Filters & Solar
Herschel Wedge
A prism that dumps 95% of the light out of the back — full aperture, true colour, no film.
A Herschel wedge is safe only on a refractor within the manufacturer's stated aperture limit — typically 150 mm and never on a reflector or catadioptric, where internal concentration of heat will damage the instrument and can cause injury. The wedge is the primary attenuator and must always be used together with the specified ND3 or polarising filter at the eyepiece. The rear of the wedge emits a concentrated beam carrying roughly 95% of the collected solar energy: never look into it, never put anything in front of it, and never block it. Check that the housing is firmly seated and locked before observing. Confirm your telescope is a refractor and within the aperture limit every time you mount the wedge. If you are unsure, use a certified full-aperture white-light filter instead.
Specifications
| Type | Herschel prism wedge (energy rejection) |
|---|---|
| Reflected to eyepiece | ≈ 4.5% – 5% |
| Rejected as heat | ≈ 95% out of the rear housing |
| Suitable telescopes | refractors only |
| Maximum aperture | 150 mm (check the manufacturer's limit) |
| Required add-on | ND3 or polarising eyepiece filter |
Observing parameters
| Image brightness | bright but comfortable with the ND3 fitted |
|---|---|
| Colour rendition | neutral white — unlike orange film filters |
| Resolution | full unstopped aperture, best-in-class |
| Not for | reflectors, SCTs, Maksutovs — heat concentrates internally |
| Best targets | photosphere, granulation, sunspot detail |
Overview
A Herschel wedge is a glass prism mounted in place of the star diagonal. It reflects about 5% of the light to the eyepiece and sends the remaining 95% out through the back of the housing as a heat beam. Because it uses the full, unstopped aperture, a wedge gives the sharpest possible white-light solar view and shows the photosphere in true, neutral colour rather than the orange tint of film filters.
It is a refractor-only device, and it has a hard aperture limit — typically 150 mm, sometimes less — because beyond that the heat load inside the wedge becomes unmanageable. It is also inherently a two-stage safety device: the wedge itself provides the primary attenuation, and a separate ND3 or polarising filter at the eyepiece provides the rest.
Used correctly, a wedge is the best white-light solar view available to an amateur. Used with the wrong telescope, or without the secondary filter, it is extremely dangerous.
Where it fits
Practical notes
- Confirm your telescope is a refractor and within the wedge's aperture limit before every solar session.
- Fit the ND3 or polarising filter as instructed — the wedge alone still delivers too much light.
- Never obstruct the rear heat-exit port; that is where 95% of the energy goes.
- Let the wedge cool before packing; the housing gets genuinely hot during a long session.
- Keep the rear port pointed away from people and away from anything flammable.
Pairs with
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