[ATM] Dark finish for a telescope tube

[Michael Wheatley]

You would not learn much from a densitometer or a sample in a dark 
room but it should be easy to get useful information with a 
flashlight and a spot meter.

Set the flashlight on a 90 degree arc from horizontal to above the 
sample then use the spot meter on a 180 degree arc and take multiple 
readings, 10 to 170 degrees, of the surface under test.  Endless 
refinements would be possible.  Such information should already be 
available for some surfaces.  The behavior of the surface will vary 
with incident and measurement angle.

Mike Wheatley