18" Moonsilver III
Merit Award, Astronomer's Choice Award
RTMC 2007
Ross Sackett's amateur telescope making
Following the success of the 8" and 12.5"
Moonsilvers, I decided to push the design up to
18".  The result is Moonsilver III (second from left).  
I really didn't know if it was going to work, but I
thought it could be fun trying.  From the beginning I
knew I wanted to have an interesting finish on the
scope, so as I was building I experimented with
various finishing materials and techniques.  In the
end I decided on a multilayer guitar-like shaded
blue finish (called "blue sunburst" in the trade).
I intended Moonsilver III to share its optics with a large conventional truss
dobsonian.  Moonsilver III was to be a second “grab-and-go” mount that I
could keep in the corner of the living room and take into the yard for
spontaneous quick-look visual observing.  It had to be light, compact,
carryable, and attractive. And I didn’t want to have to break it down and
reassemble it every time I took it outside.

The design evolved from my earlier 8” and 12” travelscopes.  The 18” f/4.3
primary mirror was made by John Hall at Pegasus Optics, and the 3.1”
secondary is from Astrosystems.  Despite its overall unconventional
appearance, optically the telescope is a conventional Newtonian.  The primary
is mounted in a PLOPtimized 6-point cell, with transfer-ball edge supports at

The scope is built mainly from hardwood veneer-softwood core plywood for its
combination of stiffness and lightness, with an added finish veneer of figured
maple.  There is also some solid maple in strength-critical parts.  Most
fasteners are stainless steel.  The finish is a transparent blue dye stain with
some darker shading for depth, topcoated with polyurethane.  The total weight
is 76 pounds, with an eyepiece height of 77 inches at the zenith.

The focuser board has hidden screws for squaring the helical focuser to the
secondary mirror, and a variable iris to keep stray light from reaching the
eyepiece.  The secondary mirror is supported on a curved spider to avoid
diffraction spikes.  The oval secondary baffle is in the light path to keep it
small, but since the primary sees it edge on it does not significantly degrade
the image.

The S-curved 2” diameter, 1/16” wall aluminum pole joins to the focuser board
and to the “stock” of the mirror box with stiff semi-kinematic rail joints locked
with machine screws.  The pole was sized to flex a maximum of about 1
arcminute (1-2 Jupiter diameters) during visual tracking, but in practice usually
flexes less.  The telescope will oscillate freely for 3-4 seconds after a sharp
rap, but dampens out in one second if the hand is in its usual position on the
pole.  An aperture mask converts the scope to a 6” f/13 Herschelian for
unobstructed planetary and lunar observing.

The mirror box is decked with black velvet to cut reflections, and the velvet-
lined protective lid is held open by a magnetic catch. A 12V fan brings the
primary mirror to equilibrium with air temperature.  The two adjustable primary
collimation knobs are marked with red and blue stars (“hot” and “cold,”
respectively) to ease collimation with an assistant-- “now give me a little more
hot…” The mirror box latches to the rocker so that the scope can be carried
as a unit.  The three-point hybrid altitude bearing has a conventional
Dobsonian trunnion on one side and a half-fork pivot on the other.  A knob on
the pivot adjusts the friction to prevent the scope from shifting while changing
eyepieces.  The telescope rotates horizontally on a conventional Dobsonian
azimuth bearing.  To minimize flexure the scope is balanced to move with a
light touch.

At RTMC 2007 Moonsilver III received one of only four Merit Awards by the
judges.  The next day, following polling of the conventioneers it was voted the
Astronomer's Choice Award, a kind of best in show award.  I was deeply
My telescopes and ATM Projects