Aligning images of starfields I have two images taken within 30 minutes of each other in the same part of the sky.  They are very similar but are slightly offset due to the Earth's rotation and other factors.
I know:  the X, Y coordinates and RA/dec of the center of each image, and the pixel scale of both images (eg. 1.3 arcsec/pixel).
I've tried treating the image as though it had a linear coordinate system, but the field of view is wide enough (~12 arcmin) that this isn't really accurate.
I want to align these images so the stars overlap.  How do I determine the X,Y pixel offset of these images so I can overlay them?
 A: IRAF (Image Reduction Analysis Facility) is a free program that can help you with aligning your images of stars and many more astronomy related tasks for image analysis. You can download IRAF here.
In order to analyze your images in IRAF they will need to be in the .fits (File Transfer Image System) image format. Most CCD astronomy cameras will output images in this format. Anyway once you have IRAF installed you can use the imalign function. This function will take the coordinates of one image based on a few reference stars that you select and will shift all of the other images to match the coordinates of the master image. 
A: If all you need to do is to stack the images, then you could use some dedicated astrophotography software to achieve this. You will want to search for "image stacking astrophotography" or something similar. Here are a few which I have used which do the trick ...
DeepSkyStacker
This tool is suited to deep sky objects like galaxies and nebula. It would be well suited to your star field I would expect as it uses stars in its alignment mechanism. It's available freely. I would use it in your situation to align and stack the images. Then use Photoshop or something similar to tweak the stacked image (colours, contrast, curves, etc.)
Registax
Primarily suited to making a single image from a video file (AVI) and is therefore well suited for planetary imaging. However it can also be used to stack single images as you have in your situation. It's available freely. There are many tutorials available online, but here's one specifically about single image stacking.
Registar
This is an excellent tool for stacking your images. It doesn't work with RAW files so you will need to save them as TIFF format first. There is a 30-day trial available with saving disabled ($149USD).
