[ATM] The challenges

[vorblesnak@peak.org]

A simple slumped mirror of thin material actually answers a couple of
problems with big scopes.

One of the issue in a large aperture is that the primary is massive and
must have an equally massive support to function.  Then the length of the
optical system becomes an issue as designs need to be thickened to
compensate for the vibration of longer components.

Consider this.  The 16 inch mirror I just did weighed in at just over 10
lbs.  This is a fraction of the weight of even a half thickness blank. 
Scaling supports for this is a very workable project.  The focal length
was only 55.5 inches, so 'tube' length was not an issue.  But, and f6
would be eight feet long.  I have had good results building multi-sided
tubes out of rigid Styrofoam.  The two inch, when glued, is extremely
stable and very light weight.  Mounting points are glued on and bolted
through for a double attachment.

Still, the secondary must be large for such a large light cone, UNLESS,
you change the design a bit.  The 16 inch ultra thin would make a
marvelous Gregorian.  The secondary would also be a thin slumped mirror,
figured to an ellipsoid.  A tertiary small diagonal could be mounted at
the pivot point of the scope and the viewing point would never change.

Given a 55.5 inch primary fl, and a secondary with 1/3 that fl, say 18
inches, your tube would be about seven feet long, your apparent fl would
be 155 inches or about an f10.

The more curve you have in the primary slump, the stronger the glass and
the less likely you are to get flex of any sort.

David Davis
Toledo, OR

_______________________________________________
ATM mailing list http://www.atmlist.net/