At the front of the telescope, covering the full aperture, so that the great majority of the light is rejected before entering the tube. Every maker of these products describes that arrangement, and supplies instructions covering inspection, fitting and storage.
This page describes construction and fitting. Use is governed by what the manufacturer publishes with the filter, and that is what to read.
The arrangement makers describe
A cell at the front of the tube covering the whole opening. Screw-in filters at the eyepiece end of older telescopes are not part of this shelf. They thread in where the telescope concentrates the light, and makers withdrew them because that concentrated heat can crack the glass; current manufacturers and observing organisations publish the same position, which is not to use one.
Film and glass
Film uses a coated polymer sheet in a cell: light, unlikely to unbalance a tube, and giving the disc a neutral or bluish cast. Glass is rigid and heavier, and usually gives a yellow or orange disc. Both are made across the same range of sizes.
Fit, and why makers stress it
The cell has to sit on the tube without play. Accessory makers publish ranges rather than exact sizes, and a tube at the extreme of a range is held less securely than one in the middle.
What the maker supplies with it
Instructions covering inspection before use, how the cell is to be secured, and how the filter is to be stored. Those instructions are part of the product, and this site points to them rather than restating them.
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 the limit this page does not cross: no filter, no pair of eclipse glasses and no mounting described here makes an observation safe. The page reports what a maker specifies for its own product — the standard the listing cites, the construction, the fitting, the inspection asked for before use — and what applies to your own equipment is decided by those instructions and by the condition of the filter in your hands.
The questions that keep arriving
What size do I order?
The one matching the outside diameter of the tube where the cell will sit, including any dew shield that cannot be removed. That is not the aperture of the optics, and ordering on the aperture is the commonest error here.
Why do cells have set screws or bands?
Because makers treat a cell that can be knocked loose as the failure to design against, and nearly all of them now add a positive retention method for that reason. Follow what the maker specifies for its own cell.
Last reviewed 17 September 2026