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Preparing NIRSpec Observaons

The Near- Spectrograph (NIRSpec) aboard the James Webb JWST/NIRSpec JWST NIRSpec Observaons of (JWST) will excel at deep spectroscopic observaons of the faintest objects in the universe. The mul-object mode using the the Hubble Ultra Deep Field: Micro-shuer Arrays (MSAs) increases mulplexing to acquire up to NIRSpec Target Acquision hundreds of simultaneous spectra of high redshi galaxies. The ’s Ultra-Deep Field (HST UDF) provides a foundaon for An Analysis of Target Acquision in the analyzing and simulang deep observaon cases with the NIRSpec MSAs. The target acquision method for NIRSpec MSA observaons Abstract “Empest” Deep Field uses sets of reference stars to align the MSA shuers with exquisite accuracy on faint galaxies. However, the HST UDF field was selected because it has a dearth of available bright stars that would be opmal for field acquision. This poster presents applicaon of the MSA target Overview of NIRSpec acquision method to the empest deep field and some strategies that Introducon can be adopted to accurately acquire the ultra-deep with Major Funconal Elements NIRSpec is the Near-Infrared Spectrograph for the James Webb Space Telescope. NIRSpec. ! Designed and built by the (ESA), NIRSpec is a mul- NIRSpec Performance object spectrograph that will enable observaons of up to 100 astronomical sources simultaneously. It has a large (see Figure 1) field of view (3’ X 3’) and is Analyzing the Hubble UDF for TA Sources NIRSpec Target Acquision highly sensive over its wavelength range (0.6−5 μm.) NIRSpec Target Acquision is opmally designed to work on stars. In the Hubble Ultra-Deep Field, stars are sparse. Figure 2 shows an analysis of the The NIRSpec plane assembly contains two closely bued sensor chip arrays number of available stars vs. magnitude bins within the UDF catalog. Bright Target Acquision Process (SCA) of 2048 x 2048 pixels. The Micro Shuer Array (MSA) consists of a 2 X 2 (AB mag <24) sources with stellarity profiles of 0.9 or greater are very rare. mosaic of quadrants. Each quadrant has 365 shuers along the dispersion axis Expanding TA source criteria to 0.7 Stellarity does not gain many potenal Reference Stars and 171 shuers along the spaal axis. reference objects. In total, there are only ~20 sources with Stellarity > 0.7 and AB mag brighter than 25!

9 NIRSpec MSA Planning Tool 8

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6 NIRSpec Target Acquision Preparing NIRSpec Observaons 5 NStars 4 The NIRSpec target acquision (TA) algorithm uses 8−20 reference stars NIRSpec Science Operaons 3 observed through the MSA to precisely posion the science targets within the 2 micro−shuers. The NIRSpec TA process (Figure 1) observes the reference stars 1 NIRSpec Engineering Operaons through the MSA in one exposure, then offsets the telescope by an average ½ 0 shuer pitch in x and y and takes another exposure to migate the effects of the 21-22 22-23 23-24 24-25 25-26 21-22 22-23 23-24 24-25 25-26 AB Mag micro-shuer grid. Centroids are calculated on-board the JWST spacecra for Figure 2: Histogram plots of the number of available reference stars in 1 magnitude bins from AB NIRSpec Mission Scenario each observed star in both exposures. An algorithm is used to calculate the best Mag = 21 – 22 to AB mag = 25 – 26. There are only three stars in the catalog with AB mag < 20.0. The slew and orient correcon to precisely align the science sources in their planned le plot presents stars / compact objects (stellarity >0.9) and the right plot shows sources with NIRSpec Data Processing MSA shuers. Stellarity > 0.7. Expanding the Stellarity criteria gains only a few potenal reference star targets. Tests on the UDF using the MSA planning tool Figure 1: Schemac view ’ of the MSA layout, with The Astronomer s Proposal Tool (APT) is the primary soware tool used by target acquision general observers to select astronomical objects of interest for a given exposure. reference stars shown in The MSA Planning Tool (MPT) is a NIRSpec component within APT that is used to red, and science targets (blue) shown in their open plan NIRSpec MSA mode science. The MPT can display an image of the MSAs MSA shuers (green). with shuer state informaon and potenal targets over-ploed. The NIRSpec Some science targets are MSA quadrants have many failed closed shuers, and our studies have shown not observed because they lie in the same row as that planning target acquision reference stars away from failed closed shuers other sources (spectral is very important. overlap) or because their flux is aenuated by the MSA support grid. The Figures 3 and 4 present some test cases of mulple poinngs found using the target acquision process MSA Planning Tool with target acquision reference catalogs as input. In this acquires exposures and case, the plans were made to specifically place as many of the TA reference carefully measures the reference stars to correct objects into the MSA quadrants as possible. The candidate reference objects are the poinng to the best relavely well distributed throughout spaal area covered by the UDF catalog. science posion. NIRSpec TA Uncertaines: This TA procedure imposes strict requirements on the relave astrometry between the reference stars and the science targets. The post-TA uncertainty on science source posioning depends on the input astrometric catalog accuracy

