ACS Instrument Handbook for Cycle 26
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Advanced Camera for Surveys Instrument Handbook for Cycle 26 > Chapter 4: Detector Performance > 4.2 The CCDs

4.2.1 Detector Properties
WFC Properties
The WFC/CCD consists of two 4096 x 2048 charge-coupled devices that are sensitive from the violet to the near-IR. These CCDs are thinned, backside-illuminated devices manufactured by Scientific Imaging Technologies (SITe) and are butted together to create an effective 4096  4096 array with a gap corresponding to approximately 50 pixels between the chips. The CCD camera design incorporates a warm dewar window, designed to prevent buildup of contaminants on the window that cause a loss of UV throughput. A summary of the ACS CCD performance is given in Table 3.1.
The HRC CCD is a flight-spare STIS 1024  1024 CCD and is a thinned, backside-illuminated device, manufactured by SITe. The coating uses a process developed by SITe to provide good quantum efficiency in the near-ultraviolet. The performance characteristics and specifications are given in Table 3.1
4.2.2 CCD Spectral Response
The responsive quantum efficiency (RQE) of the WFC and HRC CCDs is shown in Figure 4.1; the RQE includes corrections needed to reproduce the instrument sensitivity measured on orbit (ACS ISR 2012-01). The total spectral response of the camera (see Figure 5.7) is given by the product of the RQEs shown here and the throughput of optical elements of the camera. For example, the WFC silver coated mirrors enhance the reflectivity in the near-IR but impose a violet cutoff below 370nm.
Figure 4.1: Responsive quantum efficiency of the HRC CCD (solid line) and WFC CCDs (dashed line).
4.2.3 Quantum Efficiency Hysteresis
Based on current data, the ACS CCDs do not suffer from quantum efficiency hysteresis (QEH). The CCDs respond in the same way to light levels over their whole dynamic range, irrespective of the previous illumination level.
4.2.4 CCD Long-Wavelength Fringing
Like most thinned CCDs, the ACS CCDs exhibit fringing longward of ~7500 . The fringing is caused by interference of incident light reflected between the front and back surfaces of the CCD. The amplitude of the fringes is a strong function of wavelength and spectral bandpass. Only the F892N filter shows a fringe pattern for white light illumination. The fringe pattern is stable and is removed to first order by the F892N flat field for continuum sources.
4.2.5 CCD Readout Format
Each WFC CCD is read out as a 4144  2068 array, including physical and virtual overscans. Two amplifiers are used to read out each CCD. The final images consist of 24 columns of physical overscan, 4096 columns of pixel data, and another 24 columns of physical overscan. Each column consists of 2048 rows of pixel data followed by 20 rows of virtual overscan. The orientation of the CCD is such that for the grism spectra, the dispersed images have wavelength increasing from left to right in the positive x-direction.
Figure 4.2: The layout of the WFC detectors, including parallel and serial readout directions, physical prescans, and virtual overscans.
During readout, charge is transferred in the parallel direction to the serial register, and then shifted horizontally to the amplifiers, named A, B, C, and D. This figure is reproduced from Anderson & Bedin 2010, PASP, 122, 895.
The HRC CCD was read out as a 1062  1044 array, including physical and virtual overscans. There are 19 columns of physical overscan, followed by 1024 columns of pixel data, and then 19 more columns of physical overscan. Each column consists of 1024 rows of pixel data followed by 20 rows of virtual overscan. As with the WFC, the orientation of the HRC CCD was chosen so that grism images have wavelength increasing from left to right.
Figure 4.3: The layout of the HRC detector, including the prescans and overscans designated by dashed lines.
During readout, charge is transferred from top to bottom (parallel direction), then along the serial register to the amplifier (serial direction), labeled C here. The outline of the occulting finger shadow is also shown. This figure is reproduced from Sirianni et al. 2005, PASP, 117, 1049.
4.2.6 Analog-To-Digital Conversion
Electrons that accumulate in the CCD pixels are read out and converted to data numbers (DN) by the analog-to-digital converter (ADC). The ADC output is a 16 bit number, producing a maximum of 65,535 DN in one pixel. Before the failure of ACS in January 2007, the WFC and HRC CCDs could be operated at ADC gains of 1, 2, 4 or 8 electrons/DN. The current CCD Electronics Box (CEB-R) installed during SM4 changed the WFC's ADC operational gains to 0.5, 1.0, 1.4, and 2.0 electrons/DN.
All four new gains are available to the observer but only the GAIN = 2.0 option is fully supported by the ACS Team since May 2010. Although lower ADC gains can in principle increase the dynamic range of faint source observations by reducing quantization noise, the improvement is not significant for the WFC. To mitigate undesirable features in ACS data (e.g., cross-talk), the ACS Team strongly recommends observers only use the gain = 2.0 setting
Table 4.1 shows the gain and read noise values of the four WFC amplifiers measured during the orbital verification period after SM4 for each commanded gain available with the dual-slope integrator pixel sampling mode of the CEB-R. The read noise values apply to the imaging (light-sensitive) regions of the CCD quadrants and are about 0.1 electrons higher than the measured values in the corresponding overscan regions.
For archival purposes, Table 4.2 and Table 4.3 show the gain and read noise values of the four WFC amplifiers and the default HRC amplifier C when operated with Side 1 (March 2002 to June 2006) and Side 2 (June 2006 to January 2007) of the original CEB. The readnoise values in Tables 4.1, 4.2, and 4.3 apply to the image areas.
Table 4.1: WFC amplifier gain and read noise after installation of the CEB-R (valid after May 2009). Values apply to dual-slope integrator mode of pixel sampling.
Gain (e/DN)
Default gain. The 1σ scatter in the gain is 0.01–0.02 electrons/DN.
Valid after March 1, 2013; read noise was 4.1. electrons prior to this date.

