Moon Filters: What They Do and Which Size Fits

A 203 mm (8 in) Dobsonian gathers 3.96 times the light of a 102 mm (4 in) refractor. The same Moon looks soft grey in the small scope. In the big one it looks flat white. A moon filter for telescopes cuts that brightness and nothing else.

ND 0.9 passes 12.6% of the light. That is 10^-0.9 = 0.126. It is sold as a 13% filter. Two of them give 0.126 × 0.126 = 1.6%.

The same filter on two apertures is not the same brightness. 3.96 × 0.126 = 0.50 at 203 mm, against 1.00 × 0.126 = 0.13 at 102 mm.

So there are two decisions here, and the order matters. First the thread, 1.25 in or 2 in. That follows the eyepiece, not the telescope. Then the density, and that follows the aperture.

Three things, and only three. A neutral density filter cuts brightness. The bright limb stops flooding the field.

It cuts glare. The contrast between the lit edge and the shadowed craters grows. A dimmer image keeps the mid-tone contrast. Detail that was washed out comes back.

Three things it does not do. It does not add resolution. The detail limit comes from the aperture, and none of the filters we track changes it.

It does not widen the field. The true field is the apparent field divided by the power. It does not change the colour of the disc either. A neutral filter is neutral by definition.

The exit pupil stays the same. On f/5.9 a 25 mm eyepiece keeps its 4.2 mm exit pupil. A filter in the thread does not change that. The filter changes the light level, not the focal ratio.

A filter sold as ND D passes T = 10^-D of the light.

The Moon filter card's Transmission row lists 50%, 25%, 12.6%, 6.3% and 3.2%. ND 0.3 gives 50% and ND 0.6 gives 25%, while 12.6% is ND 0.9, 6.3% is ND 1.2 and 3.2% is ND 1.5. A filter sold as 13% is the 12.6% step.

Read that as brightness with Δm = 2.5 log10(100/T). The same five steps are 0.75, 1.50, 2.25, 3.00 and 3.75 magnitudes darker.

One magnitude is a factor of 2.51 in light. So ND 0.9 passes 7.9 times less light than an empty thread.

Aperture comes next, because the filter cuts the whole field. And the whole field gets brighter as the mirror gets bigger.

Light from a telescope scales with the square of the aperture. So (D1/D2)^2 is the ratio between two scopes.

An 8 in against a 4 in is (203/102)^2 = 3.96. A 10 in is (254/102)^2 = 6.20.

That is why a filter can look right on a 102 mm refractor and blown out on a 203 mm Dobsonian. The same 13% filter puts 3.96 × 0.126 = 0.50 through the big scope. The small one gets 0.13.

Our assumptions first, so every number in the table can be checked. The reference brightness is a 102 mm refractor. It has a 25% filter on it.

We assume the filter covers the whole field. So it cuts sky and Moon together. And we assume a bright Moon near full, when the glare is worst.

Five common steps sit on the shelf.

Now match the steps to the aperture. The transmission you want is 0.25 divided by the aperture gain, (D/102)^2.

A 152 mm scope needs 0.25 / 2.22 = 0.11, which is ND 0.9. A 203 mm scope needs 0.25 / 3.96 = 0.063, which is ND 1.2.

A 254 mm scope needs 0.25 / 6.20 = 0.040, between ND 1.2 and ND 1.5.

The 6 in and 8 in rows are the brightness of the scopes on our 6-inch telescope and 8-inch telescope pages.

The 203 mm row lands on 0.25, exactly the reference brightness. That is with the 6.3% filter.

In aperture terms, a 13% filter turns a 203 mm scope into a 72 mm one: 203 × √0.126 = 72 mm.

6.3% is a rare step in a finished filter. The darkest common single filter is the 13% one, sold as ND 0.9.

On a 203 mm scope that leaves 3.96 × 0.126 = 0.50, about twice the reference brightness. A big scope still looks bright through one moon filter.

Buy a set with several densities. Step from 50% down to 13%. Take the one that matches the phase of the Moon.

Or buy a variable filter. It turns continuously between 40% and 1%. One part covers every row of the table.

A crescent is far fainter than a full Moon. The step you need changes across the month anyway.

A 102 mm refractor is the easy case. Its row says 25%, the 25% step this table calls for.

The 80 mm row wants 50%, the least noticeable step of all. The 254 mm row wants 3.2%, the hardest one to find in a finished filter.

One check before you order: read the percentage, not the ND number.

Two listings in the catalog pair the two differently. The Alstar 2 in set lists ND96-0.3 at 50%. The Astromania 1.25 in set lists the same number at 13%.

Celestron's own set lists ND-96-0.3 at 50%. The percentage is the number to trust.

The thread is at the bottom end of the eyepiece. That end goes into the focuser or the diagonal. The filter screws onto it.

