a flying dobsonian jjgelfand.net/telescope/gelfandsandt.pdf · 2015-07-30 · telescope techniques...
TRANSCRIPT
• 'W"*.*
Y DESIRE FOR A L A R G E T E L E -
scope dates back to the
early 1970s when 1 was
work ing on spectroscopic
instrumentation at the M c M a t h Solar
telescope at Kitt Peak National Observa
tory in A r i z o n a . Th i s 60- inch reflector is
sometimes used at night, and I was able
to use it briefly to cruise the s u m m e r sky.
Needless to say, the views were stunning.
T h e Veil Nebula nearly knocked me over
and the M i l k y Way was beyond spectacu
lar. I knew right then that I had acquired
a case o f aperture fever that was going to
last the rest o f m y life.
I soon purchased a 17-inch Coulter
Dobsonian and on the day it arrived, I ea
gerly awaited sunset and the splendors of
the night sky. Unfortunately, disappoint
ment followed darkness. None o f the
views matched the magnificence of what I
had seen from Kitt Peak. Bathed in the
perpetual twilight of big-city light pol lu
tion, even the majestic A n d r o m e d a
Galaxy appeared as little more than a
bright spot in the middle of a wisp of
nebulosity. It was apparent that aperture
alone was not enough to treat my fever —
a dark sky was also necessary. M y travels
had convinced me that a suitable site
should be at least 125 miles from the
nearest metropolis, 50 miles from the
nearest town, and 10 miles from the near
est outdoor i l lumination o f any kind.
Clearly, I was going to need a scope capa
ble o f withstanding the rigors of air travel.
The Scope in the Suitcase M y first attempt at a travel-scope result
e d in a 16-inch Dobson ian that fit into a
shipping container measuring 24-inches
o n each side, a ski bag full o f truss tubes,
and a box for the p r i m a r y mirror sized
to fit under an airplane seat. It worked
well but the airlines took one look at the
9 0 - p o u n d shipping container and levied
Ji telescope techniques
A Flying Dobsonian j The sky is no limit for this flight-ready 16-inch Dobsonian with its light
weight and compact design. By Jack Geifand
Jack Geifand (above, left), ready for another observing trip. His entire 16-inch Dobsonian telescope
fits into two cases. After a journey by air and over dust-covered roads, Gelfand's scope (above)
awaits the pristine darkness of an Arizona night. The desire to travel to dark-sky sites such as this
inspired the telescope's design. Unless otherwise indicated, photographs courtesy the author.
Sky&Telescope A p r i l ! 999 123
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Gelfand's Flying Dobsonian can be assembled in the field in less than 10 minutes. Once the pieces are unpacked, the mirror box assembly goes together and is placed into the rocker box. Next, the eight telescoping truss tubes are attached to the mirror box and the diagonal cage. After refining the collimation, Gelfand is set for a night of observing.
a $50 charge for oversized baggage. Th i s
eventually led me to a very important
discovery: airlines will ship just about
anything for free so long as it looks like a
suitcase.
After visiting luggage stores I p u r
chased a 27-by-21-by-10-inch suitcase.
T h e challenge became to fill it with as
large a telescope as 1 could. Drawing o n
the lightweight compact designs of other
telescope makers, I began to formulate
m y telescope's configuration.
I wanted to bui ld a portable scope with
a 14- to 18-inch aperture, but even the
use of lightweight construction materials
resulted in a 60- to 80-pound instrument.
Putting the entire scope in a single piece
of luggage would make it too heavy for
one person to easily handle. 1 decided to
carry the mirror separately as I had in m y
prototype design. If 1 could find a way to
store the truss tubes in the main suitcase,
the unwieldy ski bag could be eliminated.
W i t h such an arrangement 1 could travel
mmm
with only two cases and a backpack for
clothes. Ultimately, this became the con
figuration 1 settled upon.
The two main components o f my Fly
ing Dobsonian are a mirror case and a
suitcase that contains the bulk o f the tele
scope's components. T h e y weigh 40 and
45 pounds, respectively. T h e telescope's
mirror box is composed of two 6- inch-
high pieces. T h e bot tom is the m i r r o r
case while the top part is stored in the
suitcase. The mirror case contains the 16-
inch-diameter, 2-inch-thick, f/4.8 p r i m a
ry mirror made by Galaxy Optics , and its
18-point flotation cell. T h e top cover o f
the mirror case is made of / :- inch birch
plywood and also serves as the g r o u n d
board for the rocker-box assembly. In ad
dition to the top half o f the mirror box,
the suitcase holds the rocker box and the
diagonal cage nested into each other, as
well as the side bearings, eight telescoping
truss poles, and a black nylon shroud.
