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). M. Bessell in the early 1990s selected colored glasses to match the Johnson-Cousins designs for CCD cameras of the time. However, Bessell’s designs were based upon colored glasses available at the time. Some of those glasses (e.g. Schott KG-4 used in conjunction with Schott RG-9 to make the “I” photometric filter have been discontinued. Lastly, PMTs of the time limited light past 900 nm, whereas modern CCDs are sensitive to 1100 nm. Thus, to truly match the Johnson/Cousins Ic filter, a dielectric coating must be used to block light past 900 nm for CCD systems. This cannot be achieved with colored glasses. We use Ic and Rc (c = Cousins) to designate that we match the Johnson/Cousins designs with our coated filters, as closely as possible.
Astrodon Photometrics U, B, V, Rc, Ic:
- 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
- Includes Ic filter (which must use a 900 nm cut-off coating)
- Edge Blackened
- Proud supplier to Las Cumbres Observatory Global Telescopes, AAVSO, Caltech/Palomar, NASA, MIT, Harvard, Apache Point, U. Valencia, and research organizations worldwide
- Proudly made in the U.S.A.
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, 70 -80%. Astrodon Photometrics 100%-coated filters exceed 95% (see images).
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. Shown in the images is a spectral compilation of actual scans of the Astrodon Photometrics UBRcIc filter set.
