所有NIRspec需求都是HST/WFC3预成像？在WFC3成像中使用Milky Way星配准近红外光谱MSA观测
All NIRspec needs is HST/WFC3 pre-imaging? The use of Milky Way Stars in WFC3 Imaging to Register NIRspec MSA Observations
摘要：
詹姆斯·韦伯太空望远镜（JWST）将是一个精致的新近红外天文台，通过欧空局的NIRspec仪器及其独特的微型快门阵列（MSA）进行成像和多目标光谱学，允许通过打开特定的0.002”宽的微型快门门将狭缝定位在天文目标上。为了确保正确的目标获取，在光谱观测开始之前，需要验证MSA的天空位置。机载质心程序在目标采集（TA）图像中记录预先识别的引导星的位置，这是通过MSA的无色散（图像模式）的短光谱前曝光，所有快门都打开。突出的问题是，相对于典型的高红移目标，在TA的正确光度范围内的银河系恒星的可用性。我们在这里使用恒星和$z\sim8$候选星系来探索这一点，这些星系是在重新电离星系巡天（BoRG[z8]）的源提取器目录中确定的，这是一个与哈勃空间望远镜宽视场相机3完全并行的程序。我们发现：（a）单个WFC 3场包含足够的银河系恒星，以满足NIRspec天体测量的要求（20毫弧秒），前提是其和NIRspec TA是WFC 3 F125 W中的$m_{lim}>24.5$ AB，（b）因此，如果需要，单个WFC 3图像可以用作前图像，（c）WFC 3镶嵌图和伴随的TA图像满足WFC 3 F125 W中$\sim23$ AB等的天体测量要求，（d）由于缺乏银河系恒星，没有特定的银河系纬度需要更深的TA图像，以及（e）如果需要与目标在同一MSA象限中的引导星，则需要WFC 3 F125 W中的$\sim24$ AB等的深度。我们以BoRG确定的$z\sim8$候选星系为例，要求在20”范围内有一颗银河系恒星。在这种情况下，需要F125 W中25.5 AB的深度（$\sim$ 97%置信度）。
英文摘要：
The James Webb Space Telescope (JWST) will be an exquisite new near-infrared observatory with imaging and multi-object spectroscopy through ESA's NIRspec instrument with its unique Micro-Shutter Array (MSA), allowing for slits to be positioned on astronomical targets by opening specific 0.002"-wide micro shutter doors. To ensure proper target acquisition, the on-sky position of the MSA needs to be verified before spectroscopic observations start. An onboard centroiding program registers the position of pre-identified guide stars in a Target Acquisition (TA) image, a short pre-spectroscopy exposure without dispersion (image mode) through the MSA with all shutters open. The outstanding issue is the availability of Galactic stars in the right luminosity range for TA relative to typical high redshift targets. We explore this here using the stars and $z\sim8$ candidate galaxies identified in the source extractor catalogs of Brightest of Reionizing Galaxies survey (BoRG[z8]), a pure-parallel program with Hubble Space Telescope Wide-Field Camera 3. We find that (a) a single WFC3 field contains enough Galactic stars to satisfy the NIRspec astrometry requirement (20 milli-arcseconds), provided its and the NIRspec TA's are $m_{lim}>24.5$ AB in WFC3 F125W, (b) a single WFC3 image can therefore serve as the pre-image if need be, (c) a WFC3 mosaic and accompanying TA image satisfy the astrometry requirement at $\sim23$ AB mag in WFC3 F125W, (d) no specific Galactic latitude requires deeper TA imaging due to a lack of Galactic stars, and (e) a depth of $\sim24$ AB mag in WFC3 F125W is needed if a guide star in the same MSA quadrant as a target is required. We take the example of a BoRG identified $z\sim8$ candidate galaxy and require a Galactic star within 20" of it. In this case, a depth of 25.5 AB in F125W is required (with $\sim$97% confidence).
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