This page presents characteristic information for ACS filters.
We provide the 95% cumulative throughput width (CTW95) as an alternative to the traditional root-mean-square bandwidth, PHOTBW. The CTW95 is defined as the interval where the wavelength integral of the filter+detector throughput curve reaches 2.5% and 97.5% of its total value.
Because most filter+detector throughput curves have steep cut-offs by design, the CTW95 is a much closer approximation to what astronomers traditionally consider the "width" of the filter. For example, the PHOTBW for an ideal top-hat throughput is only 29% of the interval between the top-hat cut-off wavelengths --- whereas CTW95 is 95% of that interval. The PHOTBW can also be unduly influenced by filter red-leak.
In contrast with the full-width-half-maximum (FWHM) statistic, the CTW95 is robust against noisy estimation of the throughput-curve maximum, and against "sawtooth" throughput curves such as ACS/WFC F850LP where a substantial amount of integrated throughput occurs at wavelengths with less than half the maximum throughput.
The PHOTBW value is calculated using the PHOTBW routine in pysynphot, which is based
on the equation given in the
synphot manual (Table 5.2, p.45). An alternative formulation
is given by Koornneef et
al. (1986, HiA,7,833) and this can be called using the function rmswidth in pysynphot.
The two formulations give widths that differ by ~ 20 A. For
consistency with synphot, CALACS uses the synphot manual equation.
Bandwidth keywords in the SCI extension of ACS images:
- PHOTPLAM: pivot wavelength.
- PHOTBW: rms bandwidth of filter plus detector.
Last modified on May 15, 2012.