Sky and Telescope - June 2017 - 40

Astronomer's Workbench

inside. It's important that the two holes
are directly over one another, and you
won't be able to see if that's the case
with the ﬁlter material in place. (That's
why you made the index marks!) Press
the sandwich ﬂat between some coffeetable books and let the glue dry.
Now glue the sandwich to the spiral
ring. I do this gluing with the ring on
the scope so it'll stay round, but be
careful you don't use so much glue that
it drips into the optics. After the ﬁrst
bead of glue has dried, you can take it
off the scope and run a thick bead all
around the edges - inside and out -
for good measure. Make sure it's glued
really well. You don't want the ﬁlter
sandwich to ever come loose off the
spiral ring.
After the glue has dried, hold the ﬁnished ﬁlter up toward the Sun and check
for light leaks. There shouldn't be any,
either within the span of the Astrosolar
ﬁlm or anywhere else. If you see bright
specks in the ﬁlm, blacken them on the
inside surface with a felt-tip marker.
Any other light leaks in the ﬁlter's
framework should be taped over and/or
ﬁlled with glue and painted.
That's the ﬁlter! To use it, just slide
the whole assembly down over the top
of the scope. It should ﬁt snugly enough
to have zero risk of blowing off in a
breeze. If there's any doubt, tape it in
place. If you have kids around, tape it
down anyway. Curious future scientists will want to take it off and look
through it, leaving the telescope open to
the unﬁltered Sun.
Make a box to store it in so it won't
get damaged. Even so, inspect it before
every use to make sure no holes have
developed in it.
This ﬁlter will give you a nice view
of sunspots, faculae, and granulation
in larger-aperture scopes. During the
eclipse you will see the ragged edge of
the Moon slowly creeping across the
face of the Sun, and it will be seriously
cool. Well worth the effort of making
this inexpensive ﬁlter.
¢ Contributing Editor JERRY OLTION
will be observing this year's total solar
eclipse from Newport, Oregon.
40

J U N E 2 0 1 7 * SK Y & TELESCOPE

Small Optic Filters

Eclipse glasses provide a wonderful zero-power view of the Sun.

º For small-aperture scopes and binoculars, here's a really low-cost and
readily available source of filter material: buy a pair of eclipse glasses (also
known as "solar shades"). They're
little cardboard-framed rectangles of
solar filter material that you wear on
your face like glasses so you can look

This pair of filters cost about $3 to make.

A solar filter for an Edmund Scientific
Astroscan, made from eclipse glasses. It
doubles as a Scheiner mask for focusing!

directly at the Sun. (Often these are
not manufactured with Baader Astrosolar film, but they are equally safe,
although they usually produce an
orange image of the Sun rather than
a white one.) If you snip a pair in two,
you have two solar filters that you can
build into filters as described above
and place them in front of the two objective lenses of a pair of binoculars.
Or you can put them side-by-side in
front of a small aperture telescope
(again being careful to eliminate any
chance of light leaks around the
edges of the individual filters). They're
not guaranteed to be as optically flat
as dedicated telescope filter material,
but they'll do in a pinch.
While you're buying the solar
shades, get an extra set or two to
keep intact for direct viewing of the
Sun. The no-magnification view of
partial eclipse phases is more impressive than you might imagine. (I get
my shades from Rainbow Symphony:
tinyurl.com/lq5uefx. Sky & Telescope
also sells its own version at tinyurl.
com/m5xgtzl.)
One final warning: Never wear solar
shades while looking through the
eyepiece of an unfiltered telescope
or binoculars. In order to observe
the Sun safely, solar filters must be
mounted on the objective side of any
optics, before the lenses or mirror
can concentrate the sunlight, and the
filter cell must completely cover the
objective opening. Failure to heed this
warning will fry your eyeballs all the
way back to your cowlick. Seriously.
http://www.tinyurl.com/lq5uefx
Sky and Telescope - June 2017

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