To: atm{at}shore.net
From: "Matthew L. Brown" 
Reply-To: "Matthew L. Brown" 


Pieter,
  This is the 'how good is good enough' question.
  If you leave it spherical, your wavefront error will be about 1/2
wave.   You will be able to notice this.  The image won't be really
sharp.  You'll be able to make out the rings of Saturn, but won't be able
to see Cassini's division.

Theory says that 1/4 wave is adequate.  My brother can't see the faults in
a 1/4 wave scope, but I can.  Then again, he can hear inadequacies in my
expensive audio equipment that I cannot hear.   I am certain that both of
us can see the faults of a 1/2 wave scope.  You owe it to yourself to get
it at least 1/4 wave accurate.

The Non Fussy Method
A knife edge tester is not hard to make.  The only accurate thing you need
is a 5mm screw.
Cut out a couder mask -- you don't have to worry about using specific
radii, as long as the pairs of holes are centered and of the same inner and
outer diameter (to 0.25mm or so) and measure them.  Use the geometric mean
of the ID and OD for calculations (download Sixtests or Tex for free to do
the analysis).  My lamp is a frosted 7 watt lamp.  My slit is two razor
blades 0.5 mm apart.  My knife edge is a piece of sheet metal.  Do 10
minutes of longish strokes (varying between 1/3 to 3/4 diameter), mirror on
top.  This will concentrate wear in the center area, depressing it toward a
parabola.  Let it sit overnight.  Measure it the next day.  Take 4
measurements of each zone and average.  Plug the numbers into Sixtests. 
Stop when it says Strehl >0.8.  Or stop when Tex says 1/4 wave.   Press
the lap for an hour and repeat if you aren't there yet.  This
will make a 'good enough' mirror.  Not great.  But my brother won't be able
to tell the difference.

=Matt




>  Hi Guys
>  Being an ABSOLUTE novice atm and meddling with something totally foreign to
>  me……MODAS, I am wondering if I should parabolize my mirror?
>  With that said here's the facts:
>  My 150mm plate glass mirror with a focal point of 980mm (?COC - where the
>  sunspot is in-focus when testing with wet mirror, sun and wall). MODAS
>gives
>a f stop of 6.53.
>
>Now, if I choose spherical in the easy design option it spews out these
>beautiful concentric spot diagrams and with parabolized its skews things
>terribly or is it terrible….?.
>
>The thing is, even after the tutorial reading I don't know which is better
>or worse and I understand about 10% of what is going on in the program.
>
>I am in the finishing stages of polishing ( after discovering if you make
>your pitch in winter [ave. 16deg. C] it doesn't work so well in summer [ave.
>35 deg. C]) and have kept a nice sphere according to Ronchi test and will
>love to eliminate figuring.
>
>Please advice
> >
> >

--- BBBS/NT v4.00 MP