• '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 jjg@princeton.edu and welcomes

discussion on portable scopes.

August 11, 1999

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