Determination of Spitzer Space Telescope focus from IRAC images without a focus slew
Determination of Spitzer Space Telescope focus from IRAC images without a focus slew
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在没有焦点旋转的情况下根据 IRAC 图像确定斯皮策太空望远镜焦点
DOI：
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发表时间：
2004
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影响因子：
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通讯作者：
J. Schwenker
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作者：
W. Hoffmann;J. Hora;J. Mentzell;C. Marx;P. Eisenhardt;S. Carey;S. Megeath;J. Schwenker
在发射之前，斯皮策太空望远镜（SST）的次级聚焦机构被设置为预期的在轨聚焦值。这一预测设定是根据双通道冷室测量和计算出的地面至轨道修正确定的，其不确定性大于所需的在轨聚焦精度。由于担心影响所有斯皮策仪器的低温机制可能出现故障，因此要求在从初始设置到所需设置的一系列移动中进行任何焦点校正。确定所需焦点移动的任务落在了IRAC（红外线阵列相机）身上，这是一种受散焦影响最大且最敏感的仪器。为了直接从在固定焦点处检查图像来确定焦点，我们开发了两种方法，“Simfit”和“Focus Diversity”（W。F. Hoffmann，et. 1）。Simfit通过在一系列焦点设置下获得观察图像和模拟图像系列之间的最佳匹配来找到焦点。焦点多样性利用焦平面曲率来找到焦平面上的变化图像模糊与模型散焦曲线的最佳拟合。在重新聚焦活动之前和期间，分析了在四个IRAC波段中的每个波段的许多现场位置上对单个星星的观测。由此产生的再聚焦移动使焦点接近指定要求，即距离所需IRAC最佳焦点在0.3 mm以内。这小于SST焦点处最短IRAC带（3.58微米）处0.52 mm的“衍射焦点单位”（λx（f/2））。聚焦的改善在星星图像的外观和计算的噪声像素两者中是明显的。
Prior to launch, the Spitzer Space Telescope (SST) secondary focus mechanism was set to a predicted desired in-orbit focus value. This predicted setting, determined from double-pass cold chamber measurements and calculated ground-to-orbit corrections, had an uncertainty greater than the required in-orbit focus accuracy. Because of concern about the potential for failure in a cryogenic mechanism affecting all Spitzer instruments, it was required that any focus correction be made in a set of moves directly from the initial to the desired setting. The task of determining the required focus moves fell to IRAC (Infrared Array Camera), the instrument most affected by and sensitive to defocus. To determine the focus directly from examining images at a fixed focus, we developed two methods, "Simfit" and "Focus Diversity" (W. F. Hoffmann, et. al.1). Simfit finds the focus by obtaining the best match between observed images and families of simulated images at a range of focus settings. Focus Diversity utilizes the focal plane curvature to find the best fit of the varied image blur over the focal plane to a model defocus curve. Observations of a single star at many field locations in each of the four IRAC bands were analyzed before and during the refocus activity. The resulting refocus moves brought the focus close to the specified requirement of within 0.3 mm from the desired IRAC optimum focus. This is less than a "Diffraction Focus Unit" (λx(f/2)) of 0.52 mm at the SST focus at the shortest IRAC band (3.58 microns). The improvement in focus is apparent in both the appearance and the calculated noise-pixels of star images.