Table 4.2: CCD gain and read noise operated under Side 1 of original CEB (March 2002 to June 2006).
Gain (e-/DN)
Default gain.

Table 4.3: CCD gain and read noise operated under Side 2 of original CEB (June 2006 to January 2007).
Gain (e-/DN)
Default gain.

4.2.7 Flat Fields
The WFC flat field reference images are constructed from both ground-based and on-orbit data. Ground-based flats with signal-to-noise ratios of ~300 per pixel were obtained for all filters. Low-frequency refinements of these pixel flats were made using in-flight dithered observations of a rich star field (see ACS ISRs 2002-08 and 2003-10).
These low-frequency flats (L-flats) initially showed a corner-to-corner sensitivity gradient across the CCDs of 10–18%, depending on wavelength. The L-flats were updated in July 2006 after the operating temperature of the WFC was lowered to -81 C (see ACS ISR 2006-06). The resulting flat fields are accurate to 1% over the WFC field of view for most broad-band filters and to 2% for F850LP and the narrow-band filters. Observations of a rich star field were obtained shortly after the ACS repair in May 2009 and have been used to verify that the L-flats remain stable. Internal observations made during SMOV SM4 show that the P-flats (high frequency pixel-to-pixel flats) are also stable.
Figure 4.4 shows the corrected WFC ground flats for several broadband filters. The 50 pixel gap between the top and bottom CCDs is not shown. Because the two CCDs were cut from the same silicon wafer and underwent similar processing, their sensitivities are continuous across the gap. The central doughnut-like structure is wavelength dependent. The pixels in the central region are less sensitive than surrounding pixels in the blue F435W flat, but they are more sensitive in the red F850LP flat.
There are a few types of artifacts present in WFC flat field images. Dust motes are the shadows of dust particles located on the CCD windows. Dust motes consist of a characteristic dark ring approximately 30 pixels across. Flat field correction mitigates their effects to <<1%. "Freckles" appear as small black dots a few pixels across. They are sparsely distributed in WFC images and stable over the history of the instrument. More information about freckles and dust motes can be found in ACS ISR 2001-11. In May 2017, three new artifacts appeared in WFC calibration data. Named flecks, they are also present in flat field images taken in December 2017. Two of the flecks are larger in size than the freckles, spanning 10–20 pixels. The flecks are believed to be debris fallen onto the surface of the CCD. A monitoring program has been established to find new flecks as they appear, but no additional flecks have yet been discovered. Analysis of these artifacts is ongoing, and an ISR will follow.
See ACS ISRs 2001-11, 2002-01, 2002-04, 2002-08, 2003-10, 2003-11, 2005-02, and 2005-09, 2006-06, 2007-01 and 2015-07for more information about the WFC flat fields.
The HRC ground flats were refined using in-flight dithered observations of a rich star field designed to track low-frequency sensitivity variations. These L-flats revealed a corner-to-corner sensitivity gradient across the CCD of 6-12%, depending on wavelength. NUV flats were constructed from in-flight images of the bright Earth (see ACS ISR 2003-02) and include both the pixel-to-pixel and low-frequency structure of the detector response.
The HRC flat fields have a signal-to-noise of ~300 per pixel and support photometry to ~1% over the full HRC field of view. Figure 4.5 shows the corrected HRC ground flats derived for 6 broadband optical filters. The doughnut-like structure in the WFC flats is not seen in the HRC flats. For further discussion of HRC flat fields, see ACS ISRs 2001-11 and 2002-04.
Figure 4.4: WFC Flat Field
Figure 4.5: HRC Flat Field

Advanced Camera for Surveys Instrument Handbook for Cycle 26 > Chapter 4: Detector Performance > 4.2 The CCDs

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