AUTHORS : J. Krist, G. Hartig, A.R. Martel.
LOCATION AND DATE : BATC : Jun 17, 2000
To verify the PSF suppression capabilities of the ACS coronagraph, on and off the spot, as well as its alignment.
The dataset consists of HRC entries 16428-16450.
The coronagraph test data utilized a monochromatic, laser-illuminated point source (6328 Ang) through RAS/HOMS. The models used in the coronagraph ISR (see ISR 00-04) included polychromatic effects and mid-spatial-frequency errors in the HST optics, neither of which is in the test setup. Therefore, some models were recomputed at 632.8 nm with no mid- frequency errors.
One problem was the flux calibration of the coronagraphic images. The non-coronagraphic data utilized two neutral density filters (ND3+2), while the coronagraphic data used one (ND3). Therefore, to derive how much light the coronagraph rejected, some of the unsaturated images were used for calibrations. These indicate that the ND2 filter had a transmission of 0.0797xND3. Using this, the flux calibration was bootstrapped.
The off-spot coronagraphic PSF has about 56% the flux of the regular HRC PSF, close to the 52% predicted by the model. The on-spot (0.9" radius spot) PSF has 4% of the regular PSF's flux, versus 1% as predicted by the model. However, this difference may be due to small shifts of the point source behind the spot and alignment of the Lyot stop. In general, the models and data agree to within reasonable limits.
The disappearance of the diffraction spikes at opposite offsets of the alignment mirror is expected. The Lyot stop is aligned between the incident and reflected chief rays of the M2 mirror, in front of which the stop is placed. When the M1 alignment mirror is tilted in one direction, the HST spiders are fully masked by the stop in the incident beam. When the mirror is tilted in the opposite direction, the spiders are masked in the reflected beam. When center-aligned, the spiders are partially masked.
Figure 1 shows an observed RAS/HOMS ACS coronagraphic PSF (CSIJ00169231949_1 - 0.9" spot @ 632.8 nm) compared with models with and without HST optical zonal errors. Clearly, the issue of diffraction spike visibility becomes a moot point once we start looking at things on- orbit. The scattered light from the HST zonal errors is at or greater than the diffraction spike intensity. The scattered light increases as you go to shorter wavelengths as well.
CEI SPECIFICATIONS :
The agreement between the observed and predicted coronagraph behavior is generally good.