Help me with wide field techniques...?

thegableguy (Chris) · #1 28-05-2016, 05:58 PM

Keen to get a better shot of the galactic core over my back yard; just not sure as to the best way to go about it.

This is a combination of two angles; fairly easy to tell where one stops and the other starts. It's a mess, not a very accurate representation because it's been stretched and skewed so hard to make the two fit together (and they still kinda don't).

Both angles taken with Nikon full frame with an 18-35mm lens (not a great lens - has a LOT of distortion and vignette).

Sky was 40 x 90 second subs atop my NEQ6. For consistency I took 20 x 90 second subs of the back yard too. Stacked both sets in DSS, edited each in Lightroom, edited further & smashed together in Photoshop.

What process would be best? Would love to hear from those who do wide fields often.

(Apologies for quality of upload - I'm on my phone right now and it's just copied from Facebook)

CapturingTheNight (Greg) · #2 28-05-2016, 09:38 PM

Hi Chris,

For what it is worth I'm a big believer in not combining tracked shots and untracked landscape shots. As you have found out if you have any terrestrial elements in your tracked shots they will blur and if they are a long sequence of shots (like yours) then that blur can really take up a large portion of the image and make it almost impossible to blend back in your untracked images. I suggest that if you want to use this technique, you limit your sky exposures such that your terrestrial elements only move a little bit. You then should be able to paste your untracked foregrounds over the top without too much trouble.
or, you could do what I do and take single fixed tripod images keeping my exposures short enough that the stars don't move noticeably. It is of course a constant trade off between noise and exposure, but it's what I love to do. Couple of examples: http://www.iceinspace.com.au/forum/s...d.php?t=139374
and
http://www.iceinspace.com.au/forum/s...d.php?t=142651

Kind Regards

Greg

thegableguy (Chris) · #3 29-05-2016, 11:36 AM

Thanks for replying. Nice shots too!

I've taken lots of single shots like those before over the years, including the attached from the same location; I'm keen to try something else, for two reasons. One, because the amount of dynamic depth you get from stacking is vastly improved over a single frame - as you can see comparing the two; and two, because though the seeing is fairly good where I live, it's not, like, crazily good like out in the country. Though my camera is a pro level one, I don't like shooting at ISO 6400 if I can avoid it.

As you suggest, I'm going to try a compromise between the two. I'll still use the mount but with much shorter exposures like 30 seconds, and maybe only a dozen or so. If they're that short I'll also be able to take an in-between angle so the distortion isn't so hard to deal with when stitching them together. It'll still take some fiddling but shouldn't be as much.

Also going to use a much nicer 24mm f/2.8 lens. Not as wide but the results should be a fair bit easier to work with. The distortion and vignetting of my wide is pretty extreme.

zenith (Tim) · #4 29-05-2016, 01:20 PM

Hi Chris,
Have you tried adjusting for distortion and vignetting in software before combining the images? For me it can help quite a bit before attempting to stitch them together. Also if you have plenty of overlap (i.e. using say 4 panels instead of 2 to cover the same area) then you should be dealing with the less distorted central parts of each image.
One more suggestion (and I know its obvious so please don't take offense) but wait for a completely still night. Much easier to deal with tree movement if their isn't any

. Given that it is your back yard, you could also take the foreground image on dusk with low ISO and fast-ish exposure, no stacking required, no movement issues, then blend that one with your stacked milky way pictures taken later that night.
Cheers,
Tim.

thegableguy (Chris) · #5 29-05-2016, 09:20 PM

Hi Tim

Because I shoot in RAW rather than JPEG, I'm stuck with whatever lens distortion I get. Stacking doesn't really work with anything less than RAW.

It was more an experiment to see what needed fixing and figuring out how to do so. Thanks for the suggestions, I've made the following changes for tonight's attempt:

- using my 24-70mm lens instead, as it has vastly less vignetting and is much, much sharper in the corners. Also shooting at f/4.5 rather than f/3.5.
- taking 10 x 1-min tracked subs, at two angles rather than just one (10 subs each angle), so the total movement in each angle is only 10 mins rather than an hour.
- also taking a static shot at each angle; I'll combine all three static shots and then will mask out the sky in the two upper shots, to replace it with the two stacked shots.
- And yes there's no wind tonight! I totally didn't take offense - I knew it was a dumb idea. I just wanted to try it all out and see what kinks needed straightening out.

A lot of work but hopefully it'll be worth it...!

zenith (Tim) · #6 30-05-2016, 12:17 PM

Hi Chris,

What I was thinking regarding lens distortion was applying a lens correction profile to each RAW image (using say Photoshop) and saving each result as a loss less tiff, then stacking those, then stitching into a panorama.

Alternatively you could stack first, and then apply a manual lens correction to the stacked tiff images, then stitch those into a panorama. This may be less effective because (in Photoshop) manual correction is not tailored to any particular lens.

Its a bit of mucking about but can help to produce panels that are more regular and much easier to stitch into panoramas using automated stitching tools like Microsoft ICE.

Cheers.

thegableguy (Chris) · #7 30-05-2016, 01:07 PM

Exporting as lossless TIF is actually a good idea! Never thought of that.

I actually went the other way and did the image correction on the stacked TIF. Seemed to work okay, but as I've learnt over the years stitching together shots from anything wider than about 50mm is going to involve a lot of stretching / skewing / warping anyway. Damn physics...

Just finished the second attempt - will post in a new thread now. It looks a lot better this time around!

janoskiss (Steve H) · #8 30-05-2016, 04:13 PM

Re stitching frames together I've found autostitch by far the best. It used to be free. Quick look at the website tells me that it's no longer free but they do have a free demo version.