GSO
GSO Moon Filter 13pct 1.25-inch
A fixed neutral density filter that makes a bright Moon comfortable to look at in a 6 inch or larger telescope. It does one job, has nothing to adjust, and costs less than almost anything else you will put in your eyepiece case.
78/ 100
Polaris Score
The simple fixed moon filter
From$11.95
Where to buy
Affiliate link — we earn a commission, you pay the same. We rank on fit, not payout.
02Be honest with meWhat this filter does NOT do.
It only makes things dimmer. That is exactly right for the Moon and wrong for nearly everything else in the sky.
02Be honest with me
What this filter does NOT do.
It only makes things dimmer. That is exactly right for the Moon and wrong for nearly everything else in the sky.
It only makes things dimmer. That is exactly right for the Moon and wrong for nearly everything else in the sky.
✕Nebulae and galaxies
Faint objects need every bit of light the telescope gathers. This filter throws away seven eighths of it, so a faint galaxy simply gets fainter. Take it out when you leave the Moon.
✕Detail the telescope cannot show
It does not add resolution, and it does not steady a shimmering atmosphere. A soft view made dimmer is still soft.
✕Small telescopes and thin crescents
Below about 4 inches of aperture, or a few days either side of new Moon, the Moon is rarely bright enough to need it, and 13 percent can leave the view dull.
✕The Sun. Never.
A moon filter is nowhere near dark enough for sunlight, and at the eyepiece end it sits in the focused beam, where it can crack. Solar viewing needs a certified solar filter over the FRONT of the telescope.
03Where it fitsWhere it lives in your kit.
A filter isn't a standalone instrument — it threads into the light path just before your eye. Here's the chain.
03Where it fits
Where it lives in your kit.
A filter isn't a standalone instrument — it threads into the light path just before your eye. Here's the chain.
A filter isn't a standalone instrument — it threads into the light path just before your eye. Here's the chain.
→
→
→
→
04Which filterHow the moon filter options compare.
They all do one thing: cut brightness. The difference is how much, and whether you can change it at the eyepiece.
04Which filter
How the moon filter options compare.
They all do one thing: cut brightness. The difference is how much, and whether you can change it at the eyepiece.
They all do one thing: cut brightness. The difference is how much, and whether you can change it at the eyepiece.
No filter
Baseline
Transmission
100%
Best for
Small scopes, thin crescents
Below about 4 inches, or near new Moon, you often do not need one at all.
ND 25%
Light neutral
Transmission
~25%
Best for
4 to 6 inch scopes
Takes the edge off without making the view dim.
This one
ND 13%
Neutral density
Transmission
~13%
Best for
Bright phases, 6 inch and up
The common fixed choice: one strength, nothing to adjust.
Variable polarizing
Adjustable
Transmission
~1–40%
Best for
Every phase and aperture
Two polarizers you turn against each other to set the brightness.
Coloured moon filter
Tinted
Transmission
Varies
Best for
Rarely the best pick
Dims the Moon but tints it green or blue. A neutral filter keeps its natural colour.
05The numbersWhat These Numbers Mean
5 numbers in plain English · full specifications
05The numbers
What These Numbers Mean
5 numbers in plain English · full specifications
5 numbers in plain English · full specifications
TransmissionND 0.9
13 percent
It passes about one eighth of the light and blocks the rest evenly across the spectrum. On a bright gibbous Moon in a six-inch or larger telescope that is the difference between squinting and comfortably studying craters.
What changesComfort
Glare, not detail
The filter does not add resolution. What it does is stop the Moon from dazzling you, which lets your eye settle and pick out subtle contrast along the terminator, and preserves your dark adaptation for the rest of the session.
ColourNeutral
Neutral density
A neutral coating dims everything equally rather than tinting the view green or grey the way cheap coloured moon filters do. The Moon still looks like the Moon.
Barrel sizeStandard
1.25 inch
Threads into the standard eyepiece size, so it works with almost any telescope out of the box and costs less than most eyepiece accessories.
When it is too muchSituational
Small scopes, thin crescents
In a small telescope, or on a crescent Moon, 13 percent transmission can leave the view dimmer than you want. A variable polarising filter suits observers who want to tune the amount of dimming.
Optical
TypeNeutral density
DensityND 0.9
TransmissionAbout 13%
ColourNeutral, no tint
UseVisual: the Moon, and bright planets in large scopes
Fit
Cell size1.25″ (M28.5 × 0.6)
FitsStandard 1.25″ eyepiece filter thread
06Pairs withCompatible gear.
2 matches for this filter
06Pairs with
Compatible gear.
2 matches for this filter
2 matches for this filter
Telescopes
Where it earns its place
6 inch and larger
The Moon is bright enough to dazzle
8 inch Dobsonian or SCT
Where it matters most
Under 4 inch
Usually not needed
Eyepieces
Fits any 1.25″
Low power, whole disc
Brightest view, most benefit
Medium power, terminator
Easier on the eye for long sessions
07Learn moreGuides worth reading next.
2 guides that cover this ground
07Learn more
Guides worth reading next.
2 guides that cover this ground
2 guides that cover this ground
Guide
Why Do People Put Filters on Telescopes?
How removing light helps you see more: the nebula filters that boost contrast, the moon filters that tame glare, and the solar filters that make the Sun safe.
Read the guide →
Guide
Why Does Aperture Matter More Than Magnification?
The same aperture that shows faint galaxies is what makes the Moon uncomfortably bright. Why bigger telescopes need a moon filter more than small ones.
Read the guide →