Re: ATM ronchi question

["AstroJack's Email!" <AstroJack@AVATM.com>]

Hi Hal,

At this point a picture is worth a thousand words.  If you can make a
drawing in and kind of paint program of what you see just inside and outside
of RoC it helps a lot.  What works best for me is to have an odd number of
lines (5 or seven total).  Center one of the lines on the mirror and then
draw a pic with a pencil, one of both inside and outside of RoC.  (Also
remember you are not inside or outside of focus!  I  think you meant RoC but
it is always best to be accurate.)

> used a 100 LPI . and at a slight distance inside of focus
 > there are many straight lines,

I like to use 5 lines, one in the middle and two to each side.

Right now what you describe is a semi oblate spheroid.  Your center is too
high and that makes the outside zones look low by comparison.  Remember a
good rule of thumb with the Ronchi test is you want the lines to curve
opposite the side of "RoC" you are on.  (Outside(roc) In, Inside(roc) Out)

>inside of focus there are many
>straight lines, but when i move the grating closer to roc where only 2
lines
>show, they curve inwards.and at 2 lines outside roc they curve out

As soon as you start to get very gentle slopes outward on the inside of RoC
and inward on the outside of RoC you will be headed in the right direction.
Right now I think it to be a good idea to head towards a sphere.  Then onto
a parabola.  Sounds as though you may not be far off.  I would suggest (3)
"trips arround the barrel" sessions using a "W" stroke with about 4" over
hang on the sides and the top/bottom.  The stroke should be wider at the
sides and closer together (narrower) in the central part of the mirror.
(This is if the lap is on top.)   Or if you prefer to work mirror on top
then do the narrow strokes on the edges and a wider "W" in the center.  Per
Tex. pg 90 & 91 method A.   Do you have Tex?  If so great, if not, no
problem I could scan you his example.  Myself,  I would pour a 4" lap and
use the sub-diameter method if the full size lap gives you trouble then
blend everything back with the big lap well pressed when you are close to
finished.  Remember do not use a lot of presure at this point and go
somewhat slow.  Also try to average more than 15 positions arround whatever
barrel ect. you are using.  After the 3 trips arround take another look with
Ronchi.  Remember lots of positions, little pressure and slower strokes will
produce a nice smooth surface, and will be easier to control as you get
closer to your desired figure.

  Good luck, it should go fast.  And don't over do it! Things can move very
quickly from here and 3 minutes of wrong can lead to 3 hours of fix it time.
Arg, been there!   You may want to see if the stroke I suggest sounds good
with others just to be on the safe side...

Any questions drop me a note!

Also if anyone else has other suggestions, or thinks the diagnosis of the
figure or strokes I suggested  are incorrect please let me know!

Jack Day