[ATM] Re: ATM A new kind of folded Newtonian

[mdholm@telerama.com]

The large, perforated, 45 degree flat style Newtonian is the same layout as the
famous Porter Turret Telescope.  The advantage, as implemented by Porter, is
that with sufficient counterweighting, a huge polar axle and some other adds and
ends, you can have a stationary eyepiece, inside a building.

One disadvantage is that you have to make a large, perforated flat to just as
high a standard as the smaller diagonal of a standard Newtonian.  No, you can't
make it to looser flatness specification and expect as good image quality.  The
fact that it is bigger doesn't change the flatness requirement.  The thing about
light, when considered as waves, is that what matters is the path length
difference over the aperture of the telescope, all the way from the star to the
image.  More than 1/4 wave P-V (a better criterion is about 1/14 wave rms) and
the image gets crappy in a hurry.  Actually, for really good images, the
criterion should be at least 2x stricter than that.

It doesn't matter what makes the path length difference.  It can be badly
figured primary mirror, primary mirror distortion caused by the cell, or by
uneven mirror temperature, unflat diagonal or folding mirror, uneven thickness
or uneven refractive index in a window or lens or uneven temperature of the air.
 Any source of path length difference contributes to image degradation.  The big
reason the Hubble Space Telescope can make such high res pictures is that it
doesn't have the uneven air temperature problem to contend with.  Once the badly
figured mirror problem was dealt with, it could do really well because all the
sources of path length difference have been minimized.

It was suggested taht the large folding diagonal mirror could tolerate some
curvature.  This isn't true. Curvature will cause astigmatism because of the 45
degree angle.

The large perforated 45 degree folding mirror will need to be about 1.5 times
bigger than the primary mirror, at least on the long axis.  This is one of the
big drawbacks of the design.  Say you have a 10 inch primary.  You will need to
make a 15 inch, or perhaps a bit more, high quality flat, and perforate it
without messing up the flatness.  That is a tall order.  For less effort, you
could make a 15 inch regular Newt and end up with 1.5 ^ 2 = 2.25 times more
light per star entering your pupil.

Mark Holm
mdholm@telerama.com
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