Astrodon - Photometric Filters

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.

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.

Astrodon Tru-Balance RGB filters have revolutionized CCD imaging. Their
popularity is due to ease-of-use, high optical throughput and great
resulting color for galaxies, star clusters and nebulae.  There are two
varieties of Tru-Balance filters - E-Series and I-Series.

Astrodon Tru-Balance RGB filters have revolutionized CCD imaging. Their
popularity is due to ease-of-use, high optical throughput and great
resulting color for galaxies, star clusters and nebulae.  There are two
varieties of Tru-Balance filters - E-Series and I-Series.