DSS won't stack a lot of frames

[vineyard]

Hello,

I was wondering if anyone might have some light to shed (pardon the pun!).  I've got about 100 lights of M101 that DSS won't stack.  I know a few of the last ones are out of focus so that's to be expected.  These were taken over 3 nights (about 170 in total!).  The second night's get stacked, the first & third night don't.  I've blinked through them all on PI and M101 is there (in fact the first night seems to have the best focus).

Has anyone experienced this before?  Should I just scrap the lights which it's not accepting.

(I don't think it was a kit problem on the third night - for example, I took some more of M81 last night too, and those get added & stacked to the earlier batches fine.  So it seems to be M101 in particular.  Do I need to up the gain on the lights?)

Thank you for any help,

Vin

[happy-kat]

Could you paste a screen grab of an elected image please?

Have you tried showing the star threshold when you try registering the images?

[vineyard]

Hi happy-kat,

Thanks.  Here are 3 randomly chosen screen grabs (JPGs) - the first is one of the ones accepted, the other two are rejects.  (The only thing I've done is put their levels to the same new setting in GIMP just so that the image can be seen - happy to send screen grabs of an auto-stretch from PI as well if that'd be more helpful).

I've set star threshold to 2% and it only finds about 7 stars.  I'm beginning to wonder whether its M101-specific (for me anyway) - whether its v sensitive to focus?  While the focus on the third night wasn't as good as the second, with the same setup on the same night DSS still stacked M81 additional images fine - so maybe there's something specific about the dimness/relative emptiness of M101 (that's probably a really stupid idea but I can't figure it out!).

Anyway, scratching my head - may try a bit of M101 again tonight (w sharper focusing) or try a different target (its too disheartening to get nowt from a night, although luckily last night I did get more M81).

Stay well,

Vin

(PS - one other thing that comes to mind is whether its the camera & this target together. On another thread, @vlaiv & @wimvb pointed out that my ASI178MC is over-sampling with my fl 880mm/ f8.6 scope (comes out at 0.56"/pixel).  Could that be a factor for this particular target?)

 

 

Edited April 20, 2020 by vineyard
added PS

[wimvb]

I don't know about dss, but PI looks for stars among fine structure details. This means that if your images are oversampled, you need to "widen your net", and increase the number of layers (level of detail) that is used to detect stars. My guess is that dss has a similar setting under a different name.

Edited April 20, 2020 by wimvb

[JamesF]

DSS doesn't like stars that don't appear fairly round as far as I'm aware.  In the second frame the brighter stars near the centre and top left appear oddly square to me.  I don't know why that should be.

James

[vlaiv]

Do you get any info back from DSS as to why subs were rejected?

It could complaint that there are not enough stars found for registration, but it can also complain for other things? I know it complains when calibration frames are mismatched and similar.

[vlaiv]

Ok, just realized that you are using color camera.

Try doing this:

That will make "tighter" stars. DSS does not like when stars are large - that happens when you have over sampled data. Super pixel mode will make them tighter as it reduces sampling rate (or rather applies proper sampling rate for OSC sensor - still high at 1.16"/px for 4" scope but better than 0.56"/px).

[vineyard]

Thanks all. Its really great the way people are so helpful in this hobby (or should I call it an obsession?  ).  I will keep my fingers crossed and try your suggestion vlaiv.  You'll be a sanity-saver if it works!

Stay safe all,

Vin

[vineyard]

No joy sadly.  Still only used the same 48 lights out of the total 173 (!).  At least there's an explanation namely over-sampling.  I guess will have to come back to M101 once I eventually get the ASI294MC!

wimvb I'll have an explore on the PI tips, thank you - I see from a google-search advice on not using BPP but using SA & II.  So I'll keep fingers crossed & try that.  (I had tried PI as well - via BPP - but it came up with RANSAC errors and only found 3 (!) stackable images - which would be consistent with what you said about PI not liking big stars).

Now I'm starting to see the problems oversampling leads to!

Many thanks & cheers all.

[vlaiv]

do you mind posting one of the actual FITS files that DSS refuses to stack?

[vineyard]

Sure thing - here is one from night one & one from night three.  Cheers, Vin

 

 

 

M 101 TV_Light_001.fits M 101_Light_112.fits

[vlaiv]

32 minutes ago, vineyard said:

Sure thing - here is one from night one & one from night three.  Cheers, Vin

 

 

 

M 101 TV_Light_001.fits 12.29 MB · 0 downloads M 101_Light_112.fits 12.29 MB · 0 downloads

In this particular case - frame that does not get stacked is out of focus. Here is same section from both files at 1:1 zoom (one screen pixel - one image pixel, or 100% zoom).

Notice star size in left image - it is quite acceptable - on the right it has at least x3 larger diameter - due to poor focus (could be other things - like particularly poor seeing or issues with tracking / guiding, but my money is on poor focus).

[vineyard]

Ah thank you.  I started with M81 and that one stacked fine, and then I just moved to M101 as the next target but didn't re-check focus.  Aargh.  Well, lesson learnt!  (And maybe an automatic focuser to do that every so often).

Thank you again vlaiv - v helpful & much appreciated.

(PS - one thing though, both of those lights were rejected in the stacking - the first image was the first night when I know my focusing was relatively good based on HFR.  Anyway, lesson learnt )

Edited April 20, 2020 by vineyard

[vineyard]

Hello, just as an update.  I tried playing with PI settings as well (learning a lot about that in the process) & still no joy.  It is the oversampling that is causing problems for sure.  I must have just gotten lucky with night that the images did work & stack b/c when I look at astronomy.tools it says that with good seeing conditions, the TV102 & ASI178MC play nicely together, but with OK seeing, it leads to oversampling.

As final confirmation, I went back to M101 with a little PL72 last night & the ASI178MC and got lots of data that stacks  (and it makes such a huge difference when you have more data!)

So this has actually been a really useful learning session for me on the perils (and consequences...) of oversampling.

Cheers again for the help all & stay safe,

Vin