Re: [ATM] 12.5 inches BVC advice for parabolizing

[Dale Eason <atmpob@yahoo.com>]

According to Suiter in his book Star Testing
Astronomical Telescopes in the Appendix A. Other Tests
page 294 ,295

He computes the pattern for a perfect F4.5 mirror
using a 150 line ronchi screen placed at -.06 inside
COC and a pattern for a 1/2 wave under-corrected
mirror at -.050.  They are identical.  His point being
that the test is not very sensitive if you cannot tell
the difference at similar distances.  It is much more
subjective than Foucault for an F4.5 mirror.

One might argue that there is a big difference in the
position from -.06 to -.050.  and that you can make a
stage to accurately set those positions.  I submit
that once you have made a stage accurate enough to the
above, it will also be accurate enough for doing a
Foucault test that will be much more sensitive and
much less subjective. With Foucault you can also
compute the surface deviation and adjust your figuring
accordingly.

I have had two people bring me mirrors to measure that
they made using the comparative Ronchi method.  They
did not like the views so they suspected that
something was wrong.  I measured each measured at
about 1/2 wave undercorrected using Foucault.  Both
were F5 mirrors.  To me that indicates that Suiter is
correct.

Suiter also talks about his experience with a 10 inch
F6 mirror that "passed the Ronchi test but gave soft
images and failed the star test in a stunning manner."
Once Foucault tested on the bench it proved to be 1/2
wave undercorrected. 

Try using Aberator or Diffract to compute simulated
ronchi images to see if Suiter is correct.  I think
either will generate ronchi images.  Other will
correct me if I'm wrong.


--- Bill Kelley <flex@commspeed.net> wrote:

> 
> Hi,
> 
> RML wants to know if carefully measuring offsets and
> using something like 
> WinRonchi can produce a good mirror. Dale Eason says
> no dice,  use Foucault.
> 
> Zonal testing and Ronchi comparisons both have
> subjective elements.
> I've done a couple (not very fast) with WinRonchi
> comparisons and carefully 
> measured offsets. One later later tested very well
> by autocollimation. Did I 
> just get lucky?
> 
> Bill Kelley
> 
> 
> 
> > I'm very surprised.
> >
> > I tought that using ronchigrams, (with one of the
> many free software, with 
> > offsets varying lets say from 0.3" inside ROC to
> 0.7 outside...) was 
> > sufficient, and that with successive testings and
> photo-shoots and 
> > comparisons between the ronchigrams and actual
> mirror images, I could make 
> > the figure look allmost exactly like the
> ronchigrams, then I would have a 
> > good mirror...
> >
> > I also have different gratings, and I'l try to
> find the most precise by 
> > trying them at the different offsets.
> >
> > Thanks all for your continuing input...
> >
> > RML


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