Re: 1/3 stroke turns to be 2/3 stroke (was RE: ATM Pressure during polishing??)

[Brad Davy <bdavy@fred.net>]

Bob Prabowo wrote:
> but were not smooth. What I mean with "not smooth" is that though they
> were straight lines, the sides of the lines were not clearly defined
> (the sides were blur).
> visit my still-under-construction astronomy and telescope making page:
> http://www.geocities.com/CapeCanaveral/Hall/2670/

There is a wealth of info on ronchi testing in the archives, including a
lot of discussion about making the edges sharp, but very little of it
refers to the surface quality.  My experience has been that the ronchi
grating at the null, especially when spherical, is the best indicator of
the overall surface quality.  Very small defects in the mirror surface
will stand out.  Put the mirror at the null, then rock the grating back
and forth slightly as with the knife edge test.  You will see very
clearly any small defects.  For me this was even clearer than the knife
edge.

There is also a chance that you have the classic "dog-bisquit"
appearance from the longer (faster?) stroke.  Shorten the strokes, slow
them down, give it an hour and see what developes.  Keep things random,
and keep them turning.

Brad
bdavy@fred.net                  http://www.fred.net/bdavy