In general the reflecting optical telescope has a large-aperture concave mirror for gathering and focusing light from astronomical bodies. As you've known, larger the size of mirror becomes, more difficult making such mirror with sufficient accuracy. Here is an alternative method to avert such difficulty;

 

http://hecoaustralia.fortunecity.com/telescope/reflect.htm

You can do the same thing with a bunch of small, fixed mirrors instead of one big one, and they don't need to scan. This concept is seen with radiotelescope arrays.

 

here's an artist's conception of one

http://hubblesite.org/explore_astronomy/skywatch/db/99/images/large.jpg

 

and a real one

http://courses.unt.edu/hwilliams/GEOG_1710/IMAGES/radiotelescopes.jpg

wouldn't the scanning mirror need to change rotation and curvature depending on where it was? i can see problems with that. also, telescope arrays already exist and are in use today.

Why do we want larger scope ?

To collect more light. This scanning telescope will not get more light.

To get higher resolving power. The resolving power will not be bigger than the resolving power of the scanning mirror.

Also you would need a mecanism with the same precission as a mirror the size of the whole telescope.

New big size telescope are all segmented mirror.

http://astro.nineplanets.org/bigeyes.html

The "easy" way to implement this with the required accuracy would be to make a normal mirror and run the small mirror back and to across it's surface.

 

Let me know if anyone finds a point to doing that.

 

Seriously, it's going to be a lot easier to make a static mirror that has the requred accuracy than to make a set of rails to run the small mirror on which retain their accuracy under the changing load of the moving mirror. Then you have to figure out how to rotate it or you just get a line in the sky rather than a circle.