Accessories
A filter never adds light. It removes some, and the question is always whether what it removes is more of the background than of the object. Moon filters, color filters, nebula filters and light-pollution filters each answer that differently.
A bright Moon dazzles. A neutral filter such as Vixen’s ND4, which passes about a quarter of the light, makes detail readable without changing its color.
By removing part of the spectrum, a color filter darkens features of the opposite color. A blue filter raises the contrast of Jupiter’s belts; red and orange help on Mars. The effect is real and modest; Neewer’s set is an inexpensive way to try. See Wratten numbers.
Emission nebulae shine at a few wavelengths. A UHC filter passes them in a broad band; an OIII filter such as Celestron’s isolates the 496 and 501 nm oxygen lines. The background darkens and the nebula stands out. See narrowband filter names.
SVBONY’s CLS blocks sodium and mercury lines at 0.1% and passes nebula lines at 90%. Under white LED lighting there is no narrow band to remove, and the gain shrinks.
An H-alpha filter such as Astromania’s is for cameras; the eye is nearly blind at 656 nm.
Show a galaxy from a city, or make anything brighter.
Certain stated passbands correspond to certain lamps. This site publishes them and promises nothing.
A colored filter alters contrast between details. It adds no light.
Narrowband filters act on a stated band. Outside it they do nothing.
| The situation | What a filter does | What it does not do |
|---|---|---|
| Bright sky | Cuts a band | Restore a dark sky |
| Planets | Changes contrast | Add light |
| Nebulae | Acts on a band | Work on everything |
| The Sun | Nothing here | Nothing here |
A filter’s coatings are fragile and are damaged by cleaning far more than by use. A filter that has lost its coating no longer filters what it states.
This site publishes stated passbands and validates no solar device.
OIII narrowband · 496 and 501 nm lines · 1.25 in · 2-year US warranty
Celestron's 1.25-inch Oxygen III narrowband filter: it isolates the two doubly ionized oxygen lines at 496 and 501 nm and blocks the rest, for the Veil, Ring, Dumbbell, Crescent and Orion nebulae.
Best for: Planetary nebulae and supernova remnants from a suburban yard, on a 1.25 in eyepiece
≈ $80
90% at 486, 496, 500, 656 and 672 nm · 0.1% at 435, 578 and 589 nm · 1.25 in · ion-assisted coating
SVBONY's 1.25-inch CLS broadband filter: 90% transmission at the H-beta, OIII, H-alpha and SII nebula lines and 0.1% at the sodium and mercury lines of artificial lighting, with ion-assisted coatings.
Best for: Imaging or observing emission nebulae from a town, with a color camera or by eye
≈ $32
ND4 neutral density (about 25% transmission) · 1.25 in (31.7 mm) eyepiece mount · 19 mm effective diameter · 10 g
Vixen's 1.25-inch Moon filter: a neutral density filter equivalent to ND4, which passes about a quarter of the light, with a 19 mm effective diameter, for eyepieces of 31.7 mm.
Best for: A 1.25 in eyepiece case where a neutral, known dimming factor matters
≈ $58
Seven 1.25 in filters · red, orange, yellow, green, blue · Moon and skyglow · CPL · no Wratten numbers
Neewer's seven-piece 1.25-inch set: five color filters (red, orange, yellow, green, blue), a lunar and skyglow filter and a circular polarizer, with a sentence for what each color brings out.
Best for: A first set of planetary filters, bought to find out which color helps on which planet
≈ $23
H-alpha narrowband · 1.25 in · hardened multi-coatings · for imaging H-II regions · no bandwidth published
Astromania's 1.25-inch H-alpha narrowband filter, sold for high-contrast imaging of hydrogen regions and as an extreme anti-light-pollution filter, with hardened multi-coatings and no published bandwidth.
Best for: Imaging hydrogen regions, by someone who will ask the seller for the bandwidth before buying
≈ $70
No. It darkens the sky around them more than the nebula itself, so the contrast rises and the nebula stands out.
For the Moon, a Moon filter. For nebulae, a broadband UHC or CLS before an OIII, which is more specialized.
No. Galaxies shine across the spectrum, so any filter dims them along with the sky.
For observing: Planetary nebulae and supernova remnants from a suburban yard, on a 1.25 in eyepiece
Where it is strong: Wavelengths published: 496 and 501 nm
Where it is weak: Bandwidth and transmission not published
For observing: Imaging or observing emission nebulae from a town, with a color camera or by eye
Where it is strong: Pass-band and off-band transmissions published at named lines
Where it is weak: Little effect on white LED light pollution
For observing: A 1.25 in eyepiece case where a neutral, known dimming factor matters
Where it is strong: Dimming factor published: ND4, about 25%
Where it is weak: About $58 for a fixed Moon filter
For observing: A first set of planetary filters, bought to find out which color helps on which planet
Where it is strong: Seven filters for about $23
Where it is weak: No Wratten numbers, so colors cannot be compared with other sets
For observing: Imaging hydrogen regions, by someone who will ask the seller for the bandwidth before buying
Where it is strong: A dedicated H-alpha filter for hydrogen nebulae
Where it is weak: No bandwidth published, the defining figure of a narrowband filter
$19 – $23
4 models
$27 – $58
5 models
The models above are taken from the catalog of this site. Specifications are the ones the maker publishes and the ones on the Amazon page; prices are the Keepa reading for the date shown, and both change. Ranking uses declared specifications, the weight of stated strengths against stated limits, and price. Nothing here has been pointed at the sky by us, and the page makes no such claim.
Last checked on
Any figure written into the body of this page was true on the day it was written. The amount shown next to each instrument is requested from the retailer as the page loads, and it is that amount, not the text, that stands.
