Sky photography through compositing images in software? I've read about and seen some rather sophisticated rigs for night sky photography that allow for ultra-long-exposure photos by counteracting the rotation of the sky by rotating the camera with it. The problem is mechanics for such rigs is rather complex, expensive and not entirely within reach of an amateur.

I was considering a different approach: 


Set the camera on a firm tripod or whatever, leaving it motionlessly.
Use intervalometer in the camera firmware to take multiple photos of moderately long exposure, short enough that the stars don't blur into lines.
Save each as RAW so that the trace amounts of light captured in each of them don't get lost.
Either using more bright stars as markers, or even calculating the misalignment of the photos basing on movement of the sky and lens geometry composite the images in software, to obtain good images of the night sky.


Obviously I wouldn't get very far with high zoom images, as the field of vision would shift away long before I'd get enough light in for a good photo, but obtaining a photo of a fair section of the sky from a relatively wide angle lens should be doable. 

Is such approach used or even viable? Is there such software "out there" already or would I need to write my own from scratch?
What you are talking about is making a 'mosaic'. This technique is commonly used in astronomy to stitch together many images that are off-set from each other of some (apparently) large astronomical object (e.g., a star cluster or a nearby galaxy). In addition to covering a large field of view, making a mosaic generally also improves signal to noise where the images overlap. 

One way to realign the images is to uses cross-correlation to find the optimal shift for consecutive images in order to align them. This can be done in common data analysis languages such as python (free), matlab or IDL (not free).

I have not used black-box software to do this, but here is a list of programs that might help: http://astronomyonline.org/AstronomySoftware.asp#CCD_Control_and_Imaging_Software:_

Hopefully some astrophotographers here can recommend some software.