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Anyone Stacked Widefield Milkyway?


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Hi

I was looking at getting an ultra wide angle lens for my APSC canon.
Then I wondered if I could successfully stack with it.

The widest Ive stacked on is 18mm. Which works out well. It seems to get progressively harder the wider you go.
Any movement between frames means you cant stack because of field curvature in the lens shifting the position of the stars in the outer corners of the image, meaning the stars wont line up in a stack.
So dithering is out. This is the only setup I use darks in, to help a little with noise.


So I'd love to hear from anyone who has successfully stacked down to 10-12mm on APSC.  - Is it possible?

I really would like to stack for a smooth image, as I find the single shot images I see of the milky way a bit to crunchy for my liking.

Thanks very much

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First thing - it's not field curvature, it's lens distortion.

Field curvature is something completely different.

In fact, what you are dealing with is perspective projection and lens distortion is just how much lens deviates from perspective projection.

image.png.b49720616c3175f26bdd03d813aae3d3.png

Telescopes are perspective projection systems - image on the right, and they maintain straight lines. Lens distortion is how much those lines bend (left image - look at lines on the concrete).

In any case - both are bad thing with wide field, and both prevent dithering when stacking if stacking software can't deal with them.

Any software that just uses linear transforms (3d matrices) to align subs will fail to properly stack images that have either of the two above (or lens that is in between).

However, I know two pieces of software that should enable you to stack images with dithering - one designed to deal with above cases, and one that handles them by chance (maybe it was designed to deal with distortion but I don't believe it was designed to deal with these kind of distortions specifically).

https://www.astropixelprocessor.com/

image.png.da71c22a0ead8fcef4aebb62ecf8b361.png

has this as a feature - it can actually model your lens and even transform image to different projection type. You'll have to pay for license but there is 30 day trial.

Maybe give that a go?

Second is DSS.

DSS does not list this as a feature and there is special processing, but from what I've seen, DSS does not use linear transforms when aligning subs. I've sometimes seen people complain that their stack gets warped by DSS and I believe this happens when DSS fails to properly identify stars and does some strange alignment of the image - one side of image is properly aligned and other side has stars mistaken for different stars so it is bent by DSS trying to match star positions.

Unfortunately I can't find example now, but I think it's worth having a go with DSS to see if it will manage to stack your subs properly. You can even do a "sim" - take few subs and add lens distortion in software and try stacking those subs to see if DSS will handle it properly.

Btw, while searching for example of image mangled by DSS, I came across another software that can handle lens distortion:

http://www.astrosurf.com/buil/iris-software.html

IRIS is free and it looks like it can handle lens distortions and align subs properly:

image.png.afa7cc1f3435cc7c45028eee05d6bc14.png

In any case - you should dither and hopefully, one of these suggestions will work good for you.

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What are you stacking in, Pixinsight tries to deal with distortion first before alignment, once all aligned, then stack it... might be worth taking a look at that.  Want to share the files? I'm sure a few of us could have a go for you to help narrow down on to a way forward....  

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Thanks for clearing up the terminology and describing what I had a dim understanding of.

I have used ICE before and yes its quite good but still struggles on edges of image. Plus I was hoping to capture the image in one go.
I usually travel for my wide field shots and I get a few subs which takes time, and taking more for mosaics will make the already long night longer.

I'll have a look at those softwares. Ive used APP before but only for deep sky.
My experience with DSS  has been that it doesnt stack dithered widefield images well.
Thats what led me to my current process of not dithering between images, so images are already perfectly lined up before they are stacked.
- do you have to select a special setting?

I currently use 'Siril' for stacking which I believe is a version of Iris, but again Ive only used it for deep sky. I'll try that also.

Thanks for the input...Have either of you imaged with a very wide lens?

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So you don't do any sort of alignment before stacking? 

Irrespective of tracking accuracy etc, you probably need to align before stacking? I expect quite a few of the software packages do some sort of distortion correction as part of the registration of images

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Thanks I'll try my older dithered data with Siril and see if it handles it first, and if Im not successful I'll put the images up.
Note that when I say dithering, I mean huge dithering on my widefield

On my usual deep sky rig I do dithering with APT and PHD2 as usual.
But when I travel and do widefield I use my Stepper motor driven Scotch mount, so Dithering ( I use the term loosely) is me stopping between frames and moving the camera as small an amount as I can and then carry on tracking. Its pretty punk and ends up with large movements between shots, so if I was to be able to stack successfully I would crop quite a lot.
But as I say its not successful so far, so now I don't 'dither'.

 

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I've not imaged much recently with an APSC camera but I, and quite a few others, use 14mm lenses on FF. As happy-cat says, Sequator is the best App for Astro landscapes ( Windows ) or Starry landscape stacker ( Mac ). Sequator is quite good on very widefield sky shots with no landscape.

I've stacked 20 widefield shots in Sequator and don't see any problems. 

Dave.

 

 

Edited by davew
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Thanks. Yes Ive tried Sequator before and its fine, but I prefer other programs for the stacking.

So you have done 14mm on FF -thats very wide indeed. Good to hear that it stacked well.
Were they tracked images or static shots of shorter length (rule of 500 imaging)

I normally aim for 14 x 3 min exposure on my kit 18-55mm lens (used at 18mm)

Looks like I'll be OK to get a wider lens although the options are a bit limited for APSC - because I also wanted a zoom range ~10-20mm for daytime landscape photography as well.

Edited by johneta
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Out of a matter of interest, and the fact I may be missing something, what program do you use for stacking with Astro landscape type images ?

My stacked images are almost entirely static but I don't use 500. Normally around 350 and may be a little longer if pointed directly north ( South for you of course ) That may help with stacking too as the stars are in better shape.

Dave.

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I use Siril now (I used to use DSS ).

I usually take  4 x approx 2 minute static shots of the Landscape ( Stars have drifted of course)

Then I turn the tracker on, and take around 14 x 3min images of the stars (Landscape is blurred).

Then I stack the 4 land shots and process a bit. Then I stack the 14 star shots and process a bit.

Now I merge them in Photoshop.  Because of the sky rotation I have to crop it a bit, but there hasn't been too much movement in the sky over 40 mins or so.

So although I've cropped and moved the sky part of the image a bit, the overall image is fairly true to how the scene looked

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Ah thanks. No I haven't missed anything. You and I go about things differently. I will track a sky only shot to make a sky shot ( Home or camp site ) and for landscapes I do static tripod shots.

I only carried a tracker up a big hill once and won't be repeating the experience any time soon. 😎

So, going back to the original question then I'd say as long as the lens you use is fairly well corrected then I doubt you'll have many problems. I've heard good things said about the Tokina 11-20 f 2.8 and the Samyang 12 f2 but I've used neither.

Dave.

 

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Ha I hear ya on the dragging the tracker up a hill. Its a commitment alright 🙄.

Yeah I was considering the Tokina. Looks good, except I was put off by the corners. I saw some images where there was crazy coma in the corners.
Stars looked very stretched and had wings!  Looked like birds around the edge. Shame, cause the middle was nice and sharp.

Also I had a peek at your widefield images, very nice :).  Particularly good foreground in your shots

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