Good Morning,
Let's start off with "Photography is an exercise in compromise". We are playing a game of "wack a mole" here. To solve for A, B and C breaks. To fix A, B and C at once - all of a sudden T, V and Z pop up as problems. I sorta of wanted to start out with this, especially with Pentax.
I have been playing around quite a bit with Astro over the last few months, and it is nearly impossible to get everything in line. Go for fast lenses, loose the wide focal length. Go with the O-GPS1 to solve the wide focal length and fast aperture - you pick up blurring of the static landscape items - but pin point stars without trails (for up to 5 minutes of exposure time - and you can mask in the blurred landscape from another image taken without astrotracking).
To put everything together, focal length as wide as possible, largest aperture possible, all with the lowest ISO possible and you get the new Sigma 18-35 f1.8 - but be prepared to have your checking account dinged for about $900+. Oh yea, right now there seems to be some optic / image quality problems.
So, where does that leave us. The difference between f4 and f3.5, really is not that much. I have been doing astro with the Sigma 8-16/f4.5-5.6, Pentax 12-24/f4, and the Voigtlander 20/f3.5. Actually I like the Zeiss 25/f2.8 ZK (or Zeiss Contax 25/f2.8 the older version with a mount swap). Actually the stellar performer is the 31/f1.8 Ltd (but you give up the wider focal length). However, with the wider aperture you loose the wider focal length. The only real way to go wider and faster (or just as fast) is with the Ronkin, Bowler or Samyang (all the same lens) 10, 14, 16 at f2.8. Bringing in the versatility of zooms - adds weight and complexity to the optics and pushes the aperture smaller.
A note with the Ronkin, Bowler or Samyang lenses. Infinity focus is not at the end of the physical (mechanical) focus range, as they focus beyond infinity. So, you are always trying to focus - in the dark on something that you can't see. Essentially there are operational aspects to using the glass.
The Pentax 14/f2.8, although a bit large can be a real winner here. Probably not as wide as you want - but it has the 2.8 aperture. For Pentax, its probably the best compromise available.
Another lens to consider is the Zenitar 16/f2.8. Although a fisheye, its VERY mild on the APS-C sensor size, so you wind up with out the bulk of the fisheye effect, but gain on the fast aperture and the price is around $200 +/-.
The other approach is to move away from Pentax. Tokina has the 11-16/f2.8 for Canon, Nikon and I think Sony. Also the Ronkin, Bowler or Samyang. Nikon has the 14-24/f2.8 which is said to be the hands down best wide angle made - but lighten your check book by $2000 - just for the lens. Also, by going full frame, you gain 1 full f stop in sensor noise reduction (and $2000 to $3500 or so for bodies). Also, the Zeiss lenses in the Canon and Nikon mount go from f2.8 down to f2 - which is a bonus (also the price goes down a bit, with their availability increasing).
However you choose to solve the problem, there is no absolute perfect solution. At best its a make due....