Full aperture, in front of the telescope, and nowhere else

The best solar filters for 2026

A solar filter is fitted in front of the telescope, covering the full aperture, so that the great majority of the light is rejected before it ever enters the tube. That is the only arrangement any maker of these products describes, and the instructions supplied with the filter and with the telescope are where the conditions of use are set out — not here, and not in a comparison table. The screw-in filters that fitted the eyepiece end of older telescopes are not part of this shelf, and the reason is worth stating plainly: 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 organizations publish the same position — do not use one. What can usefully be compared is construction. A film filter uses a coated polymer sheet held in a cell; it is light, it does not unbalance a tube, and the better films deliver a neutral or bluish disc that shows granulation well. A glass filter is heavier, more rigid, and usually gives a yellow or orange disc that many observers find more natural. Both are made in the same range of apertures, and the fit matters: the cell has to sit on the tube without play, because a filter that can be knocked loose is the single failure mode every maker warns about, and most now add a set screw or a tightening band for exactly that reason. Read the standard the listing cites and the maker’s own inspection instructions before every use. Read the aperture of the cell against the outside diameter of the tube, which is not the same as the aperture of the optics. We compare cell fit and outside diameter, film or glass construction, cited standard, mounting security, image color and supplied storage.

By Jean-Benoit Buisson