This is the first time I played with a RAW file, the RAW produced by Canon 450D cannot be open directly from Photoshop CS2, and therefore, I use Digital Photo Professional bundled from the camera to open the file. I did nothing there but "convert and save" it to TIFF. TIFF is an lossless compression format, supporting more than 8-bit. Since 450D supports 14-bit now, so by saving it to JPG, you lost some signal already.
Then I open the TIFF from Photoshop. To my surprise, the image is tack sharp! It's just taken by the kit lens hand-held! Previously, the images are found to be less than very sharp, even with the 200mm f2.8L, now I know the reason. The JPEG file produced by the camera always leave something to be desired.
Therefore, I believe that we shall use RAW whenever we're doing something serious. And I'm very sure that astrophotography would require that absolutely. Next time when I shoot the sun or the moon, I shall try RAW instead. Deep sky imaging needs RAW for sure.