Sky & Telescope - April 2023 - 75

POLARIZER: GARY SERONIK; MOON FILTER: CELESTRON
reflects about 12% of the light hitting
it - about the same as an asphalt road.
So why is it so dazzling? Simply because
you're usually looking at it at night,
when it's surrounded by a dark sky and
your eyes likely have become at least
partially sensitized to the nighttime
environment through a process known
as dark adaptation. When you train your
scope at the Moon, your poor observing
eye gets blasted with more photons than
it's ready for. You might even experience
a disorienting lunar afterimage when
you look away. So, even if the view isn't
harmful, you may well find it uncomfortable.
Thankfully, taming lunar
illumination is pretty easy.
The first thing to try is boosting
your scope's magnification. This has
the effect of spreading out the Moon's
light, making it appear less brilliant.
How much power do you need for this
to work? The more you apply, the dimmer
(and larger) the Moon appears.
The obvious advantage of this approach
is that you don't have to buy anything
extra - you probably already have the
necessary eyepieces. You don't want
to go too far down this road, though.
If the image starts looking blurry due
to excessive magnification, it's time to
back off a bit.
But what if the view is still too bright
for your tastes, or you want to see the
entire lunar disk but just more dimly?
This is where filters are your friend.
You can purchase a range of specialized
filters that screw into the base of
your eyepiece - there's even one called
a " Moon filter, " and its only job is to
dim the view. Sometimes also referred
to as " neutral density filters, " they
can be purchased in a couple of different
strengths, or transmission values
(expressed as a percentage of the light
allowed through). Just be sure to pick a
model that matches the barrel diameter
of the eyepiece(s) you plan to use
- typically either 2-inch or 1¼-inch.
Expect to spend $12 to $25 for one.
A more versatile option is a variable
polarizing filter. The advantage to this filter
is that you can dial in a wide range of
transmission values. In effect, a variable
polarizer works like a dimmer control
POLARIZING POWER A variable-polarizer
eyepiece filter consists of two halves (shown
separated in the photo above) that allow the
user to adjust the brightness of the telescopic
image over a wide range. As a bonus, the two
parts of the filter can be separated and used
as a conventional polarizer filter for daytime
viewing of the first- or last-quarter Moon.
for the Moon, allowing you to fine-tune
its brightness to precisely match your
preference. Such filters also allow you to
make the Moon much dimmer than any
conventional fixed filter can. A variable
polarizer is essentially a pair of polarizers
that rotate with respect to each
other to achieve the dimming effect.
And because you are in effect buying
two filters, it tends to be more expensive
than a regular Moon filter. Typically, a
variable polarizer costs in the range of
$25-$30 for 1¼-inch models.
What I like about this solution is
that you can usually unscrew one of the
filter elements and use it as a regular
polarizer. Such a filter offers surprisingly
good daylight views of the Moon,
when it's near first- or last-quarter
phase. Used this way, the polarizer
doesn't dim the image, rather it renders
the daytime sky an appealing dark blue,
thus improving contrast.
For the sake of completeness, I'll
also mention planetary filters. These are
colored filters designed to enhance the
contrast of surface features on planets
(principally Mars and Jupiter). Since
the Moon is essentially gray, there's
minimal color contrast to work with.
Instead, these devices render the lunar
surface a specific color. (The green one
LUNAR GOODIE Dedicated Moon filters like
the one shown here are a quick and easy way
to dim the telescopic view of the lunar surface.
Also known as neutral-density filters, they're
available in several different strengths. Using
such a filter couldn't be easier. Simply thread
it into the barrel of your eyepiece and enjoy
the view.
is handy if your goal is to convince
someone that the Moon really is made
of green cheese.) Planetary filters do
cut down lunar luminance, but unless
you want a fake blue Moon (or a red,
orange, yellow, or green one), the
options already mentioned are better.
Of course, if you have some planetary
filters already, there's no harm in trying
one out to see if you like it.
Although this discussion has focused
on improving your observing comfort,
you'll also see more detail when your
eye isn't being overwhelmed with photons.
Indeed, the finest view I ever got of
ol' Luna was through a friend's homebuilt
16-inch reflector telescope. He'd
just completed figuring the mirror but
hadn't yet sent it out to have its reflective
coating applied. Eager to test this
new optic, we aimed his new scope at
the Moon. And what a view it was! The
lunar surface was rendered with the full
(and impressive) resolution of a 16-inch
aperture, but thanks to the bare glass,
it was only as bright as it would have
appeared in a 3-inch telescope. It was
an astonishing sight - richly detailed
but wonderfully comfortable.
Mind you, for a true lunaphile,
bright or dim, the Moon is always an
astonishing sight. ¢
sk yand tele scope .o r g * APRIL 2023 75
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Sky & Telescope - April 2023

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