DSS and the wonderful science of serendipity

[Moonshed]

In the last week I have been on a steep learning curve regarding getting the most out of DSS, and as a result of the great advice I have received (many thanks to Thalestris24, Louise) I am now using Master Flats and Master Bias/Offset and Master Darks, it has made a remarkable difference. I also found the benefits of using kappa-sigma stacking. I use a Canon 1100D, an 8" Celsestron SCT, an EQ5 mount but no guiding, just ordinary tracking. So I have  been playing around with some of my previous subs to get an improved image to import into PS. I went back to some subs of Andromeda I took in December. These were 30 X 90 second subs at 800 ISO. The original 30 subs came out of DSS with this image, no post stacking processing in DSS or PS

 

 

The resulting Image after processing in PS was not great, rubbish if I am honest. So today I went back to those same subs and loaded them into DSS again BUT this time used kappa-sigma stacking and my newly created Master Frames (Darks, Flats and Offset/Bias.) This is the image I got, again with no post stacking processing at all.

I took one look and thought WOW!  However, after thinking about it for a while I thought this can't be right, that much improvement is too much with just those changes, so I thought I would try it again. Loaded DSS again, selected the same lights again and clicked the "check all" tab. That's when I noticed that I hadn't loaded 30 subs this time, (or the previous time today) but 39. So I referred  to my notebook to see what had happened. I had forgotten that after taking my 30 x 90 sec subs I had decided to do a little experiment and try some longer subs to see how long I could go before getting star trails. I took 3 X 120 secs, 3 X 150 secs, and 3 X 180 secs.  making a final total of 39 subs of course. Originally I carefully checked all the longer subs but rejected them all due to the star trails starting right away at 120 secs. But at least I now knew for sure my max exposture time is somewhere between, 90 and120 secs. So originally I only loaded the 30 X 90 sec subs into DSS, but today I forgot those 9 longer subs were in the file and loaded the lot by mistake. But what a brilliant mistake! I had no idea you could get away with that, I had thought that by loading them they would ruin the whole lot. How wrong can you be! Okay, I do realise there Is  bad comma there,  but that isn't the point here. I had always thought that  to take advantage of longer exposures they had to be taken in larger numbers, stacked seperatly and merged in Layers in PS or other similar software. Am I missing something? Or is what I have accidentally done normal practice?

As a matter of interest this final image is created with using all of the original  39 images, but without using kappa-sigma stacking and without using any Master calibration frames. Makes quite a difference, mainly due, I think, to the Master Frames.

 

I am really interested to learn of other member's experiences with mixing different length subs in one stack.

Keith

[Moonshed]

After a nights sleep I decided to return to my laptop and mess around with DSS some more to see what happens.  I decided to load into DSS only the 3 longer sets of subs, all of which show star trails . So that's 3 X 120 secs, 3 X 150 , and 3 X 180. These Longer subs I originally took just for test purposes. I used the same method, kappa-sigma stacking, no drizzle, and all 3 master calibration frames. It seemed a waste of time, after all EVERY sub shows star trails, even the shortest at 120 secs, although not a lot. Anyway this is the result. To my eye it looks every bit as good as the image where I used the 30 X 90 sec subs plus the 9 longer subs. Again no post stacking processing has been done. I don't get it. How can these 9 subs not show star trails when stacked? What then is the point in taking 30 good subs when 9 not so good subs get a better result. Of course the longer subs get more detail but it seems so odd that it works so well doing it this way. 

I find it strange that 9 mixed length longer subs produce just as good an image as using 30 shorter subs plus the 9 longer ones. There seems no point in taking the 30. Perhaps some of the eagle eyed on here can see a difference, but it seems to me that if there is any improvement in taking the 30 subs it is not worth the bother. Over to you guys.