The Pentax K-1…

Both versions share the same sensor and of course the feature set is the same, there was a lot of discussion at launch about how the "improvements" could be detrimental to some low light images (particularly Astro) but I think the general consensus worked out that there were almost no negatives to the MkII, only improvements in low light handling of images. It might be worth considering that the MkII is likely to receive firmware updates for longer than the MkI.

Also, do some research around Sigma lenses compatibility - certain models caught on the K1 prism housing causing a scrape (I believe) so some lenses were modified by Sigma to stop this - but that service went a long time ago.

I've owned both and I'm quite sure that you'd be happy with the results from either - if an absolute bargain MkI came up it would probably be worth going for, but the price of the MKII (used) does seem to have dropped to a relatively low level so that's where I would put my money.

Bear in mind I've never owned a K-1, but I kept up to speed with the K-1 and the later mark II and I believe the only difference is the mark II's built-in noise reduction chip which bakes in noise reduction to the RAW files. The extent of the difference I don't know, but some purists regard this as a bad move and removing control from the photographer, with said noise reduction being rather heavy-handed. This might be an over-reaction, I don't know, but for astro there might be a bigger impact than for other genres of photography.

Personally, regardless of the brand or model, if I were to get into astro then I think I'd invest in one of the programs that offers state-of-the-art AI noise reduction and do it that way rather than in-camera. From what I've read this method of NR is the best there is and certainly better than a hardware chip from 2018. So with Pentax the mark I would be the way to go.

The baked in noise reduction does not affect base iso, it kicks in at 340 or 400iso or iso if memory serves. I am sure this is on a gradual scale as one goes up with iso.
I think it annoyed purists more so, but both K1 and K1ii are night beasts as far as light gathering capabilities and image quality is concerned, probably the best out-there.
K1ii myself but I don't do astro

Forgot too add, there's a youtube series called, Pentax Astrophograghy series by C.R Kennedy Imaging
All about the K1 on astro , 4 part series , I don't do links but should be easy to find.

Take a look at the discussions over on the other forum - I think quite a few who were of the same opinion as you changed there minds when they actually saw results. What ISO do you shoot at usually? As Flan has mentioned the NR kicks in at higher ISO settings (can't find what the starting point is off hand)!

Sounds like its the K1 for you and its significantly cheaper used and the raw files will be full to the brim.
Anything half decent will still be just under a grand.
Happy hunting, Flan

This information from dpreview could be useful:

link

Particularly look at this the 'Raw DR: ISO-invariance' image comparison tool, if you select the K1 and the K1 mark ii and look at a dark area say at ISO 640 it's apparent that what should be black looks fine in the K1 but has a purple tint with the K1 mark ii. You can play around with the ISO from 100 to 6400 and move the zoomed in area around the test image.

Quote: "In summary, given that the K-1 and K-1 II share the same sensor, we expect the K-1 II's sensor and electronics itself to be essentially ISO-invariant, like the K-1. However, noise reduction makes the K-1 II appear less ISO-invariant, but only due to forced noise reduction at higher ISOs. In fact, to retain more detail, you may be better off shooting at lower ISOs and boosting exposure to retain more detail and apply selective noise reduction yourself. Ultimately though, that's a workaround for an arguably poor decision on Pentax's part: namely, to force noise reduction - typically reserved for JPEG - onto even Raw shooters."

This purple tint is talked about and shown in this YouTube from timestamp about 4:40.

link

It seems the Mkii applies extra NR as the ISO is increased, this cannot be turned off even for RAW file!

Indeed, it's one of the things that made me choose the K1 over the K1 mkii.

I had 3 or 4 MK1's, I had one MKII.

Wasn't a fan of the noise reduction being forced on me as I always thought I could do it better in post.