NICMOS Two Gyro Mode Coronagraphic Performance

In preparation for the Hubble Space Telescope (HST) entry into Two Gyro Mode (TGM) operations, the NICMOS coronagraphic performance was assessed during the February 21, 2005 TGM on orbit test (ID: 10448) in two filter bands (F110W and F160W). The gyroscope pair 2-4 was used for the TGM test. Observations were obtained of a previously observed calibration target (GJ517; sp.type=K5Ve) coronagraphically and with direct imaging. The F110W filter performance very closely tracks the three gyro mode (ID: 10177) results. The observed jitter of ~6.2 mas had little effect on the 1.1 μm coronagraphy. This result clearly shows that 1.1 μm coronagraphy is robust against the observed TGM jitter. The NICMOS coronagraph inner working angle (IWA) is 3.2 λ/D at 1.1 μm. Consequently, there is sufficient performance compliance to target decentering of these RMS amplitudes within the coronagraphic hole. There was a slight decline in performance with the F160W filter with respect to three gyro mode which may be due to imaging systematics; i.e., breathing and jitter. The 1.6 μm coronagraphy is more susceptible to imaging systematics primarily due its broader PSF (IWA=2.2 λ/D). At r=10 pixels, the coronagraphic diffracted and scattered light suppression factor is ~3 compared to direct imaging, while it is a factor of 4 in three gyro mode. The observed coronagraphic performance is essentially unaffected at 1.1 μm with TGM operations, while there is a marginal decline at 1.6 μm of similar amplitude to that arising from other well-known HST/NICMOS orbit-driven instabilities. The improvement in image contrast (and dynamic range) with coronagraphy and TGM is still substantial over direct imaging. Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. Instrument Science Report NICMOS 2005-00