UVBRI 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 "Is" photometric filter) have been discontinued. Lastly, PMTs of the time limited light past 900 nm, whereas modern CCDs are sensitive to nearly 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. Astrodon uses Ic and Rc (c = Cousins) to designate that they match the Johnson/Cousins designs with their coated filters, as closely as possible.
The new Astrodon Photometrics UVBRcIc filters:
- Achieve the highest transmission possible for fainter objects
- Use hard-durable coatings (no V filter fogging)
- Include A/R coating and construction that eliminates halos around stars
- Do not depend upon colored glasses being discontinued
- Include Ic filter (which must use a 900 nm cut-off coating)
- Research quality