The top end is where you look through. It has no filter thread at all.

So the interface follows the eyepiece, not the telescope. A 1.25 in eyepiece takes a 1.25 in filter. A 2 in eyepiece takes a 2 in filter.

The 1.25 in thread measures 31.7 mm across. The 2 in thread is M48 × 0.75, so 48 mm. One does not go into the other.

That last point catches people out. A 2 in focuser with a 1.25 in adapter still takes a 1.25 in filter. The filter sits on the eyepiece.

Buy the filter that matches the eyepiece, not the focuser. Our eyepiece guide starts from the same thread sizes.

Thread size and field size are two different things. A 2 in barrel allows a field stop near 46 mm. A 1.25 in barrel stops at about 27 mm.

On 1,200 mm that is 2.20° against 1.29° of true field. The wider field belongs to the eyepiece, not the filter.

Stacking works in principle, because the transmissions multiply: 0.251 × 0.251 = 6.3%, and 0.126 × 0.126 = 1.6%.

In practice a cell has room for about one filter. Most of them take a single filter with a lid.

Check the thread count before you stack. A cell with threads on one side screws onto the eyepiece and stops there. A cell threaded on both sides can carry a second filter.

The Celestron neutral density 1.25 in filter is one of them. Its maker lists it as stackable.

Five steps, and none of them takes more than a minute.

1. 1. Take the eyepiece out of the focuser or the diagonal and read its thread: 1.25 in is a 31.7 mm thread, 2 in is M48 × 0.75, so a filter for the other size will not fit.
2. 2. Screw the filter onto the thread at the bottom of the eyepiece. Turn it until it is snug, not until it stops.
3. 3. Put the eyepiece back and lock the thumbscrew. The drawtube should not move when you turn the focus knob.
4. 4. Check it against a bright target before you judge the brightness. In the dark you cannot see the thread, and it is easy to cross it.
5. 5. Store the filter in its own case. Threading it on and off every night wears the threads on both parts.

Check the thread now and then. A speck of dust on the eyepiece thread shows up as a dark ring in the view.

That is the usual reason a filter looks dirty rather than the glass. A blower bulb and one drop of lens fluid on a tissue, wiped from the centre out, is enough.

Keep the cells flat when you store them. A filter rolled on its edge in a bag picks up a scratch long before the glass wears out.

Three families sit on the shelf, and they do different jobs.

Neutral density is the plain one. It cuts light and leaves the colour of the disc alone. So it works on the Moon and on a bright planet.

The ND96 series names the density by number: 0.3, 0.6, 0.9, 1.2 and 1.5.

Green moon filters are the older style, usually a #58. The glass is green. The disc comes out green with it. The colour shift is the design, not a defect.

A variable filter turns between two limits instead of sitting at one. You rotate the ring and the brightness changes, in small steps.

The Explore Scientific 2 in variable polarizing filter runs from 40% down to 1% of the light.

Its maker calls the effect a neutral grey one. The colour of the disc stays put. That range covers every row of the transmission table with one part.

It suits a scope with several 2 in eyepieces and a Moon that changes phase all month. The cost is structure.

A variable filter is two polarising plates that turn against each other. That puts four surfaces in the light path instead of two.

At 200x on a big scope that costs a little contrast and a little sharpness. It costs more at 400x than at 100x.

So a fixed density is the simpler choice for a small scope with one eyepiece. You fit it. You get one number, and there is nothing to adjust at the eyepiece.

Colour filters are the other family. Each one passes a band of the spectrum and blocks the rest. That is why the view of a planet changes with the filter.

Three numbers cover most of the planetary work.

Wratten #12 is a deep yellow, and it is the general purpose one. Its maker calls it good for smaller apertures under 4.5 in.

The maker says it improves the contrast of features on the Moon. It also helps with the equatorial belts of Jupiter and Saturn.

Wratten #21 is orange. It sharpens the boundaries along the plains of Mars, because it cuts the blue and green transmission.

Wratten #80A is a light blue, for the cloud belts of Jupiter and the polar caps of Saturn.

The cost of every colour filter is light. A colour filter blocks part of the spectrum, so the planet gets dimmer. The usual way back is power.

On 1,200 mm a 10 mm eyepiece gives 120x and a 6 mm gives 200x. The higher step shows the detail the filter would otherwise wash out.

Stacking a colour filter with a neutral density filter is possible. The cells are threaded at both ends.

The two transmissions multiply, though, and the result is usually too dark to be worth it. Take one job at a time.

A set is the simple way to cover the three numbers. The Celestron 1.25 in lunar and planetary set holds four filters.

Those are Wratten #12 deep yellow, #21 orange, #80A light blue and a 50% neutral density.

Every cell has a 26 mm clear aperture. Each is threaded on both sides and comes in a plastic case.