Wide Field Camera 3 Instrument Handbook for Cycle 19 - Space ...

38 Chapter 5: WFC3 Detector Characteristics and Performance
5.4.4 Long-Wavelength Fringing
Multiple reflections between the layers of a CCD detector can give rise to fringing,
where the amplitude of the fringes is a strong function of the silicon detector layer
thickness and the spectral energy distribution of the light source. Like most
back-thinned CCDs, the WFC3 CCDs exhibit fringing at wavelengths longward of
~700 nm (see Figure 5.4). The amplitude of the flat-field signal for monochromatic
input increases gradually with wavelength and can reach levels of ± 50% at the longest
CCD wavelengths (fringe amplitude is the envelope of the curve shown in Figure 5.5).
An analysis of fringing effects in broadband-illuminated ground flats longward of
600nm (ISR 2010-04) has shown that F953N has the greatest fringe amplitude
(~16%), followed by the quad filters FQ889N, FQ906N, FQ942N, and FQ937N
(~10%). Other narrowband and quad filters have fringe amplitudes in the range of
0.5-4.6% (F656N, F658N, FQ672N, F673N, FQ674N, FQ727N, and FQ750N).
Although fringing is generally weak at wavelengths shorter than 700 nm, the very
narrow H alpha filter (F656N) exhibits a fringe amplitude of up to several percent in
flat fields acquired during ground testing (WFC3 ISRs 2008-17, 2008-46, 2010-04).
Note, however, that the amplitudes of fringing listed here (and in WFC3 ISR
2010-04) should be used only as an estimate of the effect in science data. Fringing will
be different for sources with spectral energy distributions (SEDs) which differ
significantly from the calibration lamp used to generate the ground flat fields. For
example, continuum sources in broad filters will effectively smooth out fringing
effects but that same filter can show strong fringes when illuminated by sources with
strong spectral lines or SEDs much narrower than the filter bandpass. Conversely, for
sources with SEDs similar to the calibration lamp, the fringes can be corrected by the
flat-fielding process.
The fringe pattern has been shown to be very stable, as long as the wavelength of
light on a particular part of the CCD stays constant, so fringing can be corrected if an
appropriate flat field is available. The fringe pattern can also be modeled, either by
interpolating between or combining monochromatic patterns previously obtained in
the laboratory, or from theoretical calculations. For a detailed explanation of ongoing
efforts to model the WFC3 fringe pattern, see Malumuth et al. (2003, Proceedings of
SPIE 4854, Future EUV/UV and Visible Space Astrophysics Missions and
Instrumentation, pp. 567–576).
Tools for correcting fringing are under development, and will be characterized with
on-orbit cluster photometry. A description of this work (“Fringing in the WFC3/UVIS
detector”) was presented by M. Wong at the 2010 STScI Calibration Workshop.