The Diagonal Cage and Mirror Box A carefully designed diagonal cage is the
secret to successfully bui ld ing a funct ion
al portable scope. There are several
points to consider. First, the thickness o f
the diagonal cage is strictly l imited. It
could be only 5 inches thick if I hoped to
fit it into my suitcase. W i t h the required
Above: The Flying Dobsonian unpacked and ready for assembly. On the left are the nesting pieces that make up the rocker box, the top half of the mirror box, and the diagonal cage, which fit together in the suitcase along with the side bearings and collapsed truss tubes. On the right is the case that contains the primary mirror in its cell. Its lid provides the ground board for the rocker box assembly — note the three Teflon pads.
Left: These two cases hold an entire 16-inch telescope. The one on the right holds the primary mirror in Its cell, while the suitcase contains the rest of the telescope.
2.6-inch secondary mirror , there isn't
m u c h r o o m left for the spider vanes and
diagonal holder. A typical diagonal
m o u n t i n g would d e m a n d a diagonal
cage 9 to 12 inches thick. Furthermore ,
most secondary holders have adjustment
screws that work their way loose dur ing
transit. To solve these problems, I m o d i
fied a Novak diagonal holder to create a
compact three-vane spider with vanes
that pass through the cyl indrical sec
ondary holder. Al l the adjustable parts
consist o f flat surfaces which rotate or
slide against each other, making it possi
ble to securely tighten them so they do
not vibrate loose. Also, the diagonal m i r
ror must be mounted securely to protect
it from rough handling. I glued the sec
ondary mirror in place with a bead o f
silicone sealant around the back.
T h e diagonal cage itself is an octago
nal structure 18 inches across and weigh
ing just under three pounds without the
eyepiece and finder. Because it must be
posit ioned far from the telescope's bal
ance point, the weight o f this structure
must be kept to an absolute m i n i m u m .
Every extra ounce in the diagonal cage
shifts the balance point farther forward
and increases the height o f the rocker
box. T h e height and weight o f the diago
nal cage indirectly determine the d i m e n
sions o f the entire instrument.
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Anatomy of a Flying Dobsonian
Diagonal cage Left: Jack Gelfand's Flying Dobsonian
might appear unconventional, but it in
cludes all the major components of John
Dobson's original design. Righit: The in
side of the carry-on mirror box. The mir
ror clamp is retracted by turning the two
wing screws on top of the box. Also visi
ble are the strap supporting the mirror in
the flotation cell and two of the latches
that attach the mirror box to its top half.
Low-profile rocker box
At the other end of the tube, the mir
ror box measures 18 by 18 by 6 inches
and is made of / : - inch birch plywood.
Inside, the m i r r o r is supported by an 18-
point flotation cell and a 2-inch-wide
strip o f automotive seatbelt webbing se
cured at the ends. I considered gluing the
m i r r o r directly to the flotation supports
to insure al ignment was maintained dur
ing transport, but wanted to keep a true
flotation mechanism. Instead, I designed
a c lamp using a V-shaped block to ho ld
the m i r r o r dur ing transport. T h e block is
fastened to the inside o f the box and
pushes the m i r r o r against the webbing
by tightening a set o f
thumbscrews. There is a
second clamp at the bot
tom of the box that push
es against the front r i m of
the mirror dur ing transit.
A major feature of the
Flying Dobsonian is the
use of telescoping truss
tubes. Due to the low-pro-
tile mirror box and the d i -
.̂ glll agonal cage, the eight truss
tubes need to be 58-inches
long but must collapse
down to about 20 inches
to fit inside the suitcase. Photographic
tr ipod legs seemed like a good choice, but
many are too short. Instead, I used Bogen
#3006 monopods . They are light, rigid,
and relatively inexpensive (under $25 at
discount photo-supply stores). There are
a number of alternatives I could have
used, but I wanted the truss tubes to be
telescoping. Otherwise, if each pole is
broken into three pieces, the bundle of 24
tubes would eat up a lot of storage space.
T h e top end of each m o n o p o d was
heated with a torch and flattened in a
vice before being fitted with the captive,
'/4-turn electronic-panel fasteners needed
to connect the poles to the diagonal cage.
These fasteners are lightweight and easily
operated even when wearing gloves. T h e
bottom ends of the m o n o p o d legs were
turned on a lathe to fit into the Tectron
c lamping blocks used in the mirror box.
