In front of the telescope, covering the full aperture, so that the great majority of the light is rejected before it enters the tube. That is the arrangement every maker of these products describes, and the instructions supplied with the filter are where the conditions of use are set out.
This page describes how the products are built and fitted. What the manufacturer publishes about using them is what governs use, and it is supplied with the filter.
The arrangement the makers describe
A cell covering the full aperture, fitted at the front of the tube. 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.
The measurement that decides fit
The outside diameter of the tube, not the aperture. A dew shield that cannot be removed is part of that figure. Accessory makers publish ranges, and a tube at the extreme of a range fits less securely than one in the middle.
Why fit is treated so seriously
Because a filter that can be knocked loose is the failure mode every maker warns about, which is why nearly all of them now add a set screw or a tightening band. Follow what the maker specifies for its own cell.
What to read before every use
The maker’s inspection guidance and storage instructions, supplied with the filter. This site reports what manufacturers publish and points to those instructions; it does not offer a judgement of its own about use.
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
What size do I order?
The one that matches the outside diameter of the tube at the point the cell will sit, including any dew shield that cannot be removed. That figure is not the aperture of the optics, and ordering on the aperture is the commonest error on this shelf.
Film or glass?
Film is light and does not unbalance a tube; glass is rigid and heavier, and the two give the solar disc a different colour. Both are made across the same range of sizes, and the choice is about handling rather than about anything else.
Last reviewed 17 September 2026