A 20 Inch Telescope By Dennis Borgman
The beginning
Aperture fever had been getting stronger by the
day. My chance to pacify this desire
came about quite suddenly in December 1992 when Ed Sczcepanski, a prolific
buyer, builder and seller of telescopes in Houston, put his home built 20"
DOB up for sale. Upon informing me of
it's availability, I immediately asked him to consider it sold and advertise no
further. I soon found out that my
speedy reply was indeed fortunate, for several Houston amateurs had heard
through the grapevine that it was for sale and would surely have snatched it up
had I not been prompt with my decision to purchase. My intentions for some time had been to purchase the optics and
build the mount, but that of course, would have meant completing construction
before I had a useable instrument.
Here, for little more than the cost of the optics, I had a useable scope
and could build my own mount at a leisurely pace. Ed's home-built scopes are seldom light-weights and this one was
no exception. The upper cage alone
weighed in at over 40 pounds! Nothing
wimpy here! The whole scope wasn't
quite ready for the one-ton club, but it was close. I sprained my back the first time I loaded it in the truck, and
decided there must be a better way.
Wheel-barrow handles and loading ramps to ease the pain of transporting
were only the beginning.
Personal background
Building and fixing things has been a passion of mine
for many years. In fact home shop
machining was my primary hobby before I became interested in Astronomy, so I
have a well equipped home shop and the basic skills to use the tools. I am a computer technician by trade and a
self taught hobby machinist and woodworker.
The design
I used Generic CADD software to do most of my design
work. CADD allows you to correct
mistakes before they are discovered after hours of construction in the
shop. I can't claim a lot of original
ideas in my finished scope, but I have diligently scoured the pages of
telescope making publications and dutifully observed the craftsmanship and
ideas of others. Through this process I
have gleaned the ideas that appealed to me most and attempted to apply them
wherever possible in my own design. So
while there are many great ideas from others in my design, the overall finished
product is my own. My primary desire
was to build a DOB that was reasonably easy to transport and assemble. Telescopes that don't meet these criteria
are seldom used, they just sit and collect dust! Secondly, I wanted a tracking telescope. Several years with a Celestron Ultima 8 had
spoiled me with the ability to study star charts and return to the scope with
the same star field still in the eyepiece. (I seemed to spend an undue amount
of time back at the charts determining how to star hop with those mirrored
images!) After talking with Andy
Saulietis about getting a copy of his 'Polar Roller' design, he informed me
that there was something better and brand new, a true alt-az drive system. This system was a 'CADD' (Computer Aided Dob
Driver) system built by Tangent Systems and purchased through Andy Saulietis of
DTG (Danciger Telescope Group). He
machined a pair of 17" worm wheels and matching steel worms to complete
the basic drive package. Finally, I
wanted a scope that was easy to maintain and align. Easy maintenance was accomplished by rugged construction and easy
alignment would be augmented with electric collimation motors for the primary
mirror and an "Easy Tilt" secondary holder.
The rebuild
First to be rebuilt was the upper cage assembly and
serrurier truss system. Both cage rings
are identical octagonal segmented solid oak, with biscuit joinery to insure
strength. After glue-up of all but
one of the eight joints, the inside radius of the ring was formed by fastening
it to a one quarter inch thick backup panel to provide a center pivot and then
band-sawing. After removing the ring
from the band-saw and the remaining backup panel, the final joint was glued and
clamped. The eight separating struts
between the two cage rings are one(1) inch diameter .062" wall aluminum
tubing with epoxied solid aluminum filler pieces where cross holes or tapped
fasteners needed to be located.
Recesses, matching the tube diameter, are milled .062" deep into
the oak cage rings to positively locate the tube struts. To provide fastening points for the spider
vanes that were rotated 90° to the eyepiece position, four cross bars were
mounted horizontally between adjacent tube struts. Short .25T x .5W x 1.5L bars fastened vertically at the center of
each of the four cross bars provided a pair of fastening holes for the threaded ends of the spider vanes. [Photo
X] A standard 20" Novak spider was
modified to handle a beefier secondary mount rod. The center piece was replaced with one containing a 5/8" x
20 TPI threaded hole. I machined the
secondary holder according to a Steve Watkins designed "Easy Tilt"
with a steel 5/8" x 20 TPI threaded shaft to match the spider.
