An overview of the IR spectral elements and of the process by which they were selected was given in Section 2.3. This section gives details of the IR filters and grisms.
Table 7.2 lists the IR channel’s filters, with a general description and fundamental parameters of each. Figures
7.2 and
7.3 show the effective throughput curves, including the filter transmission multiplied by the throughput of the OTA, WFC3 optics, and detector response.
More detailed information on the throughput curves of all of the filters is given in Appendix A; in particular,
Section A.2.1 describes how to generate tabular versions of the throughput curves using
synphot. All measurements of the IR filters which involve wavelengths, as tabulated in
Table 7.2 and plotted in Figures
7.2 and
7.3 and in
Appendix A, were done in helium rather than vacuum. It should be noted that the laboratory measurements were done at a temperature of –30
°C, whereas the filters are operated on orbit at –35
°C; this may lead to wavelength shifts which are expected to be very small.
The IR channel’s versions of the ground-based J and
H filters are F125W and F160W, respectively. The F125W filter has a width somewhat wider than that of a typical
J passband used in ground-based cameras. The F160W filter’s bandpass has been modified relative to ground-based
H in order to give a better fit to the QE curve of the IR detector. Specifically, the WFC3
H filter’s bandpass has been narrowed to approximately 1400-1700 nm, in order to limit thermal background and to have the filter define the bandpass on the red side rather than the detector sensitivity cutoff. By contrast, NICMOS
H filter (NICMOS F160W) covers about 1400-1800 nm. This narrowing for WFC3 reduces photometric errors due to spatial variations in the detector’s QE cutoff.
The wide F140W filter covers the gap between the J and
H bands that is inaccessible from the ground. F105W has a central wavelength similar to ground-based
Y, but is considerably wider. The IR channel also includes a very wide filter, F110W, spanning the ground-based
Y and
J bands. This filter can be used for deep imaging, with a bandpass fairly similar to that of the corresponding wide-band filter in NICMOS (also called F110W).
The other medium filters span absorption bands of water and methane (F139M) and water and ammonia (F153M), with an adjacent continuum filter (F127M). These filters were intended for compositional studies of planets searching for water vapor (WFC3 ISR 2000-09). Solar system objects with visible inventories of these gas species are too bright to observe with the medium-band filters, and WFC3 lacks occulting hardware to access the high contrast ratios and small angular separations that would enable direct imaging of exoplanets. However, the high sensitivity of WFC3 enables compositional studies of the atmospheres of cool stars, brown dwarfs, and transiting exoplanets with the medium-band filters.
Cosmological emission lines can be detected across a range of redshifts within the bandpasses of the narrow-band filters. Table 7.3 lists the redshifts that can be probed using the specified emission lines. These redshift ranges are offered as a guide; exact values depend on the wavelengths of the filter cutoffs. Filter cutoffs used in
Table 7.3 were defined using the passband rectangular widths (defined in
4 of
Table 6.2). For consistency with
Table 6.2, passband cutoffs were not centered on the filter pivot wavelengths
λp (defined in
Section 9.3). Instead, a central wavelength for each filter was determined by maximizing the wavelength-integrated product of a rectangular passband of the specified width with the actual system throughput for the filter. For the IR narrow-band filters, these rectangular passband equivalent central wavelengths are within 0.3% of the pivot wavelengths.
The IR channel has two grisms that provide slitless spectra (see Chapter 8 for more details). The “blue” G102 grism provides a dispersion of 2.5 nm/pix (or a resolution of ~210) over the 800-1150 nm wavelength range. The “red” G141 grism has a dispersion of 4.7 nm/pix (resolution of ~130) over the 1100-1700 nm range. In most cases, a grism observation will be accompanied by a direct image, for source identification and wavelength calibration (see
Section 8.3).
Table 7.4 presents estimates of the blue-leak effect, listing the fraction of detected count rate expected from 710 to 830 nm for each filter. The throughput calculation includes transmission of the filter, the throughputs of the
HST OTA and the IR optics, and the QE of the IR detector.