landscape (eg mountains) or smaller objects (trees), either as silhouettes or with some illumination,
starry sky and/or nebulae
shot with short shutter speeds to prevent star trails, as RAW, same exposure, aperture, on a tripod, at an equivalent focal length of 20–25,
meant to be combined as average/median for lower noise
then post-processed creatively, ideally in Darktable
What is the recommended software stack for doing this on Linux? Is this even possible? I found that this is not so simple, I first thought of using align_image_stack, but stars move and the landscape doesn’t.
I am not experienced in astrophotography so sorry if the answer is obvious. I am more of an opportunistic sky photographer, if hiking takes me to a place which has relatively low light pollution and I can combine it with the landscape. Currently I am trying to figure out the workflow so that I can get it right next time I have the opportunity.
I’m not experienced in astrophotography either but the only tool I’ve heard of which will separate land and sky is Sequator, which doesn’t run on Linux. Maybe I’m wrong but if not you’ll have to take multiple exposures: one longer for the land and a series of shorter subs for the sky. Then you can stack the subs in Siril or some other tool that runs on Linux and composite them in Gimp.
I would use Siril to produce two stacked images, one with star registration (so stars are aligned) and one without (so the landscape stays still). The second can also be replaced by a single long exposure. Then I would compose both with Gimp.
If the total sequence time is too long you could have problems with the sky part, as the horizon may cover a good chunk of the stacked stars image. So this is mainly useful for small sky rotations.
Sequator runs under Wine. That’s how I’ve been using it under Linux for years. It’s great for “freezing” the ground while stacking the sky. I know Deep Sky Stacker is migrating to QT eventually, but it doesn’t work under Wine at all the last time I tried. I’ve tried Siril and it works well for stacking the sky then blending. I haven’t tried what @guille2306 is suggesting with stacking foreground I just use image-magick, enfuse, or gimp+g-mic for average/medium stacking the foreground.
Here’s my basic flow with Sequator:
Create a tiffs of the images you want to stack. I use DT to apply wb, lens correction, CA corrections, exposure*, output linear SRGB/linear Rec 709 and dither (disable filmic/sigmoid).
*exposure is optional I shoot and stack at iso640, my camera’s invariant point to reduce noise, it’s dark. I trick I paint the sky with extra exposure to make it easier to see the foreground then re-export without the extra exposure.
I keep the simple in Sequator. I select images to be stacked, freeze ground if not tracked/paint the sky, enabled remove dynamic distortions, and set color space to linear.
Process the stacked tiff in DT as if linear raw. Sequator doesn’t set the color space so explicitly set it back to linear sRGB in the input color space.
There’s a lot of nuances and lessons learned from these steps. I’m still not sure they’re entirely right for keeping everything linear. White balance can still be a bit weird. Not sure if I should do wb + color calibration before or after (old versions of DT didn’t allow wb module on tiffs), or split wb before stacking + color calibration after stacking. Sequator also isn’t the best at alignment. Recently I noticed a tracked 4 minute shot had more details then a tracked 16x60s stack.
I’ve also used some of Siril’s tools like further noise reduction, deconvolution sharpening and mostly unsuccessfully background extraction (I don’t think it’s intended for landscape astro) on the stacked image from Sequator.
I’m still trying to figure out Siril, but honestly I have had some issues with color space management and keeping it linear. Maybe it’s better to not keep it linear and do final touches in Gimp. But I rather like diffuse & sharpen, color balance rgb, etc. from DT.
Siril is geared for deep space stacking rather then landscape shots. I’d suggest Sequator if you want a easy and quick way to stack shorter untracked shots while freezing the ground.
There’s also a play raw if you want to try your hand at tracked sky + blended foreground shot I recently posted. I use near identical process of processing raw and post-stacked tiff files.