Astrodon - 50mm square R Filter - unmounted
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
- Proud supplier to Las Cumbres Observatory Global Telescopes, AAVSO and research organizations worldwide
- 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 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 perfomanace, hard and durable dielectric coatings, both high transmission in excess of 95% and durability can be achieved. Bottom Line: Filters last longer and transmit more light.
The Sloan Digital Sky Survey (SDSS) filters were designed by Fukugita et al. (Ast. J., 411/4, April 1996, p. 1748-1756) to include five mostly non-overlapping filters covering 300 nm to the sensitivity limit of silicon CCD cameras near 1100 nm. They combined colored glass filters and short-pass dielectric coatings to steepen the low wavelength side of the bandpass. The [O I] skyline at 557.7 nm occurs between the g' and r' filters, and is thus reduced.
The SDSS photometric system is the most common filter set used today. The Hubble Space Telescope is equipped with an SDSS set that provides a large reference database for research. Much of photometry up to magnitude 23 will be done in this system with meter-class telescopes. The upcoming large collaborative survey projects (Large Synoptic Survey Telescope - LSST; Panoramic Survey Telescope and Rapid Response System - Pan-Starrs) will also use SDSS filters. The following is a summary of the 50% transmission points of a variety of SDSS filters from the Large Filter Consortium. The new Astrodon Photometrics Sloan set is included for comparison. The Astrodon Photometrics Sloan filters use only dielectric coatings to achieve steep cut-on and cut-offs and peak transmissions in excess of 95%.
To minimize internal reflections that produce halos around brighter stars, the entire bandpass and blocking layers are deposited on one side of the substrate and a high-quality broadband anti-reflective coating is placed on the other side. These filters are parfocal with all Astrodon LRGB and narrowband filters. Specifications:
- 50% transmission points provided above (+/- 3nm typical)
- Peak transmission guaranteed > 95% (>90% for u')
- <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
- 49.7 x 49.7 mm square unmounted