Astronomy - 06.2019

16 ASTRONOMY • JUNE 2019

O

ver the years, I’ve

always been an

advocate for the

small-aperture

telescope, tout-

ing its remarkable capabilities

in my writings and through

talks at astronomy conven-

tions and club meetings.

During my salad days as an

amateur astronomer, back in

the 1970s, my telescopic arse-

nal was made up of a 3-inch

f/10 ref lector and a 2.4-inch

f/11 refractor. Modest in size

though they were, these high

focal ratio instruments were

remarkably capable — espe-

cially on double stars, my

favorite night-sky targets.

In the early 1980s, I took

a giant leap and purchased a

13.1-inch Dobsonian-mounted

reflector. This “fast” (f/4.5)

light-gobbling beast revealed

deep-sky wonders well beyond

the grasp of my little scopes.

But it didn’t do well with dou-

ble stars. The bright binary

star Castor, whose component

stars were separated by 2" at

the time, was resolvable in

the 3-inch, but not so in the

13.1-inch. Instead of two sepa-

rate stellar images, my eye was

greeted by an elongated, scin-

tillating blob of light.

Fortunately, the big scope

came with directions to make

an off-axis mask that would

convert it into an unob-

structed 5-inch f/12 long-focus

scope that promised to yield

sharper stellar images. When I

placed a cardboard prototype

over the front of the tube and

pointed the scope at Castor,

the fuzzy blob was indeed

transformed into two

beautifully separated stars.

Encouraged by my success, I

fashioned a more permanent

model out of ⅛-inch plywood.

A few years ago, I sold the

13.1-inch Dobsonian (reluc-

tantly!) for an easier-to-handle

10-inch f/5 instrument.

Although there were no direc-

tions for making an off-axis

mask, I used my previous

experience to cobble together a

crude prototype in less than an

hour. To do the same, all you

need is a sheet of corrugated

cardboard greater in diameter

than the telescope tube, a pen-

cil, a hobby knife, a tape mea-

sure, and a roll of duct tape.

Here’s the process:

1. To make sure the mask is
   the correct size, hold the card-
   board against the top of the
   tube and trace out a circle that
   matches the telescope’s out-
   side diameter.


2. Use the tape measure
   to determine the diameter of
   the largest circle that will fit
   between the secondary mirror
   and support vanes. I found
   that to be a 4-inch diameter
   hole for my 10-inch scope.


3. Use a compass (or an
   appropriately sized can) to
   trace out the opening for the
   mask. To compensate for the

OBSERVINGBASICS

BY GLENN CHAPLE

Make a mask

for double stars

Some stellar pairs are too bright to separate with a large

scope. An off-axis mask can help you easily split them.

thickness of the telescope tube,

place the opening a half-inch

from the edge of the mask.

4. With the hobby knife,
   cut out the aperture mask and
   the opening.


5. Attach the mask to the
   telescope with a few strips of
   duct tape, and you’re ready to
   head outside and use it.
   On an evening of fair seeing
   conditions, I tested the mask
   on the striking near-twin dou-
   ble star Epsilon (ε) Arietis,
   whose diamondlike magnitude
   5.2 and 5.6 component stars
   were 1.3" apart. To my scope’s
   credit, the pair was reasonably

split at 212x without the mask.

But with the mask taped in

place, I noticed a slight

improvement. Although the

seeing conditions didn’t war-

rant it, I decided to go for

broke with a 3.2mm eyepiece

that yielded a magnification

of 397x. This time, the view

using the mask was a definite

improvement over what I saw

without it.

There are limitations to a

“new” masked scope, however.

For me, the theoretical resolu-

tion limit was reduced from

0.5" for my 10-inch scope to

a little over 1" for its 4-inch

alter ego. Still, it allowed me

to shift my attention from

faint fuzzies to close double

stars without having to put

the 10-inch away and bring

out my 3.5-inch f/11 refractor.

I made my off-axis mask

last January when Epsilon

Arietis was conveniently

located in the evening sky. If

you make one now, you might

want to test it on Mu (μ)

Draconis (mags 5.7 and 5.7,

sep. 2.3"), Xi (ξ) Ursae Majoris

(4.3 and 4.8, 1.9"), or the clos-

est components to the neat

triple star Xi (ξ) Scorpii (4.

and 5.2, 1.1"), whose third

member (mag 7.3) is 7.6" away.

Once you’ve made a work-

able prototype, you can move

on to a more durable model. As

mentioned, I made mine out of

⅛-inch plywood, then spray-

painted it f lat black on the

inside, and secured it to the

main scope with small metal

L-brackets. I’ll leave it to you to

come up with a final design of

your own once you’ve created a

workable prototype.

Questions, comments, or

suggestions? Email me at

[email protected]. Next

month: We honor an event

that happened 50 years ago.

Clear skies!

BROWSE THE “OBSERVING BASICS” ARCHIVE AT http://www.Astronomy.com/Chaple.

Limiting the brightness

of double stars with an off-

axis mask can significantly

help you separate otherwise

indistinguishable stars. The two

large, bright stars of Castor (which

is really six stars that appear as one)

are seen in this sketch. JEREMY PEREZ

Glenn Chaple has been an

avid observer since a friend

showed him Saturn through a

small backyard scope in 1963.

A mask allowed me to shift my attention

from faint fuzzies to close double stars.