Note: The
"Easy Tilt", for those unfamiliar with it, provides a simplified
means of adjusting the secondary mirror.
While many secondary holders use a push-pull adjusting/locking design
that can be a bear to adjust even in the daylight, Steve's design is based on a single socket head cap screw at each
of three evenly spaced radial points, with enough built in friction to
eliminate secondary movement under telescope use or transport. Belville spring washers hold the adjustment
screws under constant tension to the mounting plate fastened to the secondary
support rod. Small threaded brass balls
between compression plates allow for tilting of the portion of the holder glued
to the secondary mirror with silicone adhesive. This sounds complicated, but it is in fact quite simple. The whole design depends on friction forces
applied after the whole unit is assembled and works extremely well. In practice, the screws are adjusted with a
small hex wrench and are simply turned clockwise or counter-clockwise at each
of the three adjusting points to align the secondary mirror WHILE you are
looking through the focuser, alignment tube or autocollimator. Some nice pictures of the holder may be seen
in Sky and Telescope magazine, Vol 83, #1, January 1992, page 14.
I use a JMI 2" NGF focuser mounted to a
3/16" aluminum plate fastened to adjacent cage struts. The TELRAD mounts to a 1/16" plate
fastened between the lower oak cage ring and one of the horizontal spider mount
braces. The finder scope is an ORION
Big Eye 11 x 80 with an illuminated reticule and a 45° amici prism. A PULSE-GUIDE lights up the reticule in
short, medium intensity bursts and along with the right-side-up views provided
by the amici prism, is extremely pleasant to use.
The serrurier
truss system was rebuilt at the same time so I could use the new cage on the
original 20" rocker box. The poles
use a combination of innovations, again designed by others. I'm not sure who needs to get credit for the
following ideas, so I'll leave this up to the editor. The cage end of the poles are a ball and clamp design. The ball ends are solid aluminum and were
lathe turned with a ball turning attachment.
They are turned for a sliding fit to the truss pole tube ID and epoxied
in place. The clamp brackets each hold
two poles and have matching socket recesses machined to positively locate and
hold the ball ends. A non-removable
knob locks each clamp during telescope assembly. The lower end of each pole has a cam-lock design machined from
aluminum solid and epoxied in the truss pole tube. The cam has a total offset of about .060" and locks securely
with a quarter turn twist in closely machined steel sockets fastened in the
mirror box. The bottoms of the steel
truss pole sockets are adjustable to compensate for slight differences in their
placement and therefore allow truss tube interchangability. The construction so far was completed in
time for TSP 1993 and that is how the telescope arrived at the Prude Ranch with
it's proud new owner!
After a successful TSP trip, I turned my attention to
the design and replacement of the telescopes lower end, the mirror and rocker
boxes. Since my design of the telescope
lower end was to include a drive system, I had to plan for room to mount
17" worm drives and clutches for both altitude and azimuth. With this drive system the telescope is used
like any traditional non-tracking DOB, just grab the telescope and point
it. Initial setup is very simple and
requires only two steps. After turning
on the drive computer, point the telescope straight up by using a bubble level
and press a "zenith button".
Second, using an accurately aligned finder, point the telescope at the
North Celestial Pole and press the "NCP button". From here on, the drive system uses encoders
to feed back telescope position and tell the drive computer the precise sky
position being pointed to, and therefore allow precise determination of
altitude and azimuth drive rates.
To provide a sturdy light weight foundation, my idea
was to build the rocker box as an aluminum frame in place of the more
traditional wood. Telescope movement
friction would be controlled by clutch plates on the drive gears. With the telescope drive clutches
completely released, telescope movement should be frictionless. While the telescope would never be used with
the clutches completely released, it made sense to start frictionless and use
the clutches to provide full control of friction. Before I could complete the design, I needed to know the balance
point of the main telescope assembly.
