Astrodon Johnson/Cousins Rc* Red Filter, 50 mm round

Astrodon Photometrics Filter Rc - red, 50 mm round

UBVRI 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.

Internal absorption of light in the colored glasses used to make traditional filters limits the final transmission. For example, the U filter typically does not transmit more than 60-70% of the light. The Rs filter, 70-80%. New Astrodon Photometrics coated filters exceed 95%.

Schott BG-39 glass (with Schott GG-495 glass) 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.

Astrodon Photometrics UBVRcIc filters adhere to the Johnson/Cousins bandpass designs as closely as possible.

These filters are parfocal with all Astrodon LRGB and narrowband filters. You can add an Astrodon Clear or Ha filter and they will be parfocal within the limits of your optical system. Astrodon Photometrics set a new standard for photometric filters used for variable star and other research. They are 100%-coated on 1/4-wave fused silica substrates and do not use colored glass laminates. As a result, these filters have the best durability available for photometric filters.