the brightening).

I'll get back to you on settings - it's the ZWO software default "zero gain" setting, but I don't know the actual values off hand.

Thanks,

Billy.

[vlaiv]

1 minute ago, billyharris72 said:

This was 25 x 300s subs, with 15 darks (matched settings, shot about a week previously). Both shot at 0C. 

I did shoot flats (and they look fine) corrected with flat darks, but these were not applied to the image - I have an issue with field brightening near the edges, probably due to reflection, and the flats seem to make this worse (I think they are correcting for vignetting that is less than the brightening).

I'll get back to you on settings - it's the ZWO software default "zero gain" setting, but I don't know the actual values off hand.

Thanks,

Billy.

Ah, ok.

There has been discussion on this type of noise several times, and I don't think there is definitive consensus on what is causing it (I might be wrong here), but I know for a fact it is related to read noise.

With above settings and the way you are calibrating, read noise can be an issue, and hence artifact related to it can appear. Let me explain. Ha images need long exposure because dominant noise term is read noise. With using gain 0 you are using setting that has the most read noise associated with it. By using only 15 darks, and since you are not dithering, and probably working on small resolution (short FL) coupled with good guiding, creates additional read noise - when you calibrate your images with darks, you are effectively "injecting" some more read noise coming from darks, and since count of darks is too low this additional noise is not reduced (it is roughly square root of number of darks less than single sub). Also by not dithering, you are hitting same pixels with same noise value (making it non random).

I would suggest following to deal with this:

1. Use unity gain

2. Check offset value and whether your subs clip to the left - just take single dark sub and measure it - no pixel should have value of <=16 ADU - just look at stats for sub and look at minimum pixel value, it should be higher than 16.

3. Shoot much more darks - you can do it when it's cloudy, on several nights/days, just build up your dark lib. How much? I believe more than 60 to be adequate, but would personally go for even greater number - like couple of hundred - it simply cannot hurt to have as much as possible.

4. Look into dithering - with narrow band and length of sub it is worth dithering after every sub, it helps with sigma reject techniques, but it helps even if you are not using them (spreads injected noise from master flat and master dark and averages it further).

[dph1nm]

Looks more like fringing to me.

NIgelM