The balance point would dictate the vertical dimensions of the rocker
frame for clearances and the placement of altitude pivot bearings. This need necessitated that the mirror box
be completed first. My mirror box
design includes ample room for electric collimation drive motors and a cooling
fan to speed equalization of primary mirror temperature to atmospheric
temperature. Oak veneer plywood panels
with solid oak corners were used for the box construction. Building close fitting joints for the 12
solid oak corners turned out to be the most challenging aspect of the mirror box
construction. I was forewarned by a
close friend and cabinet maker, that this would be difficult. Never the less I stumbled on and was for the
most part successful. I paid close
attention to precise measurements and wound up with only a couple of joints
that could have been improved upon. The
box is held together with glue only, and has shown no signs of failure at any
of the joints. The mirror box is
finished with 3 coats of semi-gloss marine polyurethane and seems to protect
the wood well against moisture. The
primary mirror cell is a slightly modified Novak unit. Electrically driven primary mirror
collimation necessitated the use of a different adjustment bolt scheme. The lower casting normally used to fasten
the cell to the bottom of the mirror box and carry the collimation screws was
discarded. The upper portion of the
cell was modified to add ball and socket extension arms to the underside of the
three main casting arms. [Photo X] A
.75" semi-spheroid socket is machined into the upper extended surface and
a .625" through hole is centered in the depression. The upper half of the socket is machined
from a smaller piece with a similar semi-spheroid depression and through hole. Three #10-32 machine screws fasten the upper
socket half to the extension arm and are used to retain and control the captive
tension on .75" brass pivot balls.
The brass balls have a .5" x 20 TPI internal thread. The 4"L x .5"D x 20 TPI adjustment
screws pass through the upper captivation plate, through the threaded brass
ball and through the extension arm. The
collimation screws turn freely in ball bearings on each end supported by a
bracket fastened to the mirror box. The
threaded collimation screws pass through the mounting bracket and bearing on
one end to provide a stub to mount a small toothed drive pulley. The motors are mounted to the side of the
collimation screws and coupled with a 3:1 precision toothed belt/pulley
system. One turn of the 20 TPI screw
will move the mirror cell assembly .050".
Coupled to a 100 step per revolution motor with a 3:1 drive reduction,
provides .00017" movement per motor step.
Close tolerances were required in the collimation screw assemblies to
minimize lateral mirror cell movement as the telescope is tilted.
The weight of the entire telescope is carried by
altitude ball bearings in the rocker frame and 1 inch stainless steel shafts
locked into aluminum plates fastened to the side panels of the mirror box. The altitude balance point turned out to be
about 1 inch above the top of the mirror box.
All the while I was building the mirror box, I was thinking about and
refining my rocker frame design. I
pre-machined many parts that I new dimensionally would not change based on the
rocker frame size. Lots of small pieces
like stainless steel pivot shafts fit to the altitude and azimuth bearings,
clutch plate assemblies made from old 1/4" thick computer disk platters,
bearing mounts, worm wheel hubs, stepper motor and worm gear mount assemblies,
etc. filled the shorter evenings and left time on weekends for the big pieces.
My original intention to use aluminum rectangular
tubing for the frame, posed another problem ... I had the MIG welder but I had
no expertise to weld aluminum. I could
cut and fit all the parts and have it professionally welded or I could choose
another construction material. With TSP
1994 right around the corner, I decided to use steel tubing instead so I had a
chance of getting the telescope back in useable condition in time for the
trip. This added significantly to the
total weight of the telescope. It
weighs 340 pounds fully assembled. (So much for light weight!) One delay led to another and finally with
only a few days left, I scheduled some additional vacation so I could work
continuously on the frame and remaining parts.
In three days the rocker frame parts were cut, fitted, and welded. A number of other parts had to be fabricated
as well, and I worked late each night to complete these. The rocker frame pivots in azimuth on a
heavy computer disk drive bearing and is further stabilized by three outrigger
bearings fitted into the steel tube "ground board". These 3 bearings ride on the underside of a
three inch wide high density polyethylene ring. This ring is fastened to the underside of the rocker frame.
[Photo X] HDP was chosen to provide a
smooth tough surface for the outrigger
bearings. This ring is about .75"
thick and proved tough to cut. I
finally found the best cutting setup was a jig saw with a very coarse blade,
running at it's slowest speed. Friction
from the blade even at the lowest cutting speed melted the HDP and welded the
kerf back together behind the blade.
Boy! Talk about lots of effort and no progress! I finally used a small trickle of water
flowing from a garden hose to keep the material cool at the cut. Cutting was still very slow and it took over
two hours to cut the inner and outer circles of the 27" diameter
ring! Saturday noon rolled around (I've
already missed my scheduled travel day to TSP) and I was ready to assemble the
telescope to the rocker frame for the first time! I can't remember the last time I was so elated by success! But wait!
