5.9 Large Angle Scattering

Analysis of the WFPC2 saturated star images indicate that the large angle scattering (>3" from a star) is significantly higher than expected.

Three data sets were used to determine the WFPC2 scattering. The first set was from the SMOV Ghost Check Proposal 5615, in which 100-second images of Cas (V=2.7) were obtained at the center of each chip in F502N. The second set was a series of 6-second exposures of Vega (V=0.0) centered on WF2 through F410M (WFPC2 GTO Proposal 5205). The third set was Eridani (V=3.73) centered on the PC and taken through F631N (500s each) and F953N (2200s each). These were from GTO Proposal 5611.

WFPC2 scattering was determined by computing the azimuthal average and azimuthal median profiles. The regions near the diffraction spikes and saturated columns were not used. The profiles were determined using images corrected for horizontal smearing.

The measurements indicate that the average scatter in WFPC2 is an order of magnitude greater than in WF/PC-1. The increase is due to scattering in WFPC2, not due to the OTA. In the WFPC2 images, the pyramid edge shadow is not visible in the scattered light; the light is spread out to the chip edges, indicating that most of the scattering occurs after the pyramid. However, the light level in adjacent channels is back down at the WF/PC-1 levels as shown in Figure 5.13.

The scattering does not show any strong dependence on wavelength between 410 nm and 953 nm, within the uncertainties of the measurements.

The scattered light is not uniform. There are high frequency spatial structures in the form of streaks radiating outwards from the star. These features are probably both wavelength and position dependent, and so cannot be readily subtracted.

The source of the WFPC2 scattering may be the CCDs. The WF/PC-l CCDs were back illuminated and had shiny surfaces. The electrode structure was not visible over most of the wavelength range. The WFPC2 CCDs, however, are front illuminated, so the electrode structure is visible and may be scattering the light. There was a large ghost in WF/PC-l due to a reflection between the CCD and filter, but no such feature has been seen in WFPC2. The flux from this missing ghost may instead constitute part of the scatter. (See also related material in Observing Faint Targets Near Bright Objects.)

Figure 5.13: Large Angle Scattering. The proportion of the total flux in F555W falling per square arcsecond as a function of the distance from the peak of a saturated stellar image. These curves are for a target in the PC. Note the large drop in the scattered light level when looking in an adjacent camera.