1 National Space Institute, Technical University of Denmark2 Astrophysics, National Space Institute, Technical University of Denmark
Optic response model comparison to NuSTAR on-ground calibration data
The Nuclear Spectroscopic Telescope ARray (NuSTAR) is a NASA Small Explorer mission carrying the first focusing hard X-ray telescope (5 − 80 keV ) to orbit. NuSTAR is slated for launch in 2012. Through a leap in sensitivity, the realization of focusing optics holds promise of heralding in a golden age for hard X-ray astronomy similar to the one introduced by XMM-Newton (ESA) and Chandra (NASA) in the soft X-ray range (0.1−15 keV ) twelve years earlier. The leap in sensitivity over a broad energy range is achieved using depth-graded multilayer coatings. In order to predict and interpret the optic response for both on- and off-axis NuSTAR observations, detailed knowledge of the as-coated multilayer is required. The purpose of this thesis is to establish a multilayer reference database. As an integral part of this effort, a hard X-ray calibration facility was designed and constructed. Each of the nearly five thousand substrates installed in the NuSTAR optics are coated with a unique multilayer. The uniqueness derives from multilayer deposition induced non-uniformities and run-to-run variations. As mapping the multilayer of each substrate is not feasible, this thesis presents a detailed study of the coating uniformity and run-to-run variations. The uniformity of multilayers deposited on representative flight spare substrates is mapped out as a function of deposition chamber location. The uniformity map describes the relative deposition for a given chamber location compared to a witness sample. A similar witness sample was included in all flight coating runs. From the flight witness samples, the multilayer run-to-run variations are determined. Combining the uniformity map with the witness multilayer provides an estimate of the as-coated multilayer for all substrates mounted in the optics. To couple the as-coated multilayer to the actual optics, ray tracing is carried out in a detailed geometric model of the optic, including in-situ measured figure error for the mounted substrates. The effective area as a function of energy estimated from ray tracing is compared to NuSTAR on-ground calibration data. The on-ground calibration and flight witness sample investigations were carried out at a hard X-ray facility constructed for the same purpose. This thesis established the NuSTAR multilayer reference database and found that it provides a good description of the as-coated multilayers of the NuSTAR optics. A thorough quantitative study of the NuSTAR effective area requires the utilized ray tracing tool to mature further. Currently, the effective area estimated from the multilayer reference database represents an optimistic upper limit. Along with a conservative estimate derived from on-ground calibration data, the expected effective area of NuSTAR has been constrained. The multilayer reference database estimates the effective area to be on average 10% higher than the NuSTAR level 4 requirements.