Full size door shortened to 4 feet tall.

Since we build the observatory low (yes, there is plenty of headroom inside) we had to make a smaller "Hobbit" door from a full sized door.   We looked for something at a window/door salvage yard nearby but they wanted a lot of $$ to cut the door.  Home Depot had steel doors pre-hung on a frame, so that was the answer - shorten it ourselves!

Reducing the door
frame. 

The top hinges were in the right place, so those stayed.  The frame was cut down and reassembled to the 4 foot rough opening height of the door opening.  Then the fun of cutting the steel door.  To make a long story short, just look at this web site on how to do it.  Pretty much what we did, but used a chisel rather than a router.  

Door frame in place.  It's proper style to wear an "Astronomer"
t-shirt when building an observatory. 

It all fit nicely when done!  The frame was mounted and then the door fell right in place.  The doorknob holes will be filled/covered with a kick plate later.  A hole will be made for deadbolt on the top of the door - probably get an electronic keyless thing with buttons. More on that later - I'll have photos of the process when it happens. 

It fits perfectly!  

I stood on a chair, for this photo. It's a short door, but
plenty of headroom inside.