Re: ATM Refractor building questions

[Anthony Stillman <atmer@flash.net>]

>...optical flat is one of the hardest thing...usually requires three blank

I thought this too, I was wrong.  Flats, at least at the level the ATM is
concern with, arn't any harder than any other curve.  As for the three flat
test, well, the purpose of using three flats to test is to avoid the need
for a good reference flat.  If you have a good reference flat or can borrow
one, you don't need the other two pieces of glass, just a tool.

Just to be informational, the three flat test for power goes like this.

Test surface A against surface B, count the fringes, call this no. AB
Test surface B aginast surface C, count the fringes, call this no. BC
Test surface C against surface A, count the fringes, call this no. CA

A+B=AB
B+C=BC
C+A=CA

   A+B
 - B+C
_______
   A-C


   A-C
 + C+A
_______
    2A

therefore   A=(AB-BC+CA)/2
            B=AB-A
            C=CA-A

For those who don't recognize this, its straight out of ATM I, but from
memory, so please give me a break.  Do I have to mention that SA holds the
copyright?
Also, remember one fringe is half a wave of whatever light your using.


>...(fold) flat...(for) refractors...can be a little
>convex or concave unless it is being used at an appreciable angle.
>Don

Power in a fold mirror introduces stig.  I've seen a few diagonals in Newts
which were responciple for more than just a little stig.  As Don points
out, for small power and small angles this isn't relavent.  It is possible
to make use of this stig to correct for residual stig in the rest of the
system either from the design or from manufacturing errors.  I plan to do
this with the fold flat of my off axis solar telescope.  Also, this is done
in the tri-sheep sprinkler.

Anthony