They remove part of the spectrum, which lowers the brightness of features that reflect it, so a feature lost against a bright background stands out against a darker one. Nothing is added, and in a small aperture a deep filter leaves too little light to see.
The mechanism is simple and often misdescribed, which is why these filters disappoint people who expected them to add something.
How they work
By subtraction. Removing part of the spectrum lowers the brightness of whatever reflects that part, so a feature that was invisible against a bright background becomes visible against a darker one. The total light reaching the eye always goes down.
Why aperture decides
Every filter costs light, and a small telescope has none to spare. A deep blue that transforms a planet in a large aperture leaves a small one with an image too dim to read, which is why the lighter grades of each colour are the right first purchase.
Why the Wratten number matters
Because names are applied loosely across catalogues and the number is not. When comparing two filters from different makers, the number is the only field worth comparing directly.
What to expect
Subtlety. These are not dramatic accessories. They shift contrast, and the shift is worth having on a night of good seeing and invisible on a poor one.
If this page and the manufacturer’s own documentation disagree, follow the documentation. Every aperture, focal length, payload rating, field, eye relief, transmission figure and thread size on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And for any filter: the name is not the bandpass. Two filters sold under the same three letters can pass windows of very different width, and the figure that decides what a filter does is the transmission at the emission lines together with the width of the window. This page reports the band and the percentage where a maker publishes them, and says they are missing when they are not.
The questions that keep arriving
Which colour for which planet?
The pairings are well established: blue raises the contrast of the belts of Jupiter, red and orange darken the blue-grey regions of Mars, yellow tames the glare of Saturn. How much is visible depends on the aperture.
Is a set better value than singles?
A set is nearly always cheaper than three singles and nearly always contains at least one colour you will never use. Whether that is good value depends on which three you would have bought.
Last reviewed 17 September 2026