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 along their long dimension 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 at 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
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 2007-06
). 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.
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.
Like most thinned CCDs, the ACS CCDs exhibit fringing longward of
~7500 Å. 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.
Each 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.
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.
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 new 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 after SM4, but only the GAIN=2.0 option is fully supported by STScI in Cycle 19.
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.
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.
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
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.
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. See ACS ISRs 2001-11
, and 2005-09
for more information.
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 4.3
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
HRC flat field.