DSS - Cat's Eye Nebula

[dynasty99]

Hi All,

I'm trying to stack some frames of the Cat's Eye Nebula (NGC 6543), and I'm having an issue:

I bring a lot more S/N out by doing the stack on the background stars, but the nebula itself gets completely ruined - it looks a lot better in my individual frames than in the stack.  

In my subs I can only see the core of the nebula, none of the surroundings (Alt-Az near NCP = very short exposures), but I do see a lot more detail in the subs that gets completely washed away in the stacking. I've tried using Entropy-Weighted, and KappaSigma(2.00,5) stacking for the light frames, both to a similar result (sidebar: any recommendation for my best setting in my limit of many short exposures? I generally just play around and see so far)

It looks like DSS is identifying my nebula as a star since it is so small.. could this be what ruins my stack? Is there any combination of settings that would stop counting this without me manually deselecting it in my 120+ frames?

I attached a heavily cropped (maybe 30% width and height of true field) single sub, and the stacked result (in this case using KappaSigma), if that will help illustrate my issue.

 

[nmoushon]

Is your stacked photo straight out of DSS or have you PS it some? (Besides just cropping) The stars look much larger than the individual sub so it looks like you have just blown the cores out. You might have to do a seperate process for the nebula itself and then one for the stars.

Also have you looked through your single subs indivually yet? There might be a bad sub that is causing the stacking problem. This has happened to me a couple times.

Also what equipement are you using and what are the sub details?

[dynasty99]

It is straight of of DSS that way.  Could I blow the cores of the stars out just by poor choice of DSS settings?

Yes I went through all subs individually (I always do to delete any poor ones and save DSS the trouble). All subs used in the stack are essentially of the same quality as the one I posted. In the #s below, these are the number of frames after I have pruned out bad ones (about 40% with bad tracking or vibration got thrown away)

Equipment:
Nexstar 6SE (AltAz)
Nikon D3100 @ prime focus
123x14s light frames (stacked best 90% = 111 frames) @ ISO 800
1 master flat used to get rid of a few dust pieces on my sensor, but normally I do an artificial flat in PS (I haven't done this yet in the images I posted, so not related)

 

[nmoushon]

Have you tried messing with the colors and curves a bit IN DSS before you save it as a TIFF and open PS? You can get to see what data is there a bit easier and quicker before you do a save as. If its blown out in DSS before you save it as a TIFF then you knows its either a setting or on of your subs is messing it up. If you get it to show a "normal" looking image, though not fully processed, then you know you should be able to fix it in PS. Thats all I can think of trying anyways. Hope it helps.

[dynasty99]

Thanks for the suggestions, I will try that out tonight