Computer Science – Performance
Scientific paper
Jul 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001hst..prop.8979s&link_type=abstract
HST Proposal ID #8979
Computer Science
Performance
Hst Proposal Id #8979
Scientific paper
As originally designed the NICMOS coronagraph had two focii at conjugate points in the optical path with the coronagraphic hole at the f/24 focus of the input OTA beam, and the detector at the reimaged f/45 focus in Camera 2. Because of the forward displacement of the cold optical bench holding the Camera 2 detector, as a result of the larger-than-expected expansion of the solid N2 cryogen {as described and documented by the 'Dewar Anomoly Review Board'}, the two focii are now aconjugate. For direct imaging this is of little concern, and the HST/NICMOS Camera 2 focus interface is established by co-locating the f/45 image plane on the detector. This is done by de-spacing the relayed focus through a translative motion {with compensating comal tilt correction} of the Pupil Alignment Mechanism {PAM}. The mirror which this mechanism drives is upstream of the field divider mirror upon which the coronagraphic hole resides. Therefore, achieving a "best" focus at the detector results in a "soft" focus {in the f/24 image plane} at the coronagraphic hole. This leads to a wavelength-dependent increase in the diffracted energy in the now-defocused unocculted wings of a PSF from a target placed inside of the coronagraphic hole {as the f/24 image plane will fall behind the surface of the camera 2 field divider mirror} increasing the scattered and diffracted background around the target and lowering the field contrast at the detector image plane. In principal the coronagraphic stray light rejection would be most efficient by minimizing the spot size of an input PSF in the hole. This, however, is traded against a small degree of defocus at the detector. Ultimately, the best coronagraphic performance is achieved where the image contrast between an unocculted target and the residual background from an occulted source {both affected differently by focus and subsequent scattering} is maximized. The purpose of this test is to find the optimum PAM position to maximize the coronagraphic image:background contrast ratio. Further details and background information may be found in the SMOV/7157 test report "NICMOS Optimal Coronagraphic Focus Determination" available from the NICMOS IDT.
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