Sky & Telescope - March 2019 - 37

Unfortunately, when using the scope, its primary mirror
is facing upward, so any dust in the air can settle on it. With
closed-tube scope designs this isn't a big problem, but with
open-truss scopes, whose primary mirrors are right next to
your feet as you scufﬂe around in the dark, you can put on a
hefty coat of dust in a single night. The trick, then, is to not
kick up so much dust in the ﬁrst place. That means setting
up on grass or a hard surface if possible, or laying down a
ground cloth if you can't.
It also means not letting the mirror dew up if you can help
it. Dew moistens the dust particles and effectively glues them
to the mirror so they can't simply be blown off later. It's not
a disaster if the mirror dews up, but you'll need to clean it
more often if it happens frequently.
How you store the scope will also have a big effect on how
clean the mirrors stay. Obviously, use a dust cap, but also put
a bag or a shower cap around the base of the scope so dust
can't enter around the mirror cell. A full-length cover is even
better. If possible, store solid-tube scopes sideways or topdown so no dust that does get inside can fall on the primary.
Store any scope in as dust-free an environment as possible. In
the shop next to the table saw is probably not the best place.
When your mirrors do become dusty, the ﬁrst step is to
gently blow them off with compressed air. Be careful when
using mechanical air compressors; the air coming out of
them can sometimes be oily or wet, which will make matters
worse. Use a can of compressed air instead. Compressed air
won't get everything off, especially if it was dew-glued on,
but it'll help considerably. Eventually your mirrors will need
cleaning, but with a little preventative care that won't happen for several years.
Figuring out when it's necessary is the real trick. Any
telescope that's been used more than a couple of times will
fail the ﬂashlight test, in which you shine a light straight

down the tube and gasp
at all the dust on that
once-pristine surface. By
the time it's truly ready to
clean, the ﬂashlight test
will make you want to use
a chisel.
Your best indicator isn't
by examining the primary
mirror at all, at least not
directly. You're better off
looking at a bright star
p CELL PROTECTION A simple
through a clean mediumshower cap prevents dust from
to high-power eyepiece. Is
entering through the bottom of the
telescope around its mirror cell.
there a lot of ﬂare around
the star? If so, then that's
probably from dust on the primary. (Be sure to check your
eyepiece ﬁrst - the view through a dirty eyepiece mimics that
of a dirty mirror.) The scope may have a dirty secondary mirror, but since secondaries point downward most of the time,
that's less likely. If so, though, the following procedure will
work for cleaning a secondary just as well as a primary.

Hands-on Cleaning
Many people (maybe even most) are afraid of their primary
mirror. It is, after all, a precision instrument, and the coating
is on the front surface where it can be easily scratched. While
this is true, it's not as fragile as you might think. The aluminum coating is overcoated with a layer of silicon dioxide,
which is basically glass.
I have a fond memory of Bill Atwood, owner of Uvira,
Inc., holding my very ﬁrst mirror casually in his hands as he
admonished me not to be afraid of its shiny new surface. "It's
coated with glass," he said. "I could clean it with my tie."

SCA NDA LOUSLY FILTH Y MIRROR PROVIDED BY A NDY EDELEN

q HAZY REVELATION Left: When it's time to clean your Newtonian's primary mirror, be careful removing it from its sling or cell. Here, the author
dons new gloves in order to avoid adding fingerprints and other contaminants to the coated surface. Right: This mirror is definitely ready for cleaning.

sk yandtele scope.com * M A RCH 2 019

http://www.skyandtelescope.com

Sky & Telescope - March 2019

Table of Contents for the Digital Edition of Sky & Telescope - March 2019