AAst and the number of stars or point-like sources available to use as TA reference, Nstars. The TA uncertainty is esmated as: Dark grey = Failed Closed Shutters Post TA Uncertainty on science target posions (in milli arcseconds) = 2 2 2 sqrt[(Aast + 104.0) + (sqrt((647.5 +Aast ))/(sqrt(Nstars)) + 62.0)+75.0] AAst = Catalog Astrometric Accuracy, Nstars = # of Available Reference Stars

5mas catalog accuracy with Nstars > 8 will yield science target placement Pointing 1 Pointing 2 accuracy of 20 mas (1/10th of a shuer width) or beer. Posions with less than Figure 3: Two MSA poinngs on the Hubble UDF Field with potenal reference stars shown as green Nstars = 3 can not be planned because of poor accuracy (especially on orient circles. In the Le case, the Stellarity 0.9 criteria was used for sources brighter than 25th AB magnitude. To the right, the same analysis is done at a different poinng and with stellarity > 0.7. correcon). These accuracy numbers assume S/N > 20 is reached on reference At , 5-8 Reference stars are found within the magnitude bins described in Table 1. The stars in the TA images. The NIRSpec TA Concept and uncertaines were defined zoomed view to the upper right for Poinng 2 shows a candidate reference star that must be by Peter Jakobsen in 2005. rejected because of a high probability of centroiding problems caused by failed closed shuers.

NIRSpec Esmated TA Exposure Sensivity Figure 4: This figure shows a poinng on the outskirts of the The NIRSpec target acquision concept requires that reference stars be UDF catalog field that has only three potenal reference Tracy BECK,! stars within the one of the MSA quadrant regions. Of detected in the set up exposures with a Signal to Noise of 20 or higher. The these three stars, at most two are available in the Diane Karakla,! NIRSpec instrument filter and readout paern (exposure me) defines the magnitude sensivity bins shown in Table 1. This is a sensivity in these exposures. possible poinng within a UDF observing plan where Tony Keyes! NIRSpec standard Target acquision might not be successful. Expanding the Stellarity criteria to more Leonardo Ubeda! NIRSpec target acquision exposures are acquired with one of three filters in extended sources might make TA possible, but the result the instrument: F110W, F140X or the CLEAR filter. The F110W is well matched would be lower overall accuracy in the final placement of & the STScI NIRSpec Team! science sources. Acquiring a deep NIRCam image that in central wavelength and throughput profile to the JWST NIRCam instrument’s covers a wider spaal field than the UDF could reveal more Space Telescope Science Institute, Baltimore MD! F115W filter. For bright sources, using the NIRSpec filter along with astrometric potenal TA reference objects in the outer poinngs. reference posions derived from NIRCam pre-images will result in opmal TA accuracy. The NIRSpec F140X filter has a broader profile and is useful for fainter objects. The CLEAR filter covers the 1-5µm window and can be used for target acquision on very faint reference sources. There are presently two* NIRSpec NIRSpec TA in the UDF detector readout modes that define exposure me that will be used for target We carried out an analysis of the availability of NIRSpec reference objects for acquision. NRSRAPID is a short exposure me, ~10.6 seconds, and NRS is four Target Acquision in Hubble Ultra Deep Field, which is notably “empty” of stars. groups of NRSRAPID frames, ~42.4 seconds. In both the NRS and NRSRAPID Our key findings were: readouts, three reads are acquired and compared to reject cosmic rays in the TA à Many poinngs in the central region of the Hubble Ultra Deep Field exposures. Defined in the below table are the TA exposure sensivies (AB can find suitable reference objects (to ABMag~25) for Target Acquision. Mag) grouped by Filter and Readout Paern from the saturaon limit to S/N à Only a few candidate TA reference objects are gained by relaxing =20. catalog Stellarity criteria. But every added source should improve TA results. à The number of reference stars at a given poinng is typically on the F110W F140X CLEAR order of 5-8. Uncertainty on placement of science sources should be in the 20-25mas range (or beer) if catalog astrometric accuracy is 5mas. 17.7 – 22.0 NRS* 18.8 – 23.1 19.6 – 24.0 à Poinng posions on the outskirts of the Hubble UDF have few TA (ABMag) reference objects. Expanding the available catalogs or relaxing Stellarity criteria NRSRAPID 19.2 – 24.1 20.3 – 25.1 21.1 – 25.7 to very extended objects might be necessary to execute quality target Table 1: Target acquision reference star sensivies in AB Magnitudes from the saturaon limit to S/N of acquision. 20 for each Filter/ Readout Paern combinaon. * - A new readout paern is proposed but not yet implemented that will get near to NRS sensivity but in shorter exposure overhead me.