The CCDs used in the HRC and WFC suffer from a halo that is caused by very red photons passing through the device and being scattered back into the detector by the mounting substrate. This creates a large halo in HRC images beyond 7000 Å and WFC images past 9000 Å. At 8000 Å in the HRC, the halo contains about 10% of the light. At 10,000 Å, it contains about 30% and increases the surface brightness of the PSF wings by over an order of magnitude, overwhelming the PSF diffraction rings and spikes. A discussion of this may be found in Gilliland & Riess, (2002) HST Calibration Workshop at:
http://www.stsci.edu/hst/HST_overview/documents/calworkshop
/workshop2002/CW2002_Papers/CW02_gilliland
.
In the F850LP filter, in particular, extremely red stars show a progressive loss of flux in small to moderate sized apertures as a function of color. A paper by Sirianni et al. (2005, astroph/0507614, available at:
http://adcam.pha.jhu.edu/instrument/photometry/sirianni.pdf
The paper provides a detailed recipe to correct for this effect. This halo effect is only partially treated by the Exposure Time Calculator
. Observers can use synphot
(see Section 9.3.2) to accurately calculate the photometry of red sources in the SDSS z-filter.
Long wavelength photons that pass through the CCD can also be scattered by the electrode structure on the back side of the device. This creates two spikes that extend roughly parallel to the x-axis. These spikes are seen at wavelengths longer than 9500 Å in both the HRC and WFC (see Figure 5.13 and Figure 5.14).
Figure 5.13: ACS WFC PSFs (10" x10"). FR914M images are saturated.In the UV the core of the PSF becomes rather asymmetrical due to midfrequency optical surface errors. In the SBC, a halo is created by charge migration at the microchannel plate surface. This effect, seen previously in STIS MAMA images, broadens the PSF core and redistributes a small portion of flux into a broad halo that can be approximated by a Gaussian with FWHM ~20 pixels. The peak flux for a point source centered on a pixel is reduced by 30% to 40% depending on wavelength.
The encircled energy curves presented in this handbook and incorporated into the ETC include all of the scattering effects discussed here.
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