astrodon - 1.25 inch Mounted R Filter

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. We use Ic and Rc (c = Cousins) to designate that we match the Johnson/Cousins designs with our 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

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

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 perfomanace, hard and durable dielectric coatings, both high transmission in
excess of 95% and durability can be achieved.  Filters last longer and transmit more light.  

To minimize internal reflections that produce halos around brighter stars, the entire bandpass and
spectral blocking layers are deposited on one side of the substrate and a high-quality, broadband
anti-reflective coating is placed on the other side. Thus, we only use dielectric coatings and no
colored glasses.

Astrodon Photometrics UVBRcIc filters adhere to the Johnson/Cousins designs as closely as possible. 
The following is a spectral compilation of actual scans of the Astrodon Photometrics UVBRcIc filter
set. 

These filters are parfocal with all Astrodon LRGB and narrowband filters. You can add an Astrodon
Clear or H-a filter and they will be parfocal within the limits of your optical system.

Specifications:

    * Curves designed after UV, B, V, Rc, Ic Johnson/Cousins
    * 100% coated filters - no colored glass
    * Peak transmission guaranteed > 90% 
    * <0.1%T out-of-band 300 - 1100 nm
    * Striae-free, single fused silica substrates
    * 1/4-wave propagated wavefront prior to coating
    * <0.5 arcminute substrate parallelism
    * 3.000 +/-0.025 mm substrate thickness
    * 1.25" mounted
    * 49.7 mm dia. unmounted (for most filter wheels, e.g. SBIG)
    * 49.7 x 49.7 mm square unmounted