Taking an image mosaic - how?

I would like to take a series of images of the sky in the form of a
mosaic. No gaps between the images but with only a small overlap.

Somewhere recently I saw some software that enabled the observers to
put in the details of their set up and the area to be covered and the
software calculated how best to take the mosaic.

Any ideas of where I can find out the best way of taking these images?

Many thanks