Astrodon UV-blocked Clear-Filter Parfocaal 50mm square
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 900nm 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 900nm cut-off coating)
- Research quality
- Proudly made in the U.S.A.
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.
Astrodon Photometrics UVBRcIc filters adhere to the Johnson/Cousins designs as closely as possible. The image shows a spectral compilation of actual scans of the Astrodon Photometrics UVBRcIc filter set. They may be compared to the Astrodon-Schuler (colored glass) filters here.
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.
- 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 +/-0.025mm substrate thickness
- 1.25" mounted
- 49.7mm dia. unmounted (for most filter wheels, e.g. SBIG)
- 49.7 x 49.7mm square unmounted
- Proudly made in the U.S.A.