Sky & Telescope - March 2019 - 39

contact the surface.) This part is
optional, but it's worth doing if you
can work up the courage, because
your ﬁngertips are so sensitive that
you can feel any grit particles that
might still be stuck to the mirror.
Gently feel around with your soapy,
wet ﬁngertips to make sure the mirror is truly clean. If you ﬁnd a grit
particle, gently knock it free and
rinse it off the mirror so you won't
drag it around. If you see a stubborn stain, it's okay to rub it a little
- lightly - with your ﬁngertips
to clean it off. A fresh cotton ball
would work, too, but unless you're
a guitarist and you're using your
fretting hand to wash the mirror,
your ﬁngertips are gentler than the
cotton ﬁbers.
When the mirror is clean to
your satisfaction, rinse off all the
soapy water with warm tap water,
then rinse off all the tap water
with distilled or deionized water.
This last step ensures that no
mineral deposits will be left on the
mirror when it dries.
Water will sheet off a truly
clean mirror, so odds are you'll
only have a few drops remaining.
You can dry these drops by blotting
them with a soft towel, but there's
no need. The distilled/deionized
water will dry perfectly clean.

t ROUND TWO Top: Rinse off the soapy
water with lots of warm tap water. Middle:
Flood the mirror with distilled water to
prevent mineral stains. Bottom: The
cleaned mirror may still have some defects. This one is beginning to have coating issues but is still perfectly useable.

When to Recoat a Mirror

¢ Contributing Editor JERRY
OLTION loves cleaning mirrors,

coating is going bad. Pinholes in
the glass overcoating let oxygen
and other contaminants reach
the aluminum layer, and that
can eventually lead to corrosion.
These spots usually look like an
irregular patch of algae on a rock
or an ice cream cone splat on the
pavement, and they won't clean
off. When more than a small percentage of the mirror is affected
with this corrosion, it's time to
have it recoated. (See below for a
list of coating vendors.)
Assuming your mirror is in
good shape, though, you're done!
Put it back in its cell, remembering not to overtighten the mirror
clamps. You want them to just
hold the mirror from tipping
forward when the scope is tilted
horizontal, not push down on it
(not even a little bit). Too much
pressure will pinch the optics.
Now reassemble the scope and
collimate it. We'll cover the ﬁner
details of collimation in the next
installment.

If you notice a lot of splotches on a
clean mirror, it's possible that your

especially other people's.

Majestic Optical Coatings
152 Willow Way,
Clark, NJ 07066
888-278-8308
majestic-coatings.com

Optical Mechanics Inc.
2224 Heinz Rd.,
Iowa City, IA 52240
319-351-3960
opticalmechanics.com

Precision Optical
320 Kalmus Dr.,
Costa Mesa, CA 92626
949-205-7346
precisionoptical.com

Spectrum Thin Films
135 Marcus Blvd.,
Hauppauge, NY 11788
800-815-8184
spectrumthinﬁlms.com

Optiques Fullum
1111 Rt. Harwood #6,
Vaudreuil-Dorion,
QC J7V 8P2 Canada
optiquesfullum.com

Ostahowski Optics
P.O. Box 390440,
Anza, CA 92539
951-763-5959
ostahowskioptics.com

Reynard Corporation
1020 Calle Sombra,
San Clemente, CA 92673
949-366-8866
reynardcorp.com

Spectrum Coatings
1165 Ring St.,
Deltona, FL 32725
386-561-9779
spectrum-coatings.com

sk yandtele scope.com * M A RCH 2 019

http://www.majestic-coatings.com http://www.spectrumthinfilms.com http://www.opticalmechanics.com http://www.precisionoptical.com http://www.optiquesfullum.com http://www.ostahowskioptics.com http://www.reynardcorp.com http://www.spectrum-coatings.com http://www.skyandtelescope.com

Sky & Telescope - March 2019

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