CALIBRATION OF THE KERATOMETER

Adjusting the Drums

If the readings on the calibration balls 
are different than the ball radii you must 
reset the drums.  Assuming that readings 
on all the balls are off an equal amount, 
for example each one reads 0.50 D. too 
steep, then you do the following:

Two small set screws hold the drum on 
the axis.  Using an Allen wrench 
(see photo) or small screw driver, 
depending on the instrument, loosen the 
one that is at an angle to where 42.50 D. 
is on the drum.  Then with the 42.50 D. calibration ball on the unit, focus and align the mires properly on it.  Holding the drum in this position (do not let it rotate), loosen the second set screw.  The measuring drum will now rotate on the axis without the mires images changing.  Rotate the drum so that it reads 42.50 D.  Look through the eyepiece and be sure the mires are still properly aligned. If they are, tighten the set screws being careful not to rotate the drum.  Once tightened, recheck to see that the mires are still properly aligned and the drum reads 42.50 D.

Repeat for the vertical meridian if it did not give the proper reading on the calibration ball.