Re: [ATM] More on aperature stops

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

To begin with, Herschellian scopes and Newtonian scopes are two
different beasties.
The Herschellian scope doesn't have a secondary to work with so
the EP necessarily needs to point to the primary itself.
Another misconception that you have is that Herschellian scopes
use a paraboloid mirror on-axis.  The truth of the matter is that
the Herschellian scope uses the same basic paraboloid surface
that the Newtonian scope uses but you only look at an off-axis
part of that paraboloid.  Putting the paraboloid with the cneter
of the paraboloid in the center of the mirror just makes the
paraboloid look at an off-axis part of the sky with the
commitment part of the coma and so forth appearing in the image
you view.  The herschellian scope gets away with using a
spherical mirror due to the longer focal lengths that such scopes
are used at where the sphere approximates the off-axis part of
the parabolic mirror that the whole proper surface would consist
of if you filled out the aperture and used a secondary to bring
the light out to the EP.
On that image of the blur, I'd more suspect that the EP is
causing the blur rather than the primary.  This can be easily
determined by watching what the blur does as you move the EP
about relative to the light coming in.  Move Mars to another part
of the view and I'll bet that the blur moves to a different part
of the view.  This sort of thing will be from the EP having
ghosting problems from internal reflections (remember that an
uncoated surface will reflect about 4% of the light!) and some
designs will put that reflected light close to focus at the eye.
If you want to see what the shape of your scope is, a knife-edge
test is a good test to do.  Put an edge at the focal plane of the
scope and point it at a bright star.  The primary will be seen
fully illuminated and using the KE to cutoff the light from that
star will do basically the Foucault Test (n it's original form
that we don't do today!) and you will see the shadows just like
you do with the Foucault Test on a spherical surface and you will
see where all of the light is coming from as well as the surface
shape and roughness of your mirror in actual usage.  Seeing
conditions will aslo be quite visible that your view is subject
to at the moment.
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
Replace the obvious words with the proper character.

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