The Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) is fitted with three grisms for slitless spectroscopy.
In the UVIS channel there is one grism, G280, for the near-UV to visible range (200 - 400nm;
1.4nm/pix). The IR channel has two grisms: G102 for the shorter (800-1150nm; 2.45nm/pix) and G141 for the
longer (1100-1700nm; 4.65nm/pix) NIR wavelengths. Using Servicing Mission Observatory Verification (SMOV)
and Cycle 17 calibration data we have assessed the performance of the grisms. We have measured the fielddependent
trace locations and dispersion solutions and determined the throughputs. The trace and wavelength
solutions for the IR grisms were found to be linear functions, varying smoothly across the field of view. The UVIS
grism exhibits a highly bent trace and significantly non-linear dispersion solutions. The maximum throughputs
for the G102 and G141 grisms, including the telescope optics, are 41% at 1100 nm and 48% at 1450 nm, respectively.
Limiting magnitudes at S/N=5 and a 1h exposure are JAB=22.6 and HAB=22.9 for the G102 and G141
grisms, respectively. The calibration results are published in the form of sensitivity and configuration files that
can be used with our dedicated extraction software aXe to reduce WFC3 slitless data.
We present a project aimed at establishing a set of 12 spectro-photometric standards over a wide wavelength range from
320 to 2500 nm. Currently no such set of standard stars covering the near-IR is available. Our strategy is to extend the
useful range of existing well-established optical flux standards into the near-IR by means of integral field spectroscopy
with SINFONI at the VLT combined with state-of-the-art white dwarf stellar atmospheric models. As a solid reference,
we use two primary HST standard white dwarfs. This ESO "Observatory Programme" has been collecting data since
February 2007. The analysis of the data obtained in the first year of the project shows that a careful selection of the
atmospheric windows used to measure fluxes and the stability of SINFONI make it possible to achieve an accuracy of 3-
6% depending on the wavelength band and stellar magnitude, well within our original goal of 10% accuracy. While this
project was originally tailored to the needs of the wide wavelength range (320-2500 nm) of X-shooter on the VLT, it will
also benefit any other near-IR spectrographs, providing a huge improvement over existing flux calibration methods.
The Advanced Camera for Surveys (ACS), installed in HST in SM3B,
is fitted with a grism and three prisms for slitless spectrometry.
The Wide Field Camera (WFC), with a field of 3.4x3.4' and
0.05" pixels, allows slitless grism spectrometry at a dispersion of 40Å per pixel over the range 0.55 to 1.05 μm. The High Resolution Camera (HRC) provides optimally sampled imaging above 0.6μm over a field of 26x29" (0.026" pixels), uses the same grism for slitless spectrometry at 24ÅA/pixel, and a prism covering the 1600-3300Å range with a 4maximum dispersion of 5Å/pixel. The Solar Blind Channel (SBC), with pixels of 0.030x0.034" (field size 31x35"), also has two prisms giving a maximum dispersion of about 2Å/pixel and covering the 1200-1800Å range; one prism excludes the geocoronal Lyman-α for lower background.
The ground and in-flight calibration and astrophysical use of the grism and prism modes of ACS is described. Particular problems for slitless spectrometry include the dependence of spectral resolution on image size, determination of wavelength zero points and removal of flat field. The in-flight calibrations have been derived to extend and strengthen the ground calibrations taken using line and continuum lamps which will not be available in orbit. A software package is available to extract ACS spectra, based on a catalogue of images in a direct image, and has been extensively tested with a
dedicated simulator. The wide range of astrophysical programs,
for which the ACS spectral modes are well suited, will be outlined, with examples from accepted programs.
The Homunculus nebula, surrounding the massive star system Η Carinae, is a bipolar dust nebula which is outflowing at up to 700 km/s. The bipolar lobes display high linear polarization in the optical and near-IR, which is consistent with an origin in dust scattering from the central source. Extensive imaging and spectropolarimetric studies have not however been able to provide a consistent picture of the dust which has been ejected in the mass loss events, the most important of which occurred in the 1840's. The magnitude of the linear polarization shows very little change with wavelength, suggesting very small grains. On the other hand, models of the IR emission suggest a mixed grain population. The scattering
properties of feasible dust mixtures do not however well match the
observed optical and near-infrared polarization behaviour. Three possibilities are advanced to explain the dust grain population in the Homunculus: optical depth effects within a clumped distribution; the presence of many small clouds with grain size dependent on depth into the cloud; large-scale grain alignment. The last suggestion is supported by observation of 10μ polarization. Visible light circular polarization observations and refined geometric dust-scattering models are presented to advance the picture of the dust in the Homunculus. Since the dust ejected from Η Carinae is several solar masses, this study is also relevant to the understanding of ISM dust.
Imaging surveys of the bright 5007 angstrom line in nearby early-type galaxies and the bulges of spirals have catalogued many planetary nebulae. Planetary nebulae arise from the late stages of evolution of low mass stars and are thus representative of a large fraction of the stellar population by number. In about 80 percent of planetary nebulae the abundances of the well observed lighter elements are not affected by the nucleo synthesis which occurs on the Asymptotic Giant Branch, so the nebular abundances can be related to those of the progenitor star. Planetary nebular abundances compared with those of H II regions in spirals, as indicators of abundance gradients and enrichment history. Planetary nebulae provide point probes of the stellar abundance and, in contrast to integrated line of sight stellar spectra, can be used to measure the abundance spread.
High angular resolution adaptive optics data have been obtained on (eta) Carinae and Homunculus nebula, with the ESO ADONIS system. In the broad band filters, J, H and K, the central source, (eta) Car was allowed to saturate in order to maximize the signal for the extended Homunculus nebula. Unsaturated images were also obtained in a narrow continuum filter. The purpose of these observations was to explore the 3D shape of the Homunculus through high spatial resolution imaging polarimetry. The polarization of resolved knots and filaments, will allow their position along the line of sight to be determined when combined with scattering models. Here the observational details, data reduction and preliminary results are presented.
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