An overview of the UVIS spectral elements was given in Section 2.3. This section gives further details of the UVIS filters and grism.
Table 6.2 contains a complete listing of the available spectral elements in the UVIS channel. Figures
6.3 through
6.6 show the effective throughput curves, including the filter transmission convolved with the OTA, WFC3 optics, and detector response. All of the UVIS filters are contained in a multi-wheel mechanism—identical to the mechanism on WFPC2—called the Selectable Optical Filter Assembly (SOFA). Values in
Table 6.2 have been calculated for UVIS chip 2, which has a higher UV sensitivity, except in the cases of quad filters which are restricted to the A and B quadrants.
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 UVIS filters which involve wavelengths, as tabulated in
Table 6.2 and plotted in Figures
6.3 through
6.6 and in
Appendix A, were done in air. The data have been converted to vacuum wavelengths using the formula given by D. C. Morton (1991,
ApJS 77, 119). It should also be noted that the laboratory measurements were done at a temperature of 20
°C, whereas the UVIS filters are operated on orbit at 0
°C. The temperature difference may lead to wavelength shifts that are no more than 0.14 nm in the worst cases, according to the filter manufacturing specifications.
The UVIS filters have been chosen to cover a wide variety of scientific applications, ranging from color selection of distant galaxies to accurate photometry of stellar sources and narrow-band imaging of nebular gas. The set includes several very wide-band filters for extremely deep imaging, filters that match the most commonly used filters on WFPC2 and ACS (to provide continuity with previous observations), the SDSS filters, and filters that are optimized to provide maximum sensitivity to various stellar parameters (e.g., the StrÆmgren and Washington systems, and the F300X filter for high sensitivity to the stellar Balmer jump). There is a variety of narrow-band filters, which allow investigations of a range of physical conditions in the interstellar medium, nebulae, and solar system. A few of the narrow-band filters are also provided with slightly redshifted wavelengths, for use in extragalactic applications. Finally, there is a UV grism that provides slitless spectra with useful dispersion covering 200–400 nm (although the grism transmission spans the full wavelength range of the CCD).
While the red blocking in the WFC3 UV filters is generally very good, resulting in negligible red leaks for hot objects (typically <<1% for targets with effective temperature Teff > 10,000 K), the red leak can become significant in some filters for cooler targets (e.g., ~10% in F225W for a star with
Teff = 5000 K). More details are available in
Section 6.5.2;
Table 6.5 in that section tabulates red-leak values as a function of stellar effective temperature.
The UV filters include the shortest-wavelength F218W, intended for studies of the ISM absorption feature; the wide F225W and F275W for broad-band UV imaging; the StrÆmgren u (F336W) and Washington C (F390W) for stellar astrophysics; the extremely wide F300X for very deep imaging; and narrow bands such as F280N (Mg II) and the quad filters FQ232N and FQ243N (C II] and [Ne IV]).
WFC3’s maximum sensitivity to hot sources can be obtained by subtracting an F350LP image from an F200LP, thereby creating a very broad ultraviolet bandpass. In Figure 6.7, the blue curve shows the filter transmission for the F200LP filter, and the black curve shows the effective transmission for a F200LP minus F350LP difference image. For redder targets, some additional calibration may be necessary to account for differences in the transmission of the two filters longward of ~450 nm.
The F850LP filter is part of the Sloan Digital Sky Survey (SDSS)
griz filter set, and is the reddest of the ultra-wide filters.
The Sloan Digital Sky Survey (SDSS)
griz filter set (F475W, F625W, F775W, F850LP) is designed to provide high throughput for the wavelengths of interest and excellent rejection of out-of-band wavelengths. These filters provide wide, non-overlapping filter bands that cover the entire range of CCD sensitivity from blue to near-IR wavelengths.
The medium-band filters include the StrÆmgren set (u,
v,
b, and
y), as well as some continuum bands needed for use with narrow-band imaging (F390M, FQ422M). The four 11% passband filters were added to the WFC3 UVIS set in order to cover the ~600-900 nm wavelength region with equal-energy filters. The “11%” refers to the filter bandwidths, which are ~11% of the central wavelength.
The WFC3 UVIS channel contains 36 different narrow-band filters, covering a variety of species and most of the astrophysically interesting transitions, including Hα, Hβ, Hγ, He II, C II], [N II], [O I], [O II], [O III], [Ne IV], [Ne V], [S II], and Ca II. The methane absorption bands seen in planets, cool stars, and brown dwarfs are also covered.
Cosmological emission lines can be detected across a range of redshifts within the bandpasses of the narrow-band filters. Table 6.3 lists the redshifts that can be probed using the specified narrow emission lines (hence, no entries for broad absorption bands or continuum or “off” bands). These redshift ranges are offered as a guide; exact values depend on the wavelengths of the filter cutoffs. Filter cutoffs used in
Table were defined using the passband rectangular widths (defined in Footnote
4 of
Table 6.2). However, passband cutoffs were not centered on the filter pivot wavelengths
λp (defined in
Section 9.3), because red leaks shift the pivot wavelengths to longer wavelengths by 1-9% in some of the ultraviolet filters. Instead, the 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. In the most extreme case (FQ232N), the pivot wavelength of 241.3 nm is more than two bandpass widths to the red of the rectangular passband equivalent central wavelength (232.6 nm).
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The WFC3 UVIS channel contains five quad filters: each is a 2â2 mosaic of filter elements occupying a single filter slot, with each quadrant providing a different bandpass (typically narrow-band, although there are also several bandpasses intended for continuum measurements). The five quad filter sets on WFC3 significantly increase the number of available narrow-band filters. The WFC3 quad filters are listed in
Table 6.4 with their readout amplifiers.
