Re: [ATM] Slit and source arrangment part 2.

["Bob May" <rmay@nethere.com>]

It is when the alignment between the light source and the slit
are at an angle other than with or perpendicular to the KE that
you see the astig.  The worst case for astig. is to have an angle
of 45 degrees relative to each other - I.E. when the light is 15"
down and 15" to the side of the KE viewing point.  There is where
the false astig. will be the most obvious.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: Dale Eason <atmpob@yahoo.com>
To: David Harbour <stainless_steel@suddenlink.net>;
<atm@atmlist.net>
Sent: Saturday, April 26, 2008 8:56 AM
Subject: Re: [ATM] Slit and source arrangment part 2.


>
>
> The way I remember the discussion there were two
> conflicting assertions about what happens when the
> knife and slit are separated from each other.  I
> believe that Bob May and you asserted that you would
> see false astig.
>
> Nils Olaf asserted that was not the case if the knife
> had only horizontal separation.
>
> My experiment tried to test the two assertions.  I
> know from my testing that the knife and slit had to be
> pretty far from one another before I ever saw much
> false astig. So to make it perfectly obvious in the
> photos for all to see I used a very large separation.
> Yes much larger than most anyone would use in real
> life.
>
> My analysis of that result is that yes Nils is correct
> that strictly horizontal separation will not cause
> false astig.  But that it can be difficult to get only
> horizontal separation.
>
> I had missed Jerry's post before I posted my
> experiment.  My experiment supports his post that you
> need a diagonal separation in order to get false
> astig.
>
> The slitless tester or slited tester that is described
> on most web links and has been used by you, me, and
> many others does indeed avoid the problem.
>
>
> Dale Eason
>
> --- David Harbour <stainless_steel@suddenlink.net>
> wrote:
>
> > Dale,
> >
> > Maybe you meant to say a different figure for the
> > seperation of knife edge
> > and slit; 16" is over a foot; a little trig will
> > reveal to you that this is
> > an impossible arrangment for optical testing; even
> > testing at center of
> > curvature should not allow examining any image (when
> > you null the mirror,
> > you are "cutting" an aerial IMAGE of the slit) so
> > far off the optical axis.
> > You would see enormous "false astigmatism"- and I do
> > not see any in this
> > excellent focograph of yours.
> >
> > Would you like to imagine what a star image would
> > look like, if you examined
> > it with the traditional Newtonian, eight inches from
> > its optical axis?
> >
> > R-101
> >
> > P.S.- I have never heard of such inanity before: 16
> > inches; your blade is
> > eight inches from the optical axis. Perhaps I am
> > misunderstanding your post.
> > Please elaborate again about the seperation. By the
> > way, excellent
> > focograph.
> >
>
>
>
>
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