Re: ATM Re: Collimating Newtonians with Laser

["Anderson, Jay" <andeja@mail.va.gov>]

Bill Fries wrote:

>In reality in laser collimation there are three reflections. From focuser
>to primary requires one reflection from secondary. The second reflection is 
>from primary back to secondary, and finally the third is secondary back to
>focuser (we hope).  If each mirror surface is 90 percent reflective then
total
>loss equals 100 percent less 72.9 percent. Total loss is 27.1 percent.

How do you come to 27.1% total loss (72.9% reflected)?  Say you buy a
primary and secondary that are rated 96% reflectivity (assume they
perform at their rating) then how much starlight is being lost?  I don't
know much about it but I would say that 4% of the light that reaches the
primary is lost and then of the light reflected an additional 4% is lost
at the secondary.  Also, is there a way to test a mirror to see what the
actual refectivity is?