Herschel wedges are designed for the observation of the sun with refractors.
Functionality: About 95% of the sunlight is blocked out, the rest is further attenuated by a permanently installed ND3.0 grey filter. As the Herschel wedge polarises the transmitted sunlight, a variable polarising filter can then be used to reduce the sunlight to the desired level. The difference to objective solar filters is that the filtering takes place just before the eyepiece or camera. As a rule, the contrast and sharpness of the solar image is higher with a Herschel wedge than with an objective solar filter.
The advantages:
- Particularly high-quality surfaces for very good contrast
- Completely closed housing with integrated light trap, no escaping stray light - therefore particularly safe
- the absolutely necessary ND3.0 filter is already permanently installed
Can only be used with refractors up to 150mm aperture: As the full solar energy enters the telescope and the filtering only takes place just before the focal plane, we only recommend refractors for working with Herschel wedges. The refractor must also not have a rear corrector lens. Optical elements such as corrector lenses or the secondary mirror of a reflector telescope would be heated so much by the concentrated solar energy that damage cannot be ruled out.
This Herschel wedge from APM is equipped with a ceramic light trap, an integrated ND3 filter and a replaceable polarising filter. All filters are multi-coated, of course! The image brightness can be adjusted by rotating the polarising filter. This makes the Herschel wedge suitable for both visual and photographic use!
The use of a Herschel prism (compared to a foil filter) for solar observation is not only safer, but also of better quality. The image produced is high in contrast and sharp, and can therefore be greatly enlarged.
