NIRSpec is the flagship spectrograph for JWST in the 0.6 to 5.3 micron wavelength range. Observation with the Micro- Shutter Assembly (MSA) for multiobject spectroscopy (MOS) will use configurable shutters to form spectral slits and provide the first space-based MOS capabilities. The NIRSpec Micro-shutter Assembly Target Acquisition (MSATA) is an autonomous target acquisition scheme to acquire and position targets accurately with respect to the spectral slits. The method uses measured centroid positions of reference stars with accurately known relative positions across the target field for this process. MSATA performs not only linear offsets, but any required telescope orient (roll) correction to remove blind-pointing alignment error. The MSATA procedure can be used for most NIRSpec science and will be a prerequisite for most NIRSpec MOS mode observations. Astrometry relating the positions of science targets and candidate reference stars with a relative accuracy of 5 - 10 mas will be needed to deliver the best calibration accuracy of science sources. With this level of planning accuracy, the MSATA procedure should yield a final total pointing accuracy for NIRSpec MOS targets of <20 mas within the preselected 200 mas-wide MSA shutter. Here we present analysis of test cases using simulated datasets that were used to help define and check operations flow for NIRSpec MSATA.
Most observations with NIRSpec Spectroscopy will require high spatial resolution images of the science field previous to performing the spectroscopy. This is due to the fact that the standard NIRSpec target acquisition (TA) needs to acquire reference stars to deliver a position RMS of less than 20mas. NIRSpec TA uses 8 - 20 reference stars with accurate astrometry (< 5 mas), calculates pixel centroids of the stars, transforms their pixel coordinate positions to the ideal sky frame and calculates the slew to accurately place the science targets in the Micro Shutter Array. For some planned observations, very high spatial resolution Hubble Space Telescope images might be already available, and in other cases NIRCam observations will be performed. For a planned NIRSpec observation, we describe in detail the proposed method to generate a high resolution image mosaic to plan NIRSpec spectroscopy. We show some of the data products that have been developed using actual HST observations. We also describe the proposed procedure using simulated NIRCam images for derivation of source catalogs. Rapid availability of these two data products will be crucial for the success of many NIRSpec observations.
Proc. SPIE. 9910, Observatory Operations: Strategies, Processes, and Systems VI
KEYWORDS: Stars, Cameras, Sensors, Calibration, Spectroscopy, Spectroscopy, Imaging spectroscopy, Molybdenum, Target acquisition, James Webb Space Telescope, James Webb Space Telescope, Camera shutters
The Near-Infrared Spectrograph (NIRSpec) is the work-horse spectrograph at 1-5microns for the James Webb Space Telescope (JWST). A showcase observing mode of NIRSpec is the multi-object spectroscopy with the Micro-Shutter Arrays (MSAs), which consist of a quarter million tiny configurable shutters that are 0. ′′20×0. ′′46 in size. The NIRSpec MSA shutters can be opened in adjacent rows to create flexible and positionable spectroscopy slits on prime science targets of interest. Because of the very small shutter width, the NIRSpec MSA spectral data quality will benefit significantly from accurate astrometric knowledge of the positions of planned science sources. Images acquired with the Hubble Space Telescope (HST) have the optimal relative astrometric accuracy for planning NIRSpec observations of 5-10 milli-arcseconds (mas). However, some science fields of interest might have no HST images, galactic fields can have moderate proper motions at the 5mas level or greater, and extragalactic images with HST may have inadequate source information at NIRSpec wavelengths beyond 2 microns. Thus, optimal NIRSpec spectroscopy planning may require pre-imaging observations with the Near-Infrared Camera (NIRCam) on JWST to accurately establish source positions for alignment with the NIRSpec MSAs. We describe operational philosophies and programmatic considerations for acquiring JWST NIRCam pre-image observations for NIRSpec MSA spectroscopic planning within the same JWST observing Cycle.
Proc. SPIE. 9149, Observatory Operations: Strategies, Processes, and Systems V
KEYWORDS: Detection and tracking algorithms, Sensors, Multiplexing, Monte Carlo methods, Space telescopes, Astronomical imaging, Algorithm development, James Webb Space Telescope, Camera shutters, Hubble Space Telescope
The James Webb Space Telescope Near-Infrared Spectrograph (NIRSpec) instrument will offer a powerful multi-object spectroscopic capability enabled by the micro-shutter arrays (MSAs). The MSAs are fixed grids of configurable shutters that can be opened and closed on astronomical scenes. With this mode, the NIRSpec instrument can observe more than 100 targets simultaneously. The NIRSpec team and software developers at the Space Telescope Science Institute (STScI) have been implementing specialized algorithms in an MSA Planning Tool (MPT) to facilitate the complex observation planning process. Two main algorithms, the “Fixed Dithers” and “Flexible Dithers” algorithms, have been defined to achieve optimal multiplexing results with different observing strategies. The MPT is available to the astronomical community as part of the ASTRONOMER’S PROPOSAL TOOL (APT), an integrated software package for the preparation of observing proposals developed by STScI.
NIRSpec is the main near-infrared spectrograph on board the James Webb Space Telescope, offering multi-object
capabilities as well as an integral field unit and a number of fixed slits for studies of individual objects. In this paper, we
describe the unique challenges in calibrating this complex instrument, and the approach taken to deal with them, both in
terms of operational procedures and via automated processing of NIRSpec data. We provide a high-level description of
the sequence of processing steps required for NIRSpec science data, and the necessary on-ground calibration files. We
focus our discussion on the case of a typical multi-object observation with the MSA, in which adjacent micro-shutters
are used to sample the science object and the sky background in an alternating way. This dithering strategy is particularly
well suited for faint targets, but its guiding principles also apply to other NIRSpec modes.