Ross Sackett's amateur telescope making
My telescopes and ATM Projects
Bar spherometer
I needed a spherometer with a short bar while I was making a
4.25" mirror.  Here's one I made of 1.25 X 0.75 X 5" aluminum
bar.  The dial gage reads to 0.0001".  It sits on 1/4" artificial
sapphire balls JB Welded into conical depressions made with a
spot drill.  I null it on the granite surface plate it is resting on in the
pictures.  The first time out it hit within 0.01" the ROC of a 6" f/8
mirror with known radius.