Astrophotography (like anything else, especially photography-related) can be as expensive as you want.
There are several options for tracking equatorial mounts. First, there are the full computerized German eq. mounts made for telescopes. These are probably the best option, because they are intended for exactly this purpose, although they're probably more intended for use with an actual telescope and astronomy CCD camera. I've never seen one that's a good out-of-the-box DSLR astrophotography mount. The other option is a simple motorized eq. mount. They don't have the computer thing ("GoTo", etc.) that automatically points the mount at specific objects, but they do have the motors that allow the mount to track with the stars (sidereal) or sun or moon.
EDIT: Looking closer at Vixen's site, I found they have what seems to be a decent mount for astrophotography.
GP2 Photo Guider ~$1100, should work out-of-the-box.
The third option, which I have started out with, is the
Astrotrac system. My kit is mostly made from standard photography tripod components, rather than the Astrotrac pier/wedge system. I have the Manfrotto 055XPROB tripod, along with the 410 geared head (serves as the equatorial mount for precise polar alignment), the Astrotrac itself, and a ball head on top of that. The Astrotrac, once properly aligned, uses a worm gear to rotate its top half at the same rate as the stars, so the ball head, no matter where it's pointed, will track with the stars. It's fairly precise, especially for wide angle sky shots, and it's even possible to take longer focal length shots for up to a few minutes exposure.
Take a look at my setup on Flickr.
I went with the Astrotrac because it's mostly manual, very educational, and when I'm not doing astrophotography, I still have a very good tripod and photographic tripod heads to use. That geared head is fantastic for landscapes. All of the components considered, I probably spent about as much as I would have with a special motorized eq mount, but the versatility is an added benefit.
The other thing you should consider is a light pollution suppression filter. This is the next component on my list. These filters are specifically designed to cut down on the most common city lights (sodium vapor, etc) without restricting the light you want from the stars. Astronomik probably makes the best filters, and they even have filters that clip into the lens mount on your camera (assuming you have a Canon crop sensor camera). Look at either their
CLS or
UHC filters.
OPT is a good place to find them in the USA.
Getting back to your original question... Lens wise, if you get a good tracking system, your choice of lens doesn't matter as much, especially the aperture. For sharpness, you're probably going to want to stop the lens down a bit anyways. It's very hard to focus precisely (even at infinity) on a dark sky, so it's best to go with something around f/8-f/11 and take longer exposures. If you want something really wide, and it sounds like you're on a crop camera, I recommend the EF-S 10-22. It's a great lens, solid construction, and very wide - 16-35mm equivalent. It costs
$800 at B&H. Apart from that, your 15-85mm should do a fine job for you. (Very important, though - remember to turn off your stabilizer when it's on a tripod.)
The technique the professionals use is to take lots of medium-length exposures (2-5 minutes) and stack them using software like
Deep Sky Stacker (free program). I haven't used this program yet, but I'm going to. It aligns and combines several light frames (normal star photos) and dark frames (shutter or lens closed off) to cut down on sensor noise.
