MAPUG OMT TROPHY
Occasional Mapugger of the Times
ver. 1.0, January, 1998
SECTION ONE. Optical tube assembly (OTA)
The main optical tube is made from two one-inch PVC electrical conduit
couplings epoxy-bonded into a single unit and cut to appropriate length.
(Use of conduit material eliminated the need for a lathe). Brass declination
bearing shafts are attached to the optical tube using 4-40 studs, and
the shafts are further supported by hardwood seals and pseudoshims. The
circular seals are press-fit, but the pseudoshims are epoxy-bonded to
the main tube. Two 00-96 screws pass through each pseudoshim and the underlying
optical tube to secure the optical back and primary mirror cell. Thin
steel washers (one on each shaft) maintain clearance between the OTA and
the fork mount.
The primary mirror cell is a two part epoxy-laminated hardwood disk which
also serves as the optical tube back. The mirror cell includes a mechanism
for adjusting the primary mirror's figure by tensioning. The mechanism
consists of a brass annulus located under the outer rim of the primary
mirror and a tension-locking set screw that is accessible from the rim
of the mirror cell (after removal of the cell from the optical tube).
The figure of the primary mirror is adjusted by jamming the main mirror
baffle tube (this is epoxy-bonded to the main mirror) into the mirror
cell, with motion of the perimeter of the mirror being blocked by the
brass annulus between the back of the mirror and the front of the mirror
cell. When the primary mirror deflects to the proper curvature, the tension
lock set screw is tightened to lock the mirror at the chosen deflection.
The specified brass annulus provides an average f4 focal ratio in the
primary at maximal mirror deflection. Note that, under tension-locked
conditions, the primary mirror has deflection-limited performance. The
primary mirror cell and optical tube back are press-fit to the optical
tube and secured by four 00-96 screws in the pseudoshims of the dec. axis.
Baffles for the primary and secondary mirror are brass copies of the
Meade originals, except for the absence of interior ring-stops in the
primary baffle (an oversight in ver 1.0). A 1/8 inch thick acrylic corrector
plate press-fit into a recessed rim in the optical tube supports the secondary
mirror cell and baffles. The version 1.0 corrector plate is too thick:
it causes the secondary mirror support to project beyond the rim of the
optical tube, blocking the fit of a dust cover. A thinner version 2.0
corrector will fix that. A brass retaining ring (needed to attach the
version 2.0 corrector plate) is attached to the version 1.0 plate in a
purely cosmetic role, since the plate and ring are merely press-fit into
the optical tube.
An adapter that carries the mirror holder for the 1/3-inch star diagonal
is press-fit into the central opening of the optical tube back. A ver.
1.0 brass focuser knob press-fit into the optical tube back substitutes
for the functional focuser planned for the ver. 2.0 OTA. The ver. 1.0
knob turns, but it provides frictional heating rather than focusing action.
A 1/3 inch eyepiece shell is provided in ver. 1.0 - the eyepiece lenses
will be available in ver. 2.0.
SECTION TWO. Fork mount
The arms of the fork mount are carved from a single redwood board. Custom-fit
hardwood heads are secured to the arms of the forks by 6-32 screws, accessible
from the base of the arms. Press-fit brass bearings in the heads support
the declination axis of the OTA. The drive-side bearing shaft on the OTA
receives a 4-40 lock screw for a brass friction clutch that bears against
side of the fork to lock the altitude of the OTA. The bearing shaft on
the non-driven side of the OTA receives a 4-40 lock screw for a brass
declination/altitude setting circle.
The fork assembly rotates on a polar axis formed by the shaft of a stainless
steel Allen screw that extends from the polar axis housing (in the base
of the forks) into the base of the mount. The housing contains the polar
axis end-float adjusting screw and is covered by the polar axis plate,
which is secured by two 00-96 screws. The end-float adjustment screw is
locked in place by an Allen set screw accessible from the side of the
base of the mount. Clearance between the fork assembly and the base of
the mount is maintained by a thin steel washer located on the polar axis
shaft between the two assemblies. A brass insert with 4-40 threads is
bonded in the bottom of the base of the mount to accept the central attachment
screw on the field tripod's tensioner. Brass slow-motion knobs for the
dec. and polar axes provide frictional heat in the forks to supplement
that given by the focusing knob on the OTA.
SECTION THREE. Field tripod
The field tripod consists of telescoping brass leg assemblies that are
attached to a hardwood head with brass hinges. Hinge pins made from partially
crushed brass tubing are attached to the head with epoxy resin. Hinge
hardware on the legs is custom-fabricated from brass sheet. Locks for
leg length adjustments are made from brass tubing stock and steel key
stock tapped for 0-84 adjusting screws. A tripod leg spreader and spreadlimiting
spider are assembled from brass sheet and tube stocks, fastened with hard
solder and fragments of a protruding bed-spring wire that was extracted
from my mattress with pliers one night. End pieces for the legs are press-fit
hardwood dowels and are covered by rubber caps stolen from peg-board hook-ends.
Each inner leg tube is hollow, suitable for hiding rolled-up secret messages,
fractional-carat gems, jetsam, or what-haveyou.
SECTION FOUR. Small parts
A ver. 1.0 hand controller unit serves a cosmetic function until the
ver 2.0 electronically active unit is available. Coiled electrical cords
for the declination assembly and hand controller unit are made from 50
and 30 lb test monofilament fishing line (respectively) formed around
a brass core and thermo-set in cooking oil slightly hotter than boiling
water in a frying pan on the kitchen stove.
I have a few suggestions, but operation is left up to the award-winner's
- The tripod leg-length adjustment screws should not be tightened beyond
finger-tight, lest the key stock rip from its tube.
- Torque the central tripod attachment screw only enough to firm up
the spread-limiting spider's arms. Enormous force could be generated
toward ripping the hinges from the tripod head if too much torque is
- In general, try to scale down all torquing appropriately to avoid
destructive effects, for example while collimating the finderscope.
- WARNING!! The spacer washers for the Dec. axis shafts on the OTA and
on AstroBarbie's body-mount are different thicknesses and are not interchangeable.
Severe damage to the forks could result from failure to use the thick
washers with the body-mount and the thin ones with the OTA. The thick
ones have a "B" inscribed on them to help keep this sorted
- In order to change between the OTA and the body-mount, one or the
other of the 6-32 screws that secure the fork heads needs to be removed,
and a fork head taken off. The Dec. axis lock nut and Dec. setting circle
lock nut also need to be removed temporarily.
- If it is desired to remove or replace AstroBarbie in her body-mount,
I suggest performing that operation with the body-mount removed from
the fork assembly, in order to minimize stress on the forks and tripod.
It's not strictly necessary to follow that plan, but it helps to avoid
inadvertent high stresses on the tripod hinges, since sometimes AstroBarbie
gets jammed into the body-mount rather forcibly.
- Access to the main mirror for cleaning is by removing the four 00-90
screws in the pseudoshims and pulling straight back on the optical back
- The corrector plate and secondary assembly can be removed by pushing
it out of the optical tube using a 3/4 inch dowel or broom handle inserted
from the primary-end of the optical tube. Maintain the rotational orientation
of the corrector plate in order to maintain deflection-limited optical
performance, and beware the flimsy brass retainer ring when reinserting
the corrector plate. In ver. 1.0 it is NOT necessary to remove the screws
from the retainer ring before pushing out the corrector plate assembly.