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The Near-Infrared Spectrograph (NIRSpec) is one of the four focal plane instruments on the James Webb Space Telescope. In this paper, we summarize the in-orbit performance of NIRSpec, as derived from data collected during its commissioning campaign and the first few months of nominal science operations. More specifically, we discuss the performance of some critical hardware components such as the two NIRSpec Hawaii-2RG detectors, wheel mechanisms, and the microshutter array. We also summarize the accuracy of the two target acquisition procedures used to accurately place science targets into the slit apertures, discuss the current status of the spectrophotometric and wavelength calibration of NIRSpec spectra, and provide the “as measured” sensitivity in all NIRSpec science modes. Finally, we point out a few important considerations for the preparation of NIRSpec science programs.

Context.This is the first paper of a series describing our measurement of weak lensing by large-scale structure, also termed “cosmic shear”, using archival observations from the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST).

Following the detection of a cosmic shear signal at the 30''scale using archival parallel data from the STIS CCD camera onboard HST in the previous paper of this series, we analyzed a larger data set obtained from an HST GO pure parallel program. Although this data set is considerably larger than the one analyzed previously, we do not obtain a significant detection of the cosmic shear signal. The most likely cause of this null result is that the signal is comparable to (or less than) the random measurement of the noise. We also show that multiple systematics plague the STIS CCD data for its use in cosmic shear programs, and in particular the continuous degradation of the CCD charge transfer efficiency after 4 years in space, which is a source of additional noise that is still difficult to correct for in the absence of physical models.

We perform a weak lensing analysis on a previously reported dark-lens candidate on STIS Parallel data [CITE]. New VLT-data indicate that the reported signal originates from a small number of galaxies tangentially aligned towards the center of the STIS field but no signature for an extended mass distribution is found. We argue that we should be able to detect a massive cluster ($M\geq 3.2\times 10^{14}~M_{\odot}$) through its lensing signal up to a redshift of $z\approx 0.6$with our data. Also the double image hypothesis of two galaxies with very similar morphologies in the STIS data is ruled out with colour information. ?

We report the serendipituous discovery of a conspicuous alignment of galaxies in a field obtained through the STIS Parallel Shear Survey. This project collects randomly distributed $50''\,\times\, 50''$fields to investigate the cosmic shear effect on this scale. Analyzing the parallel observations having the Seyfert galaxy NGC 625 as primary target, we recognized over the whole field of view a strong apparent tangential alignment of galaxy ellipticities towards the image center. The field shows several arclet-like features typical for images of massive galaxy clusters, but no obvious over-density of bright foreground galaxies. We also find a multiple image candidate. On the basis of the possible strong and weak lensing effect within the data, we discuss whether this could be compatible with a massive halo with no clear optical counterpart.

We report on the marginal detection of cosmic shear on sub-arcmin scales with archive data from the STIS camera on board HST. For the measurement 121 galaxy fields with a field of view of $51'' \times 51''$are used to obtain an rms cosmic shear of ~4% with $1.5\sigma$significance. This value is consistent with groundbased results obtained by other groups on larger scales, and with theoretical predictions for a standard ΛCDM cosmology. To show the suitability of STIS for weak shear measurements we carefully investigated the stability of the PSF. We demonstrate that small temporal changes do not affect the cosmic shear measurement by more than ~10%. We also discuss the influence of various weighting and selection schemes for the galaxy ellipticities.

Following the second HST servicing mission in 1997 when the STIS instrument was installed and the capability for parallel observations was enhanced, a substantial archive of non-proprietary parallel data has been accumulating. In this paper, we discuss the use of unfiltered STIS imaging data for a project that requires deep observations along as many independent lines-of-sight as possible. We have developed a technique to determine which datasets in the archive can safely be co-added together and have developed an iterative co-addition technique which enabled us to produce 498 high-quality, deep images. The principal motivation for this work is to measure the Cosmic Shear on small angular scales and a value derived from these data will be presented in a subsequent paper. A valuable by-product of this work is a set of high quality combined fields which can be used for other projects. The data are publicly available at http://www.stecf.org/projects/shear/

The methods and techniques for the slitless spectroscopy software aXe, which was designed to reduce data from the various slitless spectroscopy modes of Hubble Space Telescopeinstruments, are described. aXe can treat slitless spectra from different instruments such as ACS, NICMOS, and WFC3 through the use of a configuration file that contains all the instrument-dependent parameters. The basis of the spectral extraction within aXe are the position, morphology, and photometry of the objects on a companion direct image. Several aspects of slitless spectroscopy, such as the overlap of spectra, an extraction dependent on object shape, and the provision of flat-field cubes, motivate a dedicated software package, and the solutions offered within aXe are discussed in detail. The effect of the mutual contamination of spectra can be quantitatively assessed in aXe, using spectral and morphological information from the companion direct image(s). A new method named "aXedrizzle" for 2D rebinning and co-adding spectral data, taken with small shifts or dithers, is described. The extraction of slitless spectra with optimal weighting is outlined, and the correction of spectra for detector fringing for the ACS CCDs is presented. Auxiliary software for simulating slitless data and for visualizing the results of an aXe extraction is outlined.

The Advanced Camera for Surveys on board the Hubble Space Telescopeis equipped with one grism and three prisms for low-resolution, slitless spectroscopy in the range 1150-10500 Å. The G800L grism provides optical spectroscopy between 5500 Å and >1 ?m, with a mean dispersion of 39 and 24 Å pixel?1(in the first spectral order) when coupled with the Wide Field and the High Resolution Channels, respectively. Given the lack of any on-board calibration lamps for wavelength and narrowband flat-fielding, the G800L grism can only be calibrated using astronomical targets. In this paper, we describe the strategy used to calibrate the grism in orbit, with special attention given to the treatment of the field dependence of the grism flat field, wavelength solution, and sensitivity in both channels.

We present a grism extraction package (LINEAR) designed to reconstruct 1D spectra from a collection of slitless spectroscopic images, ideally taken at a variety of orientations, dispersion directions, and/or dither positions. Our approach is to enumerate every transformation between all direct image positions (i.e., a potential source) and the collection of grism images at all relevant wavelengths. This leads to solving a large, sparse system of linear equations, which we invert using the standard LSQR algorithm. We implement a number of color and geometric corrections (such as flat field, pixel-area map, source morphology, and spectral bandwidth), but assume many effects have been calibrated out (such as basic reductions, background subtraction, and astrometric refinement). We demonstrate the power of our approach with several Monte Carlo simulations and the analysis of archival data. The simulations include astrometric and photometric uncertainties, sky-background estimation, and signal-to-noise calculations. The data are G141 observations obtained with the Wide-Field Camera 3 of the Hubble Ultra-Deep Field, and show the power of our formalism by improving the spectral resolution without sacrificing the signal-to-noise (a tradeoff that is often made by current approaches). Additionally, our approach naturally accounts for source contamination, which is only handled heuristically by present softwares. We conclude with a discussion of various observations where our approach will provide much improved spectral 1D spectra, such as crowded fields (star or galaxy clusters), spatially resolved spectroscopy, or surveys with strict completeness requirements. At present our software is heavily geared for Wide-Field Camera 3 IR, however we plan extend the codebase for additional instruments.