When Lunt Solar started making solar telescopes and filters, the subject of eye safety was at the forefront of design. Our designs were approved by a senior ophthalmologist professor at a leading University of Ophthalmology in Canada. A safety criteria was determined for both UV and IR transmission. This criteria basically set the bar at less than 1×10-5 (T) for any hazardous radiation.
Several standalone filters in Lunt products meet this criteria as a single unit. However, Lunt sets double and sometime triple standards for this requirement so that in the unlikely event one filter fails, the user will still be protected.
Energy Rejection Filter – The filtering of a Lunt system starts with a “true” energy rejection filter at the front of the system. This filter is unique to Lunt, and blocks both dangerous UV and IR. On smaller telescopes the ER filter is seen as a red looking filter installed at a slight angle (to remove internal ghosting), either at the front of the scope or just inside the main objective. On larger aperture telescopes, Lunt puts an additional IR blocking filter onto the front surface of the main objective in order to remove all heat load from the internal parts. Even on these large aperture systems we still provide the secondary red ERF just inside the objective.
Etalon – The next “filter” in the system is actually the etalon. While the etalon was not designed as a safety filter, it does have a very high reflective surface that rejects most UV (T), and would reject the majority of all IR if no prior IR filters were present.
BG Filter – The third filter is the Schott-designed BG (Blue Glass) filter. This filter is also created to absorb any residual IR.
Long Wave Pass Filter – The next filter is commonly called the diagonal “mirror”, however, it is not a mirror at all. Inside the diagonal is a Long Wave Pass filter that is designed to reflect a specific percentage of the 656nm wavelength at 90 degrees and pass thru IR into the backing plate.
Blocking Filter – The next filter is the blocking filter. Again, this is not a safety filter unto itself, but as the name implies, it blocks out-of-band wavelengths in order to allow h-alpha to pass.
Red Glass Filter – The final filter is another piece of the red glass (without the IR coating). This glass blocks 100% of all UV.
People ask why we incorporate so many IR and UV filters in the system. The multitude of safety features we employ insures that our customers will be protected even if they use our products improperly. For instance, should a person accidentally place a standard night time diagonal in the rear of a solar telescope, the view would be bright, but safe.
Because of the addition of multiple filters and safety features, a person simply standing in the sunlight will receive more ambient UV and IR radiation to the eye than when they are looking through one of our solar telescopes.
Lunt purchases our raw etalon glass materials from a ISO qualified company on the east coast of the United States. We grind, edge, bevel, and polish all the glass needed for the etalon and filter systems in-house. Some coatings are outsourced to a facility that maintains a coating specific to our requirements. Our coating facility has the required ability to produce AR coating at less than 0.1%R (typically in the 0.06%R range). They also hold the high reflector coatings to better than +/-1%. The ability to control the coating processes to such high accuracy has allowed us to make precision modifications to the coating formulas, which have proven to increase contrast through the reduction of background noise.
Each coating batch is provided with full scans of the coating applied and is certified to meet any and all safety requirements. Some of our precision-coated filters are provided to us from a US military-qualified company who provides full Mil certifications with every filter.
All Lunt Solar products are 100% safe when used as directed and are shipped from the factory free of any damage or defects. If a Lunt instrument is ever dropped or damaged, it should be returned to the factory for testing and re-certification.
Due to different optical arrangements in design, a Lunt solar product should never be mixed and matched with components made by other companies.