Sweet Sixteen
John Avellone

"Sweet Sixteen" is the name of a project that may result in a large club telescope for NOVAC.  Club member Bill Powers generously donated a large mirror and various other optical treasures to NOVAC.  Club president, Ed Karch, designated a "gang of sixteen" to try building a telescope from this stuff.  I am gang leader.  Other gang members, all notorious ATM nuts, are: Bob Bunge, Pete Johnson, Ed Karch, and Mike Mills.

To start with, we have a large mirror, 16" in diameter and 1.5" thick. Testing at Guy Brandenburg's mirror making session established that the mirror has a fine spherical figure and a focal length of ten feet.  The aluminum coating was dull, but the expert optical technicians at Cumberland Optical, in Marlow Heights, were able to restore it to a usable level of reflectivity.  These are good people!  Rather than accept payment for a good hour during which they applied "secret arts", they urged me to consider it their contribution to NOVAC!

At some indefinite future time, when the mirror needs recoating, the club may want to take the trouble and time to refigure it to a good parabola. For now, we intend to use it "as is".  Sort of.  A spherical mirror of this large a diameter and focal length will show aberrations when used to view stars. It may be possible to "flex" the mirror, by mechanical means, into a more nearly parabolic figure.  This method has been described in recent ATM literature, but not for this large a mirror.  Alternatively, there are optical ways to correct the images through the use of auxiliary lenses.  So, we must do some experimenting here.  Little is at risk, so far the project has only cost time and not money.

The long focal length of ten feet will allow the use of long focal length eyepieces.  This is good.  John Dobson holds that these are easier for most people to use, say at public viewing events like star parties.  However, in a standard Newtonian configuration, the eyepiece will be high off the ground. The ten foot ladder required by the observer may prove awkward in the dark. This is another area for experimentation! Alternatively, it may be possible to "fold" the optical path of the scope.

Included among the other optical treasures are some excellent quality optical flats, 6" in diameter by 1" thick.  One of these flats, aluminized and placed on the optical axis, seven feet above the mirror surface, would redirect the optical path downward.  A conventional diagonal, about 2.25" to 2.5" on minor axis, placed just a bit more than two feet down from the folding flat, would put the observer's eye just five feet above the mirror surface.  This, if it works out, is more reasonable.  Observers would require a low step stool rather than a tall ladder.

Use of a folding flat of this size also brings up some curious optical questions.  The entire center of the mirror, a zone six inches in diameter, will be obscured.  Only the outer zone of the mirror, an annulus five inches in width, will act to produce images.  This outer zone may not differ that much from the equivalent parabola of best fit to have a sensible effect on image quality, without the need to "flex" the mirror. Some calculations need to be worked out here!

So, the "gang of sixteen" has a lot of experimentation to carry out.  The club may end up with a good and unusual large telescope for public events, etc.  A first-cut at a working prototype may be ready as early as the picnic, certainly by the club star party.