What the designation fixes
A number that names an intended transmission colour. The system was created for photography — orange and yellow filters for black-and-white landscape work, among others — long before amateur astronomy adopted it, and it survives here because the alternative is a maker’s own vocabulary.
That matters more than it sounds. “Light blue” means whatever a catalogue wants it to mean; a Wratten number means the same thing across every catalogue that prints it. When comparing two coloured filters from different makers, the number is the only field worth comparing directly.
The designation fixes a colour and a nominal depth. It does not fix a glass type, a coating or a cell.
What it does not promise
A Wratten number does not publish a transmission curve. Two filters carrying it can differ at the shoulders of the band, and whether a maker publishes a percentage at all is a useful signal about the maker.
It promises nothing about how much of the effect will be visible. That depends on the aperture, because a filter removes light and a small telescope has little to spare, and on the atmosphere, which caps planetary detail regardless.
And it says nothing about the mechanical part: the thread, the cell depth and the flatness of the glass are separate specifications, and a deep cell can foul a short eyepiece however correct the number is.
What it works with
Wratten numbers appear on coloured planetary filters and on some neutral and Moon filters, and they travel with the filter into whatever holds it — an eyepiece barrel, a diagonal, a camera nose or a filter wheel.
On this site the designation appears on colour planetary filters and Moon filters, and on the eyepieces, Barlows and filter wheels through which they are used.
The questions that keep arriving
Are two filters with the same Wratten number identical?
They designate the same intended colour, and the glass behind it can still differ. Dyed glass and coated glass transmit differently at the edges of the band, and makers do not always publish a curve. The number is the best available common reference, not a guarantee of equivalence.
Which colour should I buy first?
On a small telescope, a light one. Every coloured filter costs light, and the deep grades that transform a planet in a large aperture can leave a small one too dim to read. The pages on this site say which grades suit which apertures.
Do coloured filters help on deep-sky objects?
Not meaningfully. They work by suppressing part of the spectrum so that a feature stands out against a darker background, which suits a bright planet and does nothing for a faint object that is already at the limit of visibility.
Last reviewed 17 September 2026