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The Sweet Sixteen Telescope
John Avellone
"Sweet Sixteen" is NOVAC's new large
club telescope. Club member Bill Powers generously donated a large (16" f/7.5)
mirror, other optics, and structural material to NOVAC. Club President, Ed
Karch, designated a "Gang of Sixteen", all notorious ATM nuts, to build a
telescope from this treasure hoard. Six months of work have resulted in an
unusual large Folded Newtonian reflector, with a special "flexing" mirror
design, on a modified Dobsonian mount. Here is the story:
The primary f/7.5 mirror has a diameter of 16" and a thickness of 1.5" .
Testing by Guy Brandenburg, at his AMU mirror grinding lab, established that
the mirror had a fine smooth spherical figure and a focal length of ten feet.
The aluminum coating was dull, but the expert optical engineer, Dwight
Cumberland, of CUMBERLAND OPTICAL, in Marlow Heights, Maryland, was able to
restore it to a high degree of reflectivity. These are good people! Rather
than charge me for the good hour's work using "secret arts", they urged me to
consider it as their contribution to NOVAC.
At some future time, when the mirror needs recoating, the club may want to
refigure it into a parabola. For now, we are using it "as is". Sort of. A
spherical mirror of this size will produce star images degraded by aberration.
However, using methods described in recent ATM literature, a special mounting
was built to mechanically distort the mirror into a more nearly parabolic
figure. It works! When leverage is applied to "flex" the mirror, the quality
of star images noticeably improves. Ours may be the largest mirror to be so
"flexed".
The "flexing cell" works by pulling down on the center of the mirror while
pushing the rim up. This deepens the curvature of the surface from spherical to
parabolic (a difference of about one wave). The outer shell of the cell is made
from a 15" automobile wheel rim (borrowed precision!). The outside edge of the
mirror is glued ("GOOP") and taped ("PROFESSIONAL QUALITY DUCT") to the lip of
the rim. A 1/8" thick rubber pad, 13" in diameter and with a 2" diameter
central hole, cut from dense foam "wet suit" material is glued to the back of
the mirror. An aluminum "puller plate", 1/2" thick by 13" in diameter, is glued
to the back of this pad. (The glue is "3 M" SCOTCH-GRIP #1357 High Performance
Contact Adhesive. It was messy to apply and tended to lump up). A long 3/8"
bolt runs down from the puller plate and thru the center hub of the wheel rim.
A spring, washer, and wing - nut on the end of the bolt apply compression on
the outer shell and tension on the puller plate to bend the mirror.
The long focal length of 10' allows the use of long focal length eyepieces.
This is good. John Dobson holds that these are easier for the general public
to use. Accordingly, a 2" diameter wide - angle eyepiece of 44 MM focal length
(gives 70 X) was built to match the telescope. However, a standard Newtonian
design with a 10' primary would put the eyepiece high off the ground. The tall
ladder required might have proven awkward for the public in the dark. Thus, for
our telescope, the top third of the optical path is folded back upon itself to
produce a "folded Newtonian" design.
The donated optics included some high quality (one twentieth wave!) 6" diameter
fused quartz optical flats. Cumberland Optical aluminized the best of these
flats. Our design places this "folding flat" 6' 8" above the primary mirror. A
2" diagonal (the only purchased optical component//ORION) is located 2' 6''
down from the flat to bring the prime focus out the side of the telescope 4' 2"
above the primary mirror. A simple push-pull 2" focuser hold the eyepiece. The
6" flat obstructs 37% of the primary's diameter and 14% of its area. These are
reasonable figures, comparable to standard SCT designs.
A single "top box" structure mounts the folding flat, diagonal, and focuser in
fixed relation to each other. The adjustable flat is held by a "T" spider of 1"
x 1/8" aluminum bar. The diagonal is fixed (after alignment) on a single 1" x
1/8" cross bar. The "top box" is built of lightweight 5 MM luan plywood. It
slides up out of a "rib cage" built of 1" aluminum angle. Eyebolts (1/4" x 20)
secure the top box to the rib cage. They telescope together for ease of
transport.
The rib cage rests on top of the heavy plywood mirror box. Eyebolts (1/4" x
20) fasten the two together. The box has a hinged lid to protect the mirror.
Magnets on the lid and the rib cage hold the lid open for observing. The
mirror cell is supported off the bottom of the box by eyebolts and springs.
Opposing thumbscrews lock the cell in position after collimation. Two massive
7" diameter machined aluminum rings bolted to the sides of the mirror box act
as lifting handles and altitude bearings. Felt lined saddles on the ends of the
vertical arms of the plywood azimuth turret carry these altitude bearings and
provide smooth motion. A sheet of 1/8" thick acrylic plastic is glued (pliobond)
on the lower side of the 1" thick plywood base of this turret. The turret
pivots on a 3/8" diameter bolt and slides on 1/2" thick Teflon slabs mounted on
the "T" shaped base built of 2" section square aluminum. The base is mounted on
5' diameter casters to facilitate moving the telescope.
"Sweet Sixteen" saw first light on 21 September, 2001. The following night, we
used it for public outreach at the NOVAC Star Party. Since then, it has been
campaigned at many local outreach events. The basic design features of
simplicity, low eyepiece height and long eye relief, together with the bright
images provided by the large primary, make it well suited to public events.
Although there is room for considerable detail improvements, the project has
turned out successfully.
This large instrument is a club asset. It is available for loan to any member
with an appropriate observing project. The only constraint is that it be
brought to all public outreach events. Interested individuals should apply to
the NOVAC Board.
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