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U, B, V, Rc, Ic filters have been the standard for photometric measurements for decades. They have evolved over time as technology changed. H. Johnson in the 1950s and A.W.J. Cousins in the 1970s designed these filters for photomultiplier tubes (PMT).
Astrodon Photometrics U, B, V, Rc, Ic filters:
- achieve the highest transmission possible for fainter objects
- use hard-oxide durable coatings (no V filter fogging) for durability
- include A/R coating and construction that minimizes halos around stars
- do not depend upon colored glasses being discontinued
Internal absorption of light in colored glasses used to make traditional filters limits the final throughput. For example, the traditional UV filter made with colored glasses typically does not transmit more than 60-70% at the peak. The Rs filter, comes to 70 -80%. Astrodon Photometrics 100%-coated filters exceed 95%!
Schott BG-39 glass (with Schott GG-495 glass) used in the traditional photometric V filter may degrade and become cloudy due to surface crystallization from exposure to humidity. It needs to be covered by a clear glass to protect any exposed surface, making the filter thicker.
With the advent of high performance, hard and durable dielectric coatings, both high transmission in excess of 95% and durability can be achieved. Filters last longer and transmit more light. Durability is critical for long-term, consistent research.
Astrodon Photometrics U, B, V, Rc, Ic filters adhere to the Johnson/Cousins/Bessel designs as closely as possible.