Long-term trends in the NICMOS Camera 2 obscuration pattern and aberrations
Phase retrieval measurements of well-exposed NICMOS Camera 2 star images show that long-term changes occurred in coma and the cold mask position over the 1997-1998 lifespan of the instrument. These caused time-dependent variations in the PSF diffraction pattern, notably the shape of the Airy rings and the positions of bands in the diffraction spikes. These effects are evident in both direct and coronagraphic images when two observed PSFs obtained at different times are subtracted from each other. The derived trends have been implemented in Version 5 of Tiny Tim. PSF monitoring programs for NICMOS after the cryocooler is in place should include similar exposures so that any further cold mask shifting can be tracked.
Wide-Field Infrared Survey Telescope–Astrophysics Focused Telescope Assets coronagraphic operations: lessons learned from the Hubble Space Telescope and the James Webb Space Telescope
Abstract. The coronagraphic instrument (CGI) currently proposed for the Wide-Field Infrared Survey Telescope–Astrophysics Focused Telescope Assets (WFIRST-AFTA) mission will be the first example of a space-based coronagraph optimized for extremely high contrasts that are required for the direct imaging of exoplanets reflecting the light of their host star. While the design of this instrument is still in progress, this early stage of development is a particularly beneficial time to consider the operation of such an instrument. We review current or planned operations on the Hubble Space Telescope and the James Webb Space Telescope with a focus on which operational aspects will have relevance to the planned WFIRST-AFTA CGI. We identify five key aspects of operations that will require attention: (1) detector health and evolution, (2) wavefront control, (3) observing strategies/postprocessing, (4) astrometric precision/target acquisition, and (5) polarimetry. We make suggestions on a path forward for each of these items.
Fragment Rotational State Distributions From The Dissociation Of NeIBr: Experimental And Theoretical Results
The IBr fragment rotational state distributions that result when the NeIBr van der Waals molecule undergoes vibrational predissociation have been measured in a pump–probe laser‐induced fluorescence experiment. Independent of initial vibrational state and the number of quanta of vibrational energy lost from the I–Br coordinate, the rotational distributions extend over the full range of energetically accessible states. From the observation of energetic constraints on the rotational distribution, the dissociation energy (D0) is calculated to be 65.5±1.2 cm−1 for the A electronic state, v=16. For the X electronic state, v=0, D0=71.8±1.2 cm−1. Quantum mechanical bound state calculations carried out on a model A electronic state potential energy surface are in quantitative agreement with this result. The rotational distributions are broader than that predicted by either a Franck–Condon or classical impulsive model for the dissociation. The distributions are qualitatively in accord with classical trajectory calc...