Am I right that this indicates I needed to take Darks?

[Gfamily]

Using a tracking alt az mount I took about 9 minutes of exposures (30s each) and ended up with these multiple tracks.

I assume these are hot pixels of some sort, and because the stacking matched on the stars they show as separate dots in the final stacked image. 

Would taking a 30s dark frame in the same session have helped?

[rotatux]

Hello... what is this showing exactly ?

If I assume this is the result of stacking, then these tracks may be from hot pixels moving with sub-frame registration. Then yes dark frames and/or bias should help, but not only...

I also suspect something else : as these hot pixels move with the result of registration, they should be eliminated by the stacking algorithm rather then leaving a trace. That is, if you used the correct stacking algorithm => You should activate and use some form of outlier rejection, which is often an option in stacking programs, they are called differently so I will just cite "sigma clipping" or "winsorized sigma clipping" but there are others. If you tell which program you use, maybe others using the same will be able to complement this answer.

[TiffsAndAstro]

I'm a noob, so please take what I say very lightly, at least until pros have replied, but, while darks should be taken at same temperature as lights, take 5 or 10 now and use them to restack your lights what harm can it do? I reckon hot pixels will be there whatever the temp  so recent darks might fix/reduce the problem.

Edited April 9 by TiffsAndAstro

[Gfamily]

1 hour ago, TiffsAndAstro said:

I'm a noob, so please take what I say very lightly, at least until pros have replied, but, while darks should be taken at same temperature as lights, take 5 or 10 now and use them to restack your lights what harm can it do? I reckon hot pixels will be there whatever the temp  so recent darks might fix/reduce the problem.

Thanks 

I can certainly give that a go. The main issue is that this was a set taken just after I'd spent about 25 minutes taking 15s exposures of another area of the sky, so the camera had been fairly busy.

[Gfamily]

1 hour ago, rotatux said:

Hello... what is this showing exactly ?

If I assume this is the result of stacking, then these tracks may be from hot pixels moving with sub-frame registration. Then yes dark frames and/or bias should help, but not only...

I also suspect something else : as these hot pixels move with the result of registration, they should be eliminated by the stacking algorithm rather then leaving a trace. That is, if you used the correct stacking algorithm => You should activate and use some form of outlier rejection, which is often an option in stacking programs, they are called differently so I will just cite "sigma clipping" or "winsorized sigma clipping" but there are others. If you tell which program you use, maybe others using the same will be able to complement this answer.

Thanks 

I'll have a go at trying some of the other options. 

I'm using DSS for stacking.

[michael8554]

You haven't said which camera ?

Dithering, Darks, and Flats.

Michael

[rotatux]

I'm not a user of DSS, but it definitely has outlier rejection available. From some screenshot I found it seems "Average" is the default, so as a start try one of the two options outlined below :

Edited April 10 by rotatux

[Gfamily]

On 09/04/2024 at 12:50, rotatux said:

Hello... what is this showing exactly ?

If I assume this is the result of stacking, then these tracks may be from hot pixels moving with sub-frame registration. Then yes dark frames and/or bias should help, but not only...

I also suspect something else : as these hot pixels move with the result of registration, they should be eliminated by the stacking algorithm rather then leaving a trace. That is, if you used the correct stacking algorithm => You should activate and use some form of outlier rejection, which is often an option in stacking programs, they are called differently so I will just cite "sigma clipping" or "winsorized sigma clipping" but there are others. If you tell which program you use, maybe others using the same will be able to complement this answer.

Thanks Fabien

I re-ran the stack using Sigma Clipping and it removed the effect.