I believe that you still have some studying to do in order to learn about what you are trying to do. Regardless of the camera you may be using, especially for astrophotography, you will probably need to do dark frame subtraction. DFS is a technique originated by astronomers to characterize their electronic sensors, and remove internal noise generated by them in the process of taking images. DFS can be done in a number of ways.
You can eliminate the source of the noise before it affects the image. Heat is the primary culprit. You can cool the sensor with various items like liquid nitrogen, but that really is not practical. Or you can take a "noise" image, by taking an image of nothing (with the lens cap on). That is essentially what Pentax does with DFS. You take your regular image, then if the shutter duration is over 9 seconds, it takes a "second" image, and then mathematically subtracts the value of each of the pixels of the second image from the first. The second image represents the mapping of the noise generated by the heat of the first image's processing.
The primary argument you have is when and how this is done. Pentax for the K20 does it after each frame automatically. Other camera manufacturers let you do it when ever, however this will lead to inaccuracies (taking too little noise or removing too much noise). The longer you wait to take the DFS frame, the more differences you will have. For instance if you take the DFS frame first and then 30 frames over say 5 hours, will the DFS frame accurately characterize the noise over all the rest of the images? How about taking it after wards. In the after case, the DFS will exhibit too much noise for the first few images. So every technique is a compromise to some extent. Actually, the process Pentax uses is the most accurate, however it may not be the most convent and may be a case of overkill depending upon use.
Not performing DFS would be the worst of all possibilities. An alternative would be applying some noise reduction, however the noise reduction algorithms are general in nature and not specific to your sensor. In this situation DFS would be the best. I wrote overkill previously in the sense that if your are out doing quite a few images in a row, it would be reasonable to probably do a DFS image every 7 to 15 frames, so that this could be used to subtract off the noise in post processing. However, you need to remember to actually take the DFS image, and then to actually use the DFS image afterwards to remove the noise.
With respect to light pollution, you can probably drive an hour or so (depending on where you live) and find a place that has potentially less light pollution. Here is a light pollution map...
For the best star trails, you should also eliminate the moon by only going out on a moonless night.