First, know that I am cheap! OK now thats out of the way I am thinking of the 500mm mirror lens on ebay. It is inexpensive an has the range expecially with a 2x converter. On a side note is there a web site or application that can tell me what is in the sky at what time to photograph? Any help...
The first thing you need to do is decide whether you wish to take pictures of stars, planets, the moonrise (or moonset) against a landscape, or star trails.
The easiest thing by far is moon pictures and your 500mm lens should do pretty well here. The nighttime sky is by its very nature quite dim and one of those f5.6 500mm lenses would be less than ideal. Most stuff will need an f2.8 or lower lens to capture good images. But the moon is BRIGHT and that lens should do fairly well.
If you want to take pictures in which planets are anything more recognizable than bright dots, you're going to need a telescope. Oddly enough, you can take surprisingly good shots by simply aming your camera straight down the eyepiece. This takes a little practice, so prepare to be a bit frustrated at first. A T-mount will allow you to connect your SLR directly to the telescope and this is the best bet for nice long-exposure shots using the telescope's guidance system.
As with anything else, you get what you pay for. That little 114mm (4.5") telescope at WalMart will actually provide some pretty decent views, but for really NICE images you're going to have to have a nice telescope. Remember that the size of the scope matters; you will not see any colors in the stars until you use a scope at least 10" in diameter. The bigger the scope, the faster your shutter times can be and/or the lower ISO which can be used. Check the net for a local astronomy club which might have telescopes for use to members. The Museum Of Science and Industry in Tampa has such a club with several scopes (one of them 24'!) which are set up on clear Friday and Saturday nights so the general public can use them.
If what you want is star trails on a long exposure, your best bet is actually a wide angle lens, so you can capture more of the show. It should go without saying that ANY of these shots will require a tripod and a remote shutter release.
Some folks will try to discourage you from buying that 500mm mirror lens because of its very limited usefulness, but it can also be a lot of fun to play with. Remember that you can purchase one brand new for around a hundred bucks though. Check the ads in the back of Shutterbug or most other photo mags. But don't expect to get pix in which you can discern the bands of Jupiter or the rings of Saturn with such a lens.
Sunrise and sunset are particularly good times for shooting the sky. Follow the link to a shot I took of the Evening Star on the evening of the Winter Solstice last year. In the original at 100% magnification, you can actually see that Venus is a crescent. Though this image was taken with a 210mm lens, that's about as much detail as you're going to get with any camera-mounted lens. If you want more, your best bet is to find a way to mount your camera to a large telescope.
Don't forget to have fun; the night sky is glorious - particularly in the winter!