In an effort to reduce weight as m u c h
as possible, composite construction mate
rials and techniques were used through
out. I built the side bearings and rocker
box by sandwiching a Styrofoam core be
tween layers of thin plywood with ordi
nary wood glue as an adhesive. The best
results were achieved by cutting the parts
/2-inch oversize and t r imming the fin
ished piece to the correct size once the
glue had set. The sides of the diagonal
cage, parts of the pr imary mirror flotation
cell, and the vanes for the secondary-
mirror holder were fabricated from car
bon-fiber-composite panels available from
Aerospace Composite Products. They sup
ply panels up to '/: inch thick using either
foam, Nomex, or balsa cores. These panels
come in a wide variety of sizes and pro
duce telescope elements that are both very
light and rigid. This material can be cut
and drilled like plywood. However, if you
are using screws to fasten anything to the
panels you must reinforce the screw holes
by first applying a few drops of a very thin
epoxy to the edges of the holes. T h e epoxy
soaks into the core material and enables
the panel to withstand the compression
force exerted by the screw.
In the Field Because I used captive hardware — fasten
ers that remain attached to the telescope
parts even when the instrument is disas-
Many of the telescope's final dimensions are
governed by the size and weight of its diago
nal cage. The diagonal vanes pass through
the body of the modified Novak holder. The
slot visible on the side of the holder allows
access to the mirror tilt screw for collimation
adjustment. A sliding filter holder, which can
accommodate up to four 2-inch filters, is visi
ble beneath the focuser.
Bits and Pieces The following suppliers were used to obtain the
hardware and materials used in this project.
Composite panels, fiberglass sheets:
Aerospace Composite Products
14210 Doolittle Dr.
San LeandraCA 94577; 510-352-2022
http://www.acp-composites.com
Aircraft-grade birch plywood, Styrofoam and
epoxy:
Aircraft Spruce and Specialty Company
P.O. Box 424
Fullerton, CA 92632; 800-824-1930
Plastic cabinet latches, plastic thumbscrew
knobs, V4-turn fasteners and Teflon sheets:
McMaster-Carr
RO. Box 440
New Brunswick, NJ 08903-0440; 732-329-3200^
sembled — field assembly proceeds very
quickly. M y personal record is six minutes
from suitcase to sky — in the dark! Some
minor coll imation adjustment is usually
required. T h e use o f telescoping truss
tubes greatly simplifies the procedure. For
rough collimation, the mirror's dust cover
has a target cross that coincides with the
mirror's center. After the telescope is as
sembled, I loosen the top clamp on each
m o n o p o d and adjust the position of the
secondary cage until the cross is centered
in a collimating eyepiece. Next, I remove
the dust cover, unclamp the mirror, and
give the alignment a final touchup using
the mirror mount screws.
At a dark observing site, galaxies and
nebulae in my 16-inch appear as they do
in photographs. O n e can see all o f the
detail in M 3 1 , inc luding its spectacular
dust lanes, H II regions, and faint g lobu
lar clusters. Notorious ly difficult objects
such as the Horsehead Nebula are readily
visible while the Veil Nebula resolves into
strands of nebulosity twisting a r o u n d
each other like stretched cotton candy.
Most spectacular to me are the swarms
of galaxies in Virgo and C o m a Berenices,
Us ing m y 3 2 - m m wide-field eyepiece,
"island universes" wander into view by
the score.
Al though this instrument has brought
me many nights o f enjoyment, I still con
sider it a work in process with a number
of improvements possible. For example,
the mirror box is a bit heavy for carry-on
luggage. By rebui lding it with foam-core
composites and using a thinner, light
weight mirror, the box could be more
manageable. Reducing the n u m b e r o f
truss tubes is another possibility. I have
experimented with using only six truss
tubes, and there are designs that call for
as few as two large-diameter truss tubes.
Other telescope configurations are also
conceivable. I believe that two large suit
cases alone could accommodate a 20-inch
t h i n - m i r r o r Dobson ian or even a 13-inch
Dobson ian binocular-scope for airplane
travel. W i t h lightweight designs, the sky
really is not the l imit. ^
JACK G E L F A N D IS a research, scientist in the
Psychology Department at Princeton Univer
sity. When not roaming throi4gh the moun
tains of the Southwest, Jack likes to sail and
explore nature with his four-year-old daugh
ter. He wishes to acknowledge technical ad\Hce
and encouragement from Jim Watson, Kirk
Alexander, and Roger Thorpe, and the photo
graphic expertise of Bob Matthews. Check his
Web site, www.princeton.edu/jjg/astro.html,
for further details. Geifand can be reached via
e-mail at [email protected] and welcomes
discussion on portable scopes.
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