There are three sensor properties which matter here. You want high sensitivity to light: quantum efficiency. You want low readout noise. And you want low dark noise per second.
Avoid any Leaf or Phase One back with a Dalsa sensor. It fails on all three properties. Sensor+ (pixel binning) on some backs reduces this to two, as it halves the effective readout noise...but it's still not as low as a DSLR though.
Only those Phase One P+ backs with Kodak sensors support Xpose+ (dark noise reduction permitting truly long exposures). The P30+ would be a better choice than the P45+ because this domain is really flux-starved, and the P30+ has microlenses which roughly double its quantum efficiency over the P45+. The two Kodak sensors have the same dark current characteristics and presumably Phase One's "Xpose+" is equally effective for all sensors of a given dark current characteristic.
The Pentax 645D is basically a P30+ in terms of equal quantum efficiency and sensor size. It just divides the sensor into smaller 6 micron pixels as opposed to 6.8 microns. The Kodak sensor in the 645D has even better dark current than that in the P30+, but whether that translates into a less noisy output file depends on how well Pentax have matched Phase One's "Xpose+" technology. I have not seen a comparative test of this (and would love to).
Now whether either of these 44x33mm sensors gives you that much over a 36x24mm sensor is a moot point. The larger sensors capture more photons, but their readout noise is several times higher than a full-frame Canon or Nikon, so the shadow signal to noise is still going to be poorer in a given exposure time in a night sky photography situation - and astrophotography is all about the shadow detail.
I would say: go into medium format for this only if you want to use medium format for a lot of other types of photography as well. I've been a MF user for 20 years and it's a love of the cameras, lenses and usage feeling that keeps me in it, plus its daytime image quality - but I'd be the first to admit that my 5DII - with the same M645 lenses - murders my old Kodak DCS645M digital back for astro shots longer than about 10 seconds. Once the twilight fades, it's time to either swap the Kodak DB with a film back or take out the 5DII, or both.
To better my Kodak back I'd fancy a microlensed P+ back (P21+ or P30+), with good long exposure dark noise suppression - 2 out of 3 ain't bad, as the song goes. But that 3rd thing, readout noise, will always be a problem as long as medium format backs use CCDs, and unless that changes to CMOS - which also has lower dark current - they will always lag behind FF-DSLRs as astrophotography cameras.
Now you're probably thinking "what about all those marvellous images taken with CCDs on big telescopes?" - but 1) they are actively cooled to bring dark current to negligible levels, 2) they are read out slowly to reduce readout noise (not quite as good as CMOS, but definitely better than a DB), and 3) they maintain a high quantum efficiency by eschewing the losses in Bayer CFA filters. 2.5 out of 3 ain't bad!
Ray
