Loss of definition with DSS vs Nebulosity

[allan gould]

Ive been coming across a repeatable phenomenon while processing some of my images.
Last night was a good night for imaging except for the presence of the moon but never the less I was imaging M83 again with a 10"scope at 2500mm for some definition. Thinking that I would better manage the pixel size of my QSI583 camera to that of the scope I was imaging at 2x2 to see what would eventuate.
After gettig 35x5 min Lum subs at 2x2 it came to stacking them.
Initially I used Deep Sky Stacker to process but the stars appeared to be blocky or square and so I reprocessed them in Nebulosity which appeared to give a better result.
So after preprocessing all frames in nebulosity with flats, darks and biases I then took the same frames and stacked them in either Nebulosity or DSS.
The results are clearly seen in the image attached.
Nebulosity on the left and from DSS on the right. To be honest I was amazed at how much detail I had lost with DSS. The stacked images were quickly processed in PS3 with just levels and curves.
Hope this is of interest to others.
Allan

[RickS]

That's a huge difference, Allan. I would have expected DSS to do better. I guess I'll stick with PI :)

[allan gould]

Im not sure but it only appears to happen when I shoot at 2x2. I will have to see if there is such a profound difference when I process 1x1 images.
I MUST get my head around PI one of these days as Im sure it offers many advantages.
Allan

[lazjen]

I have been wondering about DSS myself recently. I'd like to use PI, but the learning curve/brain power required - urgh! :P I think I'll eventually succumb though.

Would you mind sharing what DSS settings you used? Did you do any hot pixel removal? Drizzle settings? etc?

[allan gould]

I did hot and cold pixel removal but no drizzling etc. all other settings were at median combine.
What I particularly noticed with the same file was that DSS gave me "blocky"stars - almost looking like squares which was especially noticable with the double stars at 7 o'clock in the images.
But straight forward opening of the images showed they were round. I had noticed this previously but when it happened again I thought I would post.
My previous efforts at M83 with DSS stacking at 1x1 with the same settings shows no such effect.
I may go back and process everything (1x1) with nebulosity to see if DSS did indeed loose detail from these subs.
Allan

[allan gould]

Well I have now done a Nebulosity and DSS comparison between stacking in the two programs at 1x1 resolution for the subs and again Nebulosity comes out as a definate winner.
The DSS image was processed exactly the same as the Nebulosity stack with only a faint touch of High Pass to both of them. The rest was pure levels and curves done exactly the same.
The DSS image to me looks as if it has a haze over the final frame and there is definately less detail especially in the two faint galaxies in the image.
Allan

[Screwdriverone]

Hmm, funny you should mention this Allan,

I was just about to post a thread myself about the crappy donut hole stars I get after stacking with DSS, it seems I lose the core of all the faint stars and no amount of changing settings, AHD debayering etc or "fixing" the final stacked image by stretching do I seem to get nice star cores, they all seem to get washed out and hollow?

I have taken a comparison of one single frame and the stacked 12 frame result to show the issue, seems DSS is not cracked up to be all it should be???

I hope this thread can throw up some ideas?

Cheers

Chris

[lazjen]

I think I get this as well. Do you notice any difference if you're stacking just lights and darks, no flats?

[tlgerdes]

I have found the DSS donut stars before, this was do with the hot pixel detection settings.

[allan gould]

Have to use flats as my camera needs a really good clean, but just to sattisfy myself I did a stack without flats and got the same result except with added doughnuts and vignetting.
Allan

[allan gould]

Ive noticed this as well Trevor and came to the same conclusion.
Allan

[Screwdriverone]

Thanks Trevor and Allan,

I might turn off hot pixel detection and see if that helps.

Awesome ;)

Cheers

Chris

[lazjen]

And I'll try to remember to do the same. I'm pretty sure I have hot pixel detection on.