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The illustrations above show that the Newtonian offers
the smallest RMS spot size. The f8 Newtonian also has, by far,
the smallest central obstruction. The two Cassegrain types require
secondaries that equal 37.5% central obstruction. The Newtonian
only requires a secondary that is 20.1% central obstruction.
Compounding the central obstruction problem for an open truss
Cassegrain is the requirement for additional baffling coming
off the secondary. For the Cassegrain's this puts their final
central obstruction at 46.9%. The Newtonian's secondary diverts
the focuser position to a perpindicular arrangement and therefore
requires little to no additional baffling on the secondary itself.
The area behind the secondary, as viewed from the focuser position,
needs to be as dark and baffled as possible but this area can
be outside the optical path. |
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Because the secondary on the Newtonian is so much
smaller and lighter, it doesn't require spider vanes as thick
as those needed for the larger secondaries of the Cassegrain
types. Thinner vanes means slightly less pronounced diffraction
spikes. This is a very small advantage for the Newtonian, more
a point of difference between such systems. |
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The Newtonian is far easier to both collimate and
hold collimation than the Cassegrain styles with their f3 primaries. |
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The parabolic primary mirror on the Newtonian is one
of the least expensive mirror figures to have produced. The secondary
is also a "simple" flat. From a cost/optics standpoint
the Newtonian is by far the least expensive route. |
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The downside to the Newtonian in this focal ratio
and size is its length. At 160" of focal length the telescope
itself would be roughly 13 feet long. The other downside to the
Newtonian is the position of the focuser. For visual use, especially
at zenith, the focuser could only be reached with a tall ladder.
This is less than ideal for public use. The Cassegrain design
offers a lower/more accessible focuser location. |
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There are numerous other focuser locations for both
Cassegrains and Newtonians designs. A folded Newtonian can lower
the height of the focuser position. Cassegrains can use a Nasmyth
focuser position. Combining an alt/az mount with a Nasmyth focuser
position on a Cassegrain can yield a fixed focuser height that
only rotates in azimuth, not altitude. For imaging purposes though
the latter design would need a field derotator. |
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Specifications for the 20" f8 Newtonian: |
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