The mirror box is awful close to the azimuth pivot shaft. Darn*#!@%&! It doesn't clear! I must
have missed a calculation somewhere, because I had 1/2" clearance in the
CADD drawings! ... PANIC! ... Can I cut some off the mirror box? ... Wait!...
If I shorten the azimuth shaft and slightly redesign the clutch pressure
adjustment screw it might clear!? It
works! 1/32" to spare! Piece of cake! The worm wheel/clutch assemblies were mounted and the worm wheels
temporarily clamped to the rocker frame to provide traditional DOB positioning
friction. And so it went to TSP
1994. The drive motors weren't ready
yet, no gear covers to keep out the dirt, but at least I could use the
telescope as an undriven DOB. By 7:00pm
Saturday evening I had packed all in the truck. Now for a good nights sleep! ... Man! I've got enough adrenaline pumping in me to last a lifetime! Sleep? ... The heck with it! If I leave now, I can be at TSP by Sunday
morning! I'll sleep when I get
there. And leave I did. I arrived at the Prude Ranch about 4:30 am
Sunday. The only thing that kept me
awake, was counting the deer along I-10.
If I counted 50, there must have been at least 150! This kept the adrenaline going and me
awake. TSP 1994 was enjoyed immensely
and was a much needed rest from telescope construction.
After returning from the Texas Star Party, I busied
myself with completing the stepper motor mounts. The steel worm gear as supplied by Andy Saulietis, was mounted in
a block of PVC material machined to very close worm shaft tolerances. The block of PVC was split such that all end
play of the worm could be eliminated by squeezing the two halves together
against the ends of the worm. I wanted
a spring loaded design that would maintain full worm engagement in the PVC worm
wheel during the pressures of telescope pointing induced by the friction drive
clutches. The PVC worm wheels are
rather soft, and I was afraid that it would strip the teeth if the worm wasn't
kept fully engaged. The cautious design
has apparently paid off in that the worm wheels show no signs of failure after
two years of service. The worm is kept
centered by Teflon lined guide plates that ride on the faces of the worm
wheel. The PVC worm bearing block
slides between these same guide plates.
Adjustable guides along the edges of the block control radial
movement. A heavy spring maintains
pressure on the worm bearing block to maintain constant worm engagement. The whole worm/stepper motor assembly pivots
laterally to follow any slight irregularities in the flatness or alignment of
the worm wheel/clutch assembly. This whole system has worked well, but there is
an annoying springiness in the altitude movement. During periods of gusty breezes, it is difficult to hold the
telescope stable, especially at higher powers.
Aside from this the drive system has been exceptionally pleasant to
use. How sweet it is to walk away from
your telescope for a late night snack, and return 30 minutes later to find that
faint fuzzy object still centered in the field at 480 power!
Drive enclosures came next. A band roller was designed and built to facilitate construction
of the .125" thick x 1.5" high curved walls of the drive
enclosures. The back covers are
.062" aluminum. Plexiglas front
covers were used to complete the enclosures and provide visible access to the
drive assemblies. An aluminum enclosure was also constructed for the altitude
encoder with a thin Plexiglas front cover for visual access. The azimuth
encoder is mounted inside the azimuth drive enclosure, so no separate
protection was needed. The azimuth
encoder is driven with a thin flat belt from a pulley locked to the stationary azimuth
shaft. This allowed the encoder to be mounted off to the edge of and in line
with the azimuth worm wheel. This
maintained the much needed clearance between the mirror box and the azimuth
shaft. With an identical pulley on the
encoder, one complete revolution of the rocker frame (with encoder attached)
rotates the encoder shaft one revolution.
The only piece of my design that I have yet to
complete is the microprocessor drive for the primary collimation stepper
motors. Adjustment is currently done by
simply reaching into the mirror box from the top and turning the appropriate
collimation screw. My idea for the
drive is a joy stick control that would be hand held while looking through a
collimation Cheshire. I have used a
Motorola 6805 CMOS microprocessor coupled to MOSFET drivers in several such
projects before and it shouldn't be too difficult to do. Between Club projects and work, there never
seems to be enough time to get everything done. But I guess that's why they call this a hobby!
Clear Skies and Happy Observing
Dennis Borgman
Houston, Texas