Astrodon Photometrics- Stromgren

Astrodon Introduces Astrodon Photometrics Strömgren filters.

The Strömgren photometric system is four-color intermediate-band photometric system (plus H-beta filters) for stellar classification. It is referred to as the uvby (ultraviolet, violet, blue, yellow) system. It was pioneered by the Danish astronomer Bengt Strömgren in 1956 and extended by his colleague the American astronomer David L. Crawford at Kitt Peak in 1958. These filters are distinguished from the broadband (80-100 nm wide) UBV system by their narrower spectral widths of 20-30 nm.   Narrow and wide H-beta filters are used in conjunction with the uvby filters.

Strömgren established the uvby system to measure (1) color due to temperature differences (e.g., u-b), (2) the Balmer discontinuity, and (3) blanketing absorption due to heavy elements. The u-band filter is located entirely below the Balmer discontinuity, but above the atmospheric  cutoff.  The v-band is entirely above the Balmer discontinuity, but in a region where blanketing is strong. The b- and y-band filters are above the region where blanketing becomes important, so that the b-y color index is mostly free of blanketing effects. 

Indices include b-y, m1, c1, ß, where

m1 = (v-b) – (b-y)

c1 = (u-v) – (v-b)

ß= H-ß(narrow) – H-ß(wide)

y magnitudes are correlated with Johnson-Morgan V magnitudes, b-y is sensitive to stellar temperature (measure of Pashen continuum), c1 is sensitive to the surface gravity (measure Balmer discontinuity strength), m1 is sensitive to metallicity (measure of line blanketing).

Our center wavelength (CWL) and full-width (FWHM) (nm) are summarized in the following table:


  u v b y
CWL 350 410 470 550
FWHM 30 16 19 24


Actual spectral scans are presented below:


  • 100%-coated with hard sputtered coatings, A/R designed in
  • CWL +/- 3-4 nm
  • FWHM +/- 2-3 nm
  • Peak transmission guaranteed > 95%  (>90% for u)
  • <=0.01%Tave out-of-band 300 - 1120 nm (<=0.06% Tabs 380-1000nm for u)
  • 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 in screw-in holder
  • 49.7 mm dia. unmounted
  • 49.7 x 49.7 mm square unmounted
  • Proudly made in the U.S.A.

We have made 3 nm and 15 nm H-beta filters on a custom basis to accompany the Strömgren filters, also having >90%T at the CWL.