DSS stacking problem

[Ken Mitchell]

Hi,

I have a problem stacking lights from various nights.

First when I took the images I tried to align the camera as much as possible, so I don't believe alignment is the problem.

I grouped them, the lights from each day are in separate groups, calibration frames are in the main group. So there are 3 groups + main group.

I have eliminated all the bad light frames before importing them into DSS.

There are a total of 337 light frames(+11hours)

When I try to stack them in DSS it only takes around 150-200 images depending on what reference frame I use.

Are there any specific settings I have to apply for this? Can't seem to find the problem.

DSS is also not computing the offsets for several images. Probably those that aren't included.

Also when registering and stacking each folder separately it doesn't have any problems doing so.

I hope someone can help me with this

 

Ken
 

(star detection threshold is set to at 80%, all images have a star count ranging from 150 to 250 stars)

[newbie alert]

I use a star count of 120-150..otherwise it takes too long to stack as it uses the stars to align the frames 

[The Admiral]

Welcome to the forum Ken.

It's a while since I used DSS but I'd have thought that the main group would have contained calibration frames and the lights of one set, and the remaining 2 sets would have been in 2 other groups. May be that isn't critical though. Also, is it calculating the quality factor for the subs which don't appear to be used? I take it all the frames have been 'ticked'. Can you tell us whether DSS just grinds to a halt, or whether it appears to complete its task but with fewer than expected subs?

Ian

[Ken Mitchell]

Thanks for the suggestions,

 

I also don't think it will change much in what group you put the lights but I will try and what difference it makes. 

It is calculating the score (quality factor I believe) and stars for each frame in every group.

DSS completes its task, producing the final image, but with less subs than initially where added.

I know it's not unusual that a few subs don't get added because of its quality I guess, but now there are 150 subs missing in the final image, that's a lot no. 

The subs that don't get stacked all have a "NC" in columns "dX, dY and angle". 

 

I have stacked each folder/day separately without any problem and without loosing any subs. Also tried to stack these 3 "master stacks" but it doesn't seems to do much 

at the detail or quality. Wouldn't a total of 10 hours give you better result than 5 hours?

Maybe it would also be helpful to add that the total subs stacked depends on the reference frame I choose. So when I set the reference frame at a sub 

from group/day 1, it will stack all the subs from that group but only a couple from the other 2 groups. The same counts if I use a sub from group 2 or 3 as reference 

frame.

 

Ken

[kens]

NC means Not Calculated. I think that if the number of stars found is less than some percentage of the number found on the reference frame the image is not stacked. That number is indicated by the Quality Factor. Plus there is the input value for % of frames to stack. I'd suggest to try selecting a lower quality frame as reference. Another option is to use a different registration threshold for each group of frames so their quality factors are more similar. And make sure the % of frames to stack value is not too low. I think it defaults to 80% but may have been set lower by mistake.

[Stub Mandrel]

That's a high threshold. It probably isn't finding enough starts in common to register all of the frames

I usually use a threshold less than 10%.

It just need to be high enough to avoid confusing noise with stars

[Ken Mitchell]

Thank you all for the suggestions and help!

I've been trying different settings but doesn't change a lot. The fact that DSS isn't calculating the offsets of certain images is, I believe, caused by an alignment issue during imaging.

It is the first time I tried to image during several nights and as i said the camera wasn't perfectly aligned(frame alignment not PA). Still learning here.

I was just thinking of a possible reason. When I started imaging the second and third day, for the first frame of that day I used the last frame of the previous to match for frame alignment. In the end the images were so much misaligned(mount drift?) that DSS just couldn't align them all? Although I could only see the drift when zoomed in.(see attachment)

Not sure if that makes any sense, but now I believe it would be better to use the first frame of the previous day(first imaging session) for reference.  

Misalignment is the only problem I can think of why the offsets aren't all calculated. Although DSS uses subs from all the 3 different sessions???  

You guys got any tips on how to frame align when imaging at various nights? For the moment I'm using a simple Star Adventurer mount with ff dslr and 200mm lens. 

--

In the first image added, I used the first sub from the first day and the last sub of the last day. Layered them in photoshop at 50% opacity to show the difference in alignment. Maybe that's too much for DSS to calculate?

The second is the result of after PP, total of 5hour24min. 162 frames of 337 used by DSS. 

 

Ken

edit: Is there a way to crop the subs before stacking them? maybe that way DSS has fewer problems aligning them?

[kens]

Looking at your attachment the stars are doubled up but radiate from a point and the separation grows with distance from the centroid which is presumably the guide star?

That suggests a slight difference of focal length over the two images. Is your 200mm lens fixed focal length or a zoom lens?

[Ken Mitchell]

I'm sorry that was my fault. I moved both images to match the middle star.

I will add the same both images but just layered without moving. First attachment are subs from day1 and day3, just as in the previous post. You can see left below the same issue.

The second attachment are 2 subs from day3.  

I'm not using any guiding. 

