Monitoring of the occulting spot positions using earth flats shows that they move over weekly, and even daily time scales in an unpredictable manner. The cause of this motion is unknown. Their locations typically vary by ~0.3 pixels (8 milliarcseconds) over the span of one week, but on occasion they have shifted by 1 to 5 pixels over 1 to 3 weeks. While inserted in the beam, however, they remain stable to better than ±0.1 pixel, and when cycled within an orbit they return to the same position within ±0.25 pixel.
The uncertainties in the locations lead to star-to-spot registration errors. After the acquisition exposure the star is moved to the predefined aperture position of the spot, which is measured on the ground from on-orbit flats and may be out of date. Due to the layout of ACS, it is not possible to determine the spot location automatically before a coronagraphic observation, as can be done for NICMOS. Also, unlike STIS, the star cannot be dithered around until the stellar flux within the occulter is minimized.
Star-to-spot registration errors affect coronagraphic imaging. If the star is significantly offset from the spot center (> 3 pixels), then one side of the spot interior and edge will be brighter than expected, and may possibly saturate much earlier than predicted. A large offset will also slightly degrade the coronagraphic suppression of the diffraction pattern. Most importantly, even slight changes in the spot position will alter the residual diffraction pattern, introducing mismatches between the target and reference PSFs that may result in large subtraction residuals. This means that an observer cannot rely on reference PSFs taken in other programs or at different times.
To reduce the impact of spot motion, observers using the ACS coronagraph are required to obtain a reference PSF in an orbit immediately before or after their science observation. A single reference PSF can be used for two science targets if all three objects can be observed in adjacent orbits and are of similar color (note that it is difficult to schedule more than five consecutive orbits). Otherwise, if multiple science targets are observed, each one will require a reference PSF. The additional reference PSF orbit(s) must be included in the Phase 1 proposal.
STScI is using a revised procedure to update the coronagraphic spot positions shortly before a proposal executes to provide better registration. In this procedure, the last measured offset of the spot from its defined aperture location is uploaded to HST a few orbits before a coronagraphic observation executes. By including a USE OFFSET special requirement for each coronagraphic exposure after an acquisition, the target is shifted by the appropriate amount. The spots are measured weekly from earth flats, so this method provides up-to-date positions than relying on the aperture location. This procedure adds approximately 40 seconds to each visit. This procedure is required for all coronagraphic observations. More details will be provided on the STScI ACS Web
site and in Phase II Proposal Instructions
.
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |