Stack star photos without trails but WITH sharp foreground?

You can do this in Sequator (free for Windows; there is no Mac Version as of July 2020).

In Composition, make sure that both Align Stars and Freeze Ground are checked. In Sky Region, you then have to check Irregular Mask and paint on the sky with the brush. According to the Sequator Q&A,

"Freeze ground" does not need a precise mask. The algorithm has the ability to detect moving pixels & select stable pixels, perform intelligent interpolation. [...] You only have to specify the mask roughly, around the trees.

I have found Sequator to work very reliable and fast. If your start images are very noisy, then apparently Sequator can have some problems with reducing noise in the stack - see my question and answer here for more information.

Unfortunately, Sequator does not support shots that were shot using a star tracker.

I agree with Randy's response but believe I have a better solution for case a.

If noise reduction of the sky background is your intent, you can take a few short exposures and stack them in DSS with alignment on to increase the overall S/N, without the foreground motion becoming excessive and blurring into the area where your composited foreground long exposure shot is.

The issue with stacking multiple sky exposures is that the terrestrial objects do become blurry. Fixing this up is tricky as the blur extends beyond the foreground of a single exposure specifically for the foreground.

In my YouTube video Widefield astrophotography stacking with terrestrial foreground I cover a process whereby I stack the sky using Registar on Windows (DeepSkyStacker, Photoshop align layers or other alternatives will also work), the foreground using Starry Landscape Stacker on the Mac and then composite these two images together very simply in Photoshop.

My process yields great results because:

• Starry Landscape Stacker prioritises the foreground at the expense of alignment to the outer edges of the frame. Sky close to the static objects are well aligned and stacked
• Registar prioritises star alignment but leaves blurred static objects
• A layer mask covering just the foreground and the blurred portions of the Registar foreground allow the aligned and stacked Starry Landscape Stacker to show through providing a seamless and low-noise final composite

I also have a D5100 and the Tokina 11-16 which I use for wide angle night shots.

There are several separate cases that I have encountered in my experience: a) Night shot with stars, and terrestrial foreground elements (trees, buildings etc) b) Night shot with terrestrial foreground elements where I DO desire star trails c) Night shots with terrestrial foreground elements where the sky contains some element that benefits from long/accumulated/stacked result (eg Orion Nebula, Andromeda Galaxy, Milky Way)

Case a) is the simplest - take separate exposures for foreground and background (with different focus) and composite them together. Nothing is really gained by stacking multiple short exposures for single point stars. The longer foreground exposure will almost surely be long enough to blow out the stars, and the same effect will result from stacking. If noise reduction of the sky background is your intent, you can take a few short exposures and stack them in DSS with alignment on to increase the overall S/N, without the foreground motion becoming excessive and blurring into the area where your composited foreground long exposure shot is.

Case b) No need for much explanation here - when I want the trails, while shooting on a static tripod I use StarTrails to stack multiple images (as from a sequence intended for a timeplapse) and composite in the single best foreground image. (I've found that the lighten screen blend mode for foreground elements can still be a problem if I allow the foreground from all images to be stacked in StarTrails, because shadow detail may seem to blur with the motion of whatever ambient light source is illuminating it - a better result is obtained from a single adjusted image)

Case c - is what I imagine you're most interested in. Shots of the Milky Way moving across the sky for a timelapse. In that event you need to keep individual exposures short enough to prevent too much star trailing (it's a tradeoff, and the shorter the fL the longer exposure you can get away with, just settle on something that is acceptable to you. The longer exposure will allow a slightly lower ISO setting, and less noise at the expense of a tiny amount of trailing in each frame. This is actually not too bad for producing the timelapse, because it adds to the perceived smoothness of the motion.

But at the end, you'd like to take that entire set and turn it into a single image. Stacking in StarTrails will cause the undesired long star trails you hope to avoid. Stacking in DSS with alignment of the stars (to improve the S/N of the dimmer parts of the Milky Way) will result in motion blur of the foreground, as if you were on a tracking mount.

The solution is much like case a, but requires more planning.

I still would stack for the sky background in DSS, but first I will examine each individual frame and compare the position of the Milky Way relative to my foreground element(s) of interest. KNOWING IN ADVANCE that the stacked result will make it appear as though my foreground elements have translated and rotated throughout the duration of the shots, I try to select a single frame where the sky background is in a position that will allow me to composite in a single shot for the foreground that covers all of that apparent foreground motion.

In DSS, you can select any frame as the reference frame - then the the rest of the frames are registered and aligned to THAT frame. If you don't intervene, DSS will select the highest scoring frame (lowest star FWHM, lowest background sky brightness, highest # stars etc) as the default reference frame. For this method, you don't have to use ALL the frames you shot, and you'd probably be wise not to. More than likely what would happen is that artifacts from the edge of each frame would creep into the stacked result and have a negative impact on your sky. If you have no choice, due to relative positions of foreground/background, you can always take the last image in the sequence of frames you stacked, and carefully re-blend in a portion to cover up the framing artifacts.

Sorry, a little hard to describe in words, but hopefully that helps point you in the right direction.

Deep Sky Stacker has a mode that allows the detection of a meteor and stars, and for it to de-blur the stars (by rotating them to stack (after substracting the meteor)) and merge the meteor back in, I've not used it in this fashion, but it might be able to handle the foreground object.

I don't know of any software that does it automatically, but simply producing a mask of the foreground should allow you to keep it sharp by combining the foreground and background separately and compositing them back together.

In theory, since the foreground should remain the same within the stack, I would expect it to be pretty easy for software to use a difference map to isolate the foreground and not move it, but I don't know of anything that does it automatically.