The lens is the nikkor 80-200. To me it also seemed like there was a difference in focal length, as you suggest(spiral stars).  

I haven't touched the zoom during sessions, left the camera and lens on the mount. Images were taking on 3 consecutive nights.

The metadata is showing 200mm also.

 

Ken

 

[Stub Mandrel]

I'm afraid @kens  is right - it does look like you have two very slightly different focal lengths.

[kens]

1 hour ago, Ken Mitchell said:

I'm sorry that was my fault. I moved both images to match the middle star.

I will add the same both images but just layered without moving. First attachment are subs from day1 and day3, just as in the previous post. You can see left below the same issue.

The second attachment are 2 subs from day3.  

I'm not using any guiding. 

The lens is the nikkor 80-200. To me it also seemed like there was a difference in focal length, as you suggest(spiral stars).  

I haven't touched the zoom during sessions, left the camera and lens on the mount. Images were taking on 3 consecutive nights.

The metadata is showing 200mm also.

 

Ken

Can you attach the two unlayered frames? Or alternatively plate solve them separately at astrometry.net to get the pixel scales from which you can calculate the focal length fairly accurately. The thing you want to know is whether the focal length shifted gradually or was just different on each night. If it's the latter there is an outside chance you can rescale the offending frames so they align. 

[Ken Mitchell]

Thanks guys! You are correct. Just rechecked the metadata from each night and on the third night the focal length was 185mm(the whole session). Day one and two are all 200mm.

No idea how this happened as I didn't touch the camera, maybe with moving the setup inside and outside. 

I will now try to stack day 1 and 2 just to see if DSS takes the frames. When layering subs from 1 and 2 it doesn't show the spiral 'starring'.

I've also added the two subs to the astrometry group. Is it ok when I receive the data from astrometry to post it here kens?  

How does this rescaling works? Will DSS allow different image size?

Thanks again guys! Really appreciate it. I've posted the problem on different fora but without any help. 

Thanks for taking your time!

 

Ken

Edit: From the 192 frames(day1 and 2) DSS stacks 178 frames. I haven't re-registered the subs or change the reference frame, so the total may still vary. 

I believe its a big improvement for now!

[The Admiral]

Zooms often suffer from 'creep', where the lens collapses under its own weight, when pointed upwards (and vice versa). Positive steps need to be taken to prevent it, such as gaffer tape!

Ian

[Ken Mitchell]

1 minute ago, The Admiral said:

Zooms often suffer from 'creep', where the lens collapses under its own weight, when pointed upwards (and vice versa). Positive steps need to be taken to prevent it, such as gaffer tape!

Ian

Thanks Ian, the lens has an internal zoom and the zoomring is pretty stiff. I use this lens for outdoor photography and astro, never encountered this issue. But indeed I can't find another cause for it to why the FL has changed. I believe it must have happened while taking the setup outside(bumped it maybe?).

If it was a creep issue the focal length would change during the whole imaging session no? I mean it would show the focal changing incrementally(step by step) on several images?

Ken

 

[carastro]

I don;t think DSS copes with images of different sizes.  I think unless you want to buy/learn a software will will re-scale the images, you probably won;t be able to stack them together.

I think nebulosity will do it, as will Registar, Pixinsight and Maxim, and I think I heard recently that Astroart will do it.

Carole 

[kens]

3 hours ago, Ken Mitchell said:

Thanks guys! You are correct. Just rechecked the metadata from each night and on the third night the focal length was 185mm(the whole session). Day one and two are all 200mm.

No idea how this happened as I didn't touch the camera, maybe with moving the setup inside and outside. 

I will now try to stack day 1 and 2 just to see if DSS takes the frames. When layering subs from 1 and 2 it doesn't show the spiral 'starring'.

I've also added the two subs to the astrometry group. Is it ok when I receive the data from astrometry to post it here kens?  

How does this rescaling works? Will DSS allow different image size?

Thanks again guys! Really appreciate it. I've posted the problem on different fora but without any help. 

Thanks for taking your time!

 

Ken

Edit: From the 192 frames(day1 and 2) DSS stacks 178 frames. I haven't re-registered the subs or change the reference frame, so the total may still vary. 

I believe its a big improvement for now!

No reason you can't post the astrometry.net data here. I've seen it done often enough.

To rescale you'll need some other software. That then depends on your file format. There's a tool I've used called Xnview ehich has an option for batch processing a number of files. I've not used it for astro work but it works well with TIF files. I've not used it to convert FITs. ImageMagick can also batch process from the command line.

The only issue is how accurately the resized frames match the scale of the others. If there is less than a pixel of error it should be ok. 

[Ken Mitchell]

Just an update and thanks for the help. 

I finally had some clear nights to see if the problem still existed. 
This time DSS stacked all the images from 2 nights without any problem. 
So to confirm(and if someone encounters the same issue), the problem was at my side. Change in focal length during different nights. Probably bumped the lens, camera or tripod which slightly changed fl.

Ken