Whirlpool attempt

[Marky1973]

Hello again. Had a shot at the Whirlpool last night. Again not an easy target and the sky was not properly dark but I was pretty happy with the result. Stacked about 34 of the original 40 90s ISO 1600 shots. Took out a few with sattelites and trailing and then DSS kicked the others. Once more, as my kit is not the best, the result is not mind-blowing but I was happy with what I got - and this image is pretty heavily cropped from the original where the galaxy was a tiny spot in the middle. Used the rectangle selection/stacking in DSS to just stack that area of the image and got the following:

What might be apparent in that image is what I am finding comes out with a number of my shots, a kind of brush effect across the image. I've attached an over-processed image below so you can see what I mean.

I have been playing with the settings in DSS to try and get rid of them and have had varying success, but manage to minimise them to a great extent, so the final image is bearable. My understanding is that they are largely unavoidable as they are typical of the "noisy" behaviour of a DSLR and can be exacerbated by LIGHTS and DARKS being taken at differing temperatures - which is largely unavoidable with long runs on a DSLR unless you want to be shooting LIGHT/DARK/LIGHT/DARK/LIGHT/DARK - which would be a bit of a faff.

So, while I can process this out to an extent, I presume I am right in thinking that this is something I am going to have to putting up with until I move onto a cooled CCD?

Does anyone else see these patterns coming up in their images? Any tips for minimising their impact during stacking or in processing?

[Davey-T]

You could try a trial version of Russel Cronins gradient exterminator. Think you may need to have Photoshop as well.

Dave

[Kropster]

I get those brush marks too from time to time.

Not sure why, but maybe something to do with the DSLR as you say.

Perhaps shooting at a lower ISO might help?... you still get all the photons, but less amplification of the signal, which may be exacerbating the problem.

Just a stab in the dark, but maybe worth trying.

[sharkmelley]

As you rightly guessed, the brush effect is caused by the darks not matching the lights perfectly.  However, DSS can scale dark frames to match light frames when "Dark Optimisation" is switched on.  It calculates a multiplier (in the range 0 to 1) for the dark frame using "Entropy-based Dark Frame Subtraction". Further details and a link to a technical article are found here:

http://deepskystacker.free.fr/english/technical.htm

Since the darks are always scaled down, make sure they are taken at a slightly warmer temperature than the lights e.g. shoot the darks indoors.

Some folk suggest dithering.  This can certainly help but it doesn't always do a good job when the sky background levels change from sub to sub.

Mark

[Marky1973]

You could try a trial version of Russel Cronins gradient exterminator. Think you may need to have Photoshop as well.

Dave

I wondered about that Dave, but wasn't entirely sure whether it was gradient related and also don't have photoshop at the moment - too many other shiny things to buy! 

I get those brush marks too from time to time.

Not sure why, but maybe something to do with the DSLR as you say.

Perhaps shooting at a lower ISO might help?... you still get all the photons, but less amplification of the signal, which may be exacerbating the problem.

Just a stab in the dark, but maybe worth trying.

Thanks Kropster - yeah, I suspect 1600 was probably pushing it a bit, was just thinking it was a faint object so give it a bit more sensitivity! Will try something similar with ISO 800 next time I get the chance! It is crazy this hobby - drives me absolutely nuts sometimes, but keep coming back for more!

As you rightly guessed, the brush effect is caused by the darks not matching the lights perfectly.  However, DSS can scale dark frames to match light frames when "Dark Optimisation" is switched on.  It calculates a multiplier (in the range 0 to 1) for the dark frame using "Entropy-based Dark Frame Subtraction". Further details and a link to a technical article are found here:

http://deepskystacker.free.fr/english/technical.htm

Since the darks are always scaled down, make sure they are taken at a slightly warmer temperature than the lights e.g. shoot the darks indoors.

Some folk suggest dithering.  This can certainly help but it doesn't always do a good job when the sky background levels change from sub to sub.

Mark

Thanks for the link Mark - will double check "Dark Optimisation" as I think I may have turned it off at some point last night when I had stacked the images for about the 8th time, trying different combinations. I thought that darks needed to be taken at the same time/temp as the lights, but perhaps not - so much to learn!   Mind you , with so many components required for a single photo, it's a wonder I ever get anything! I think dithering is on in APT, but not sure whether it has any effect on my simple set-up?

Thanks chaps!

[Marky1973]

Well I had another session a couple of nights ago and pushed the exposures up to 120 seconds, but dropped the ISO to 800. The "brush" marks are still there but, with careful adjustment and processing, they are less of an issue. I also went back to my favoured settings in DSS and it seems to okay - I guess if it ain't broke....don't fix it.

[Kropster]

Looks better this time.

Nice shot.

[Marky1973]

Cheers!

[wxsatuser]

Well I had another session a couple of nights ago and pushed the exposures up to 120 seconds, but dropped the ISO to 800. The "brush" marks are still there but, with careful adjustment and processing, they are less of an issue. I also went back to my favoured settings in DSS and it seems to okay - I guess if it ain't broke....don't fix it.

I would be looking to up the exposure to at least 300secs and more if needed, plus get at least 2hours worth as a minimum.

Use your histogram to gauge your exposures, get the data out of the read noise but don't over expose.

Surprising what more quality signal will do for images.

[Marky1973]

Hi Mike, yeah, I want to get higher with the exposure length, but I was struggling at 120 seconds on the EQ3-2. Took 40 subs and lost 11 of them mainly through trailing (presumably periodic error) so 3 minutes might be a big ask - hence the reason I was planning to upgrade the mount and get into guiding. Might get some clear skies tomorrow (fingers crossed) so, if I get the chance, I might have a go at longer than 120s. Might need to check my polar scope alignment too.

In terms of the histogram - changing the ISO doesn't seem to have changed the histogram much when using APT. Is there likely to be a difference between APT and the Camera, or am I just being daft? Presumably the main impact on the histogram will be the ISO setting? Sorry, I need to look into this a bit more, but time is so short at the moment, I'm just lucky to get the chance to take some subs!

Cheers

Mark