From: "Bill T." 
To: atm{at}shore.net
Reply-To: "Bill T." 


Unless this is a mirror for looking at Ronchi images, it's time to turn off
the opto-cad, and start looking at star tests.

Star test your spherical mirror.  See how the diffraction rings look
different inside and outside of focus.  That bright ring inside of focus is
the calling card of spherical aberration.  Notice how the point of sharpest
focus is a somehwat vague.

Now start doing a few minutes at a time of parabolizing strokes.  Let the
mirror cool down.  See how the inner and outer diffraction rings are now
more similar in appearance.  See how the focus is now more defined.

Keep going until the inner and outer diffraction rings are very similar,
and the focus point is very crisp.  Quit when you feel like it, but
definitely before the bright outer ring moves from inside focus to outside
focus.

Pretty soon you'll probably want to build a Foucault tester, and download
some data reduction software.

Bill T.

>  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
> >
> >
>
>


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