A filter collection grows quietly and becomes unmanageable at about six pieces.
Keep them in a case with marked slots
Threading a filter in the dark means knowing which one you have by touch or by position. A case with labelled positions, read once by red light, solves it permanently.
Thread them onto the eyepiece, not the diagonal
Unless the maker says otherwise. It keeps the filter with the eyepiece it suits and avoids the diagonal’s thread wearing from constant changes.
Do not overtighten
They are aluminium threads on aluminium, and a filter tightened hard in the cold can be very difficult to remove warm. Finger-tight, and no more.
Handle by the cell
Fingerprints on a filter sit directly in the light path of everything behind it. The cell exists to be held; the glass does not.
And keep the boxes
The plastic cases filters arrive in are the best storage they will ever have. Throwing them away is the commonest reason a filter ends up loose in a coat pocket.
The questions that keep arriving
Can I stack filters?
Mechanically yes, and it is rarely a good idea: each adds surfaces, and two bands multiplied together can pass almost nothing. Where a maker states a stacked combination is intended, that is different.
Why does my filter foul a short eyepiece?
Cell depth. Some cells are deep enough to reach the field lens inside a short barrel. Where a maker publishes the depth, this site reports it; where they do not, a test fit in daylight is the only way to know.
Last reviewed 17 September 2026