Is this what dithering solves?

[furrysocks2]

Got a new camera tonight (ASI120MM clone) and fired up SharpCap, learning as I go.

Took some 30 second exposures of the carpet, dark subtracted. Looking at a live-stack of 4, I see a prickle of dark spots:

I'm assuming this is because the hot pixels captured in the dark are so loud that there's little signal left after dark subtraction? Is this what "dithering" is for, so that when a new frame is aligned to the stack, these hot pixels (or "cold" I guess, after dark subtraction) move about and fade into the average, or is there more to it than that?

 

Earlier, I took a 10s max gain dark - here's a snippet, would you say this is pretty bad, or typical? Is max necessary exposure, min gain typically best? (Unless you're live stacking and can tolerate some noise for the sake of time...?) 

 

[AKB]

1 hour ago, furrysocks2 said:

Is this what "dithering" is for

That's one thing that it should help solve, especially when used with sigma stacking.

1 hour ago, furrysocks2 said:

would you say this is pretty bad, or typical?

Perfectly normal, I would say.    Personally, I don't use darks directly, simply converting a master dark into a bad pixel map and using that along with bias (and flats) to do a calibration.  That way, you don't need a different set of darks for different exposure lengths (or temperatures.)

[wimvb]

Cmos cameras need an offset to match the gain. Generally, the higher the gain, the higher the offset. Your imaging software should allow this to be changed.

The offset is needed to avoid black clipped pixels. If you do a statistical analysis on an image, you should ideally not have any pixels that have zero value.

For gain = 0, try offset = 10 - 15. For highest gain, try offset = 30, or maybe even higher. This assumes you're imaging in 12 bit mode.

Dithering can get rid of the black pixels, but it's best to avoid them all together.

[vlaiv]

Yes indeed, dithering solves this, and you are right, for long exposures some pixels in darks will be saturated, and when calibrating out darks from lights - at any pixel if you subtract maximum value - it will go to 0 (or negative) - hence black pixel.

Sigma clip stacking + dithering solves this.

Other way to solve it is to identify hot pixels in both dark and light frames and substitute them with some sort of low pass filter value - best to use median value (3x3 or 5x5 median of frame) - this is known as cosmetic correction in DSS.

[furrysocks2]

Thanks to all, great stuff! Will need to explore the likes of DSS.

[Gerry Casa Christiana]

8 hours ago, AKB said:

That's one thing that it should help solve, especially when used with sigma stacking.

Perfectly normal, I would say.    Personally, I don't use darks directly, simply converting a master dark into a bad pixel map and using that along with bias (and flats) to do a calibration.  That way, you don't need a different set of darks for different exposure lengths (or temperatures.)

Hi there 

Can I just ask a question how you convert a master dark into a bad pixel map. I'm interested to try it especially if it saves time. 

Thanks!

Gerry

[AKB]

18 minutes ago, Gerry Casa Christiana said:

Can I just ask a question how you convert a master dark into a bad pixel map.

I think a lot of software has this functionality. Personally, I use Nebulosity, but any Astro-stacking tool worth its salt should do this.