A quadrant nominally covers only 1/4 of the WFC3 total field of view or about 80"â 80", although edge effects (
Figure 6.8) result in an unvignetted field of about 1/6 of the field of view. The filter edges are out of focus on the focal plane, so light from multiple passbands reaches the detector in those areas.
Table 6.5 below summarizes the red-leak levels for the WFC3 UV filters. The table lists the fraction of the total signal that is due to flux longward of 400 nm, as a function of effective temperature. This was calculated by convolving a blackbody of the given
Teff with the system throughput in the listed filter. As can be seen from the table, red leaks should not be an issue for observations of any objects taken with F275W or F336W. The other UV filters have some red leaks, whose importance depends on stellar temperature. The red leaks in F218W and F300X, for example, exceed ~1% for objects cooler than ~6000 K, while in F225W the red leak reaches ~1% for objects with even cooler temperatures. The most extreme red leaks arise from F218W and F225W observations of objects with
Teff of ~4000 K or cooler, necessitating appropriate corrections.
Filter ghosts for the WFC3 filters were specified to be less than 0.2%, and in most cases were measured during ground testing to be less than 0.1%. A few filters however, were found during ground testing to have ghosts that exceeded the specification. Some of these, the ones deemed highest priority, were remanufactured and installed in the SOFA. Consequently, there are a relatively small number of filters that may be of concern for ghosts. These are listed in Table 6.6. They have been retained in the flight instrument either because they were of lower scientific priority, or because the ghost level was deemed acceptable in light of the otherwise excellent performance characteristics of the filters (e.g., in- and out-of-band transmission, sharpness of bandpass edges). While some scientific programs (e.g., stellar photometry) may be unaffected by filter ghosts, others (e.g., observations of extended targets or faint objects adjacent to bright ones) could be adversely affected. In such cases, extra planning and/or data-analysis efforts may be needed, e.g., combining images taken at different dither positions and/or roll angles, or applying a deconvolution algorithm.
Figure 6.6: Integrated system throughput of the WFC3 UVIS narrow-band filters covering 6000-6800 å (top panel) and the narrow-band filters covering 6600-9600 å (bottom panel). The throughput calculations include the HST OTA, WFC3 UVIS-channel internal throughput, filter transmittance, and the QE of the UVIS flight detector, and a correction factor to account for the gain sensitivity seen in SMOV4 on-orbit observations vs. TV3 ground tests.The UV filters include the shortest-wavelength F218W, intended for studies of the ISM absorption feature; the wide F225W and F275W for broad-band UV imaging; the StrÆmgren u (F336W) and Washington C (F390W) for stellar astrophysics; the extremely wide F300X for very deep imaging; and narrow bands such as F280N (Mg II) and the quad filters FQ232N and FQ243N (C II] and [Ne IV]).WFC3’s maximum sensitivity to hot sources can be obtained by subtracting an F350LP image from an F200LP, thereby creating a very broad ultraviolet bandpass. In Figure 6.7, the blue curve shows the filter transmission for the F200LP filter, and the black curve shows the effective transmission for a F200LP minus F350LP difference image. For redder targets, some additional calibration may be necessary to account for differences in the transmission of the two filters longward of ~450 nm.The F850LP filter is part of the Sloan Digital Sky Survey (SDSS) griz filter set, and is the reddest of the ultra-wide filters.The most commonly used WFPC2 and ACS wide filters are also included in the WFC3 filter set. In addition to a wide V-band filter (F606W), there is the Johnson-Cousins BVI set (F438W, F555W, F814W).The Sloan Digital Sky Survey (SDSS) griz filter set (F475W, F625W, F775W, F850LP) is designed to provide high throughput for the wavelengths of interest and excellent rejection of out-of-band wavelengths. These filters provide wide, non-overlapping filter bands that cover the entire range of CCD sensitivity from blue to near-IR wavelengths.The medium-band filters include the StrÆmgren set (u, v, b, and y), as well as some continuum bands needed for use with narrow-band imaging (F390M, FQ422M). The four 11% passband filters were added to the WFC3 UVIS set in order to cover the ~600-900 nm wavelength region with equal-energy filters. The “11%” refers to the filter bandwidths, which are ~11% of the central wavelength.The WFC3 UVIS channel contains 36 different narrow-band filters, covering a variety of species and most of the astrophysically interesting transitions, including Hα, Hβ, Hγ, He II, C II], [N II], [O I], [O II], [O III], [Ne IV], [Ne V], [S II], and Ca II. The methane absorption bands seen in planets, cool stars, and brown dwarfs are also covered.Cosmological emission lines can be detected across a range of redshifts within the bandpasses of the narrow-band filters. Table 6.3 lists the redshifts that can be probed using the specified narrow emission lines (hence, no entries for broad absorption bands or continuum or “off” bands). These redshift ranges are offered as a guide; exact values depend on the wavelengths of the filter cutoffs. Filter cutoffs used in Table were defined using the passband rectangular widths (defined in Footnote 4 of Table 6.2). However, passband cutoffs were not centered on the filter pivot wavelengths λp (defined in Section 9.3), because red leaks shift the pivot wavelengths to longer wavelengths by 1-9% in some of the ultraviolet filters. Instead, the 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. In the most extreme case (FQ232N), the pivot wavelength of 241.3 nm is more than two bandpass widths to the red of the rectangular passband equivalent central wavelength (232.6 nm).
.