We calculate theoretical isochrones, in a consistent way, for five filters in the atmospheric window between 1.9 µm and 2.5 µm, K, K′, Ks, F205W, and F222M, using the Padova stellar evolutionary models by Girardi et al. Even when displayed in the same Vega magnitude system, the near-infrared colors of the same isochrone can differ by up to 0.18 mag at its bright end, depending on the filter. We present magnitude transformations between K-band filters as a function of color from H & K band filters. Isochrones with extinction at K of up to 6 mag are also presented. We find that care is needed when comparing extinction values that are estimated using different filter sets in the K-band, in particular when comparing those between atmospheric and space filter sets: extinction values for space filters can be in error by up to 0.3 mag. To reduce this error, we introduce an “effective extinction slope” for each filter set and isochrone model, which describes the extinction behaviour of isochrones in the color-magnitude diagram more correctly than the actual extinction law. Our calculation also suggests that the extinction law implied by the observations of Rieke, Rieke, & Paul for wavelengths between H and K bands is better described by a power-law function with an exponent of 1.61, instead of 1.55, which is commonly used with an assumption that the transmission functions of H and K filters are Dirac delta functions. (Refer to PDF file for exact formulas).

Publication Date



Also archived in: arXiv:astro-ph/0505089 v1 5 May 2005 This work was supported by the research fund from Kyung Hee University. S.S.K. was supported by the Astrophysical Research Center for the Structure and Evolution of the Cosmos (ARCSEC) of Korea Science and Engineering Foundation through the Science Research Center (SRC) program. M.G.L. was in part supported by the ABRL (R14-2002-058-01000-0) and the BK21 program.ISSN:1538-3873 Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works in February 2014.

Document Type


Department, Program, or Center

School of Physics and Astronomy (COS)


RIT – Main Campus