Re: ATM More Spheroidalage

[Dominic-Luc Webb molmed <Dominic.Luc-Webb@molmed.ki.se>]

On Fri, 24 Aug 2001, Anthony Stillman wrote:

   > PS  Below is a table of minimum focal ratios required for spheroidal
   > mirrors.  Differing wavefront criteria (in waves) and diameters (in
   > millimeters) are given.  Lambda is 574 nm
   >
   >        wavefront    1/10       1/8        1/4        1/2         1
   >    diameter
   >
   >      80 mm          14.0       13.0       10.3        8.2       6.5
   >     100 mm          15.0       14.0       11.1        8.8       7.0
   >     150 mm          17.2       16.0       12.7       10.1       8.0
   >     200 mm          19.0       17.6       14.0       11.1       8.8
   >     250 mm          20.4       19.0       15.0       11.9       9.5
   >     300 mm          21.7       20.1       16.0       12.7      10.1



Well, well... look what the wind blew in... Hi Anthony.

Along these lines, I have been considering the case of the
1/10th wave parabolic primary combined with a spherical
versus hyperbolic secondary in a classical Cassegrain design.
Don't recall if it was this or another poster asking about
the folded scopes, but this is of general interest. The above
values are not new to me and always seemed like pretty radical
focal ratios for most users. In general, they are not very
realistic, aside from making some kind of fashion statement.
Some of these would have tubes over 5 meters long and the
magnification would be too high for most evenings!

For a Cassegrain, there may be some sanity to this. Perhaps
a 1/10th wave parabolic primary with a lower focal ratio,
which we can all make, combined with a properly chosen
amplification sperhical secondary would turn some Newts
fairly easily into decent Cassegrains with markerdly higher
magnification. I like this for moist areas like where I am
living where we have a lot of mediocre nights and some rare
nights when much higher magnifications are possible. A simple
change of the secondary turns a widefield scope into a high
magnification scope for viewing planets. Take for instance
the 200 mm mentioned above. A parabolic 8" F/6 should
behave similar to the F/19 above by use of a 3-amplification
cass secondary. I would be be tempted to try this with a
spherical seondary.


Cheers,


Dominic

North 59 37' 30"
East  17 48' 10"