GettingNuSTARon target: predicting mast motion

Abstract

The Nuclear Spectroscopic Telescope Array (NuSTAR) is the first focusing high energy (3-79 keV) X-ray observatory operating for four years from low Earth orbit. The X-ray detector arrays are located on the spacecraft bus with the optics modules mounted on a flexible mast of 10.14m length. The motion of the telescope optical axis on the detectors during each observation is measured by a laser metrology system and matches the pre-launch predictions of the thermal flexing of the mast as the spacecraft enters and exits the Earths shadow each orbit. However, an additional motion of the telescope field of view was discovered during observatory commissioning that is associated with the spacecraft attitude control system and an additional flexing of the mast correlated with the Solar aspect angle for the observation. We present the methodology developed to predict where any particular target coordinate will fall on the NuSTAR detectors based on the Solar aspect angle at the scheduled time of an observation. This may be applicable to future observatories that employ optics deployed on extendable masts. The automation of the prediction system has greatly improved observatory operations efficiency and the reliability of observation planning.

Observational artifacts of Nuclear Spectroscopic Telescope Array: ghost rays and stray light

“…3 (4) length of the mirrors, but as mentioned it cannot completely eliminate the GR. To further reduce the GR, baffling of some sort is required.…”

“…To complicate matters further, thermal gradients along the mast cause it to flex on an orbital time scale, 4 and the motion smears out the PSF on the detector plane, resulting in a timedependent clipping of the area. However, the problem is completely determined by knowing the relative location of the aperture stop, stationary with respect to the detectors, and the focal plane bench, with respect to the OA location, stationary in the optical bench frame.…”

“…3 (4) without weighting it by an input spectrum. This will underestimate the area at higher off-axis angles due to there being more low-energy than high-energy photons in the averaged energy interval.…”

“…The optical and focal plane benches are separated by a mast providing a focal length of 10.15 m. A laser metrology system is used to keep track of the lateral and angular displacement of the two benches, 3 which is caused by thermal motions in the mast, and a star tracker coaligned with the optics provides the absolute aspect. 4 NuSTAR is a pointed observatory and averages about one observation a day.…”

In-flight performance of the Canadian Astro-H Metrology System

“…A similar alignment system [8][9][10] is used on the NuSTAR satellite and it will continue to be required for several upcoming missions [11][12][13] in x-ray astronomy.…”