Physics – Optics
Scientific paper
Jul 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008hst..prop11414s&link_type=abstract
HST Proposal ID #11414. Cycle 17
Physics
Optics
Scientific paper
As originally designed the NICMOS coronagraph had two fixed focii in the optical path: {a} at the coronagraphic hole {image plane obscuration} in the camera 2 field divider mirror {at the f/24 focus of the input OTA beam} and, {b} at the detector {reimaged focal plane of the f/45 camera 2 relay optics}. Because of the forward displacement of the cold optical bench holding the Camera 2 detector resulting from the larger-than-expected expansion of the solid N2 cryogen early in HST Cycle 7 {as described and documented by the 'Dewar Anomaly Review Board'}, the two focii are now aconjugate. For direct {non-coronagraphic} imaging, this is of little concern and "best focus" at the detector is readily achieved by adjustment of NICMOS's Pupil Alignment Mechanism mirror. This, however, results in a "soft" focus at the upstream f/24 image plane of the coronagraphic hole and a degradation in coronagraphic contrast. Further deformation and hysteretic displacement of the NICMOS dewar and associated differential stresses on its mounting straps are likely to result from power cycling the NCS during SM4. As a result, the intended locations of the camera 2 focal planes in the cold well at the detector, and in the warm optics outside of the dewar, are likely to change. Such a change was seen following the warmup and cooldown of the NICMOS dewar following cryogen depletion in January 2000 and the installation of the NCS in 2002. If uncompensated, through informed investigation, coronagraphy will be adversely affected since the scattered and diffracted background light from a target afocally imaged in the coronagraphic hole will increase and thus reduce the achievable contrast at the detector image plane. The coronagraphic stray light rejection is most efficient by minimizing the spot size of an input PSF in the coronagraphic hole, but is traded against image defocus at the detector. The purpose of this test is to find the optimum wavelength-dependent compromise in the PAM position to maximize the coronagraphic image:background contrast ratio at the detector. Further details and background information may be found in the SMOV/7157 test report "NICMOS Optimal Coronagraphic Focus Determination" available from the NICMOS IDT. This proposal corresponds to SMOV activity NIC-11.;
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