performs the bias correction in two steps (see Section 3.4.1
: a "superbias" reference image is subtracted from the science image to remove the fixed bias structure and during-readout dark current.1
The superbias, constructed from individual bias frames obtained three times a week, samples the fixed bias structure at a high signal-to-noise ratio and is free of cosmic ray artifacts. This calibration step is applied to the image before its conversion to units of electrons, so the superbias is in units of DN.
: this step subtracts the bias level from the image, after the image has been converted from DNs to electrons. For pre-SM4 data, it fits the bias level from the physical overscan, and subtracts it from the science data. For post-SM4 data, additional steps are required to remove the fixed bias structure and "readout dark," the dark current that accrues during readout (ACS ISR 2014-02
The WFC bias frames show small differences between the bias levels of the physical pre-scans and the imaging region of the CCD (Sirianni, et al., 2002, HST Calibration Workshop, STScI, page 82
). These bias offsets vary from amplifier to amplifier and they can be as large as 3.5
DN. If these offsets were constant, a full frame bias subtraction (doBias
) would remove any differences between the pre-scans and the imaging region. Unfortunately, the offsets show random variations of about 0.3
DN that may be caused by interference between the WFC integrated electronics module and the telescope and/or other science instruments. The accuracy of the bias level subtraction in a single quadrant is limited by this random effect. Consequently, sky background levels often appear discontinuous across the boundaries of adjacent image quadrants after calacs
processing (Figure 4.2
). Automated photometry of point or extended sources that span the quadrant boundaries should therefore be considered suspect. In such cases, measuring and subtracting the sky background levels in each quadrant separately is recommended.
Since Servicing Mission 4 (SM4), the WFC bias frames exhibit two-dimensional spatial gradients of 5 DN
DN within each image quadrant (Figure 4.3
). These gradients are stable within the time spanned by each superbias reference image, and so they are completely removed (along with other fixed pattern noise) in the doBias
step of calacs
. These gradients are characteristics of the dual-slope integrator (DSI) implemented in the replacement CCD electronics to reduce the noise incurred during pixel sampling. The gradients are mainly caused by slow drifts of the bias reference voltages during and after the readout of each row of pixels. A small fraction of the gradient is also due to accumulated dark counts during the readout process. Since the time for a given pixel to read out depends on its vertical distance from the amplifier, there is a vertical gradient in accumulated readout dark counts, which adds to the bias gradient.
Because of the uniformity of the striping across WFC rows, it is straightforward to characterize and remove this low-level 1/f
noise from WFC bias frames. The amplitude distribution is well fit by a Gaussian of σG
= 0.74 e−
with an enhanced negative tail, giving an overall σ = 0.9 e−
. (See Figure 4.5
). This is under 25% of the WFC read noise. Averaging N bias images reduces the 1/f
noise by nearly a factor of N1/2
, so the total noise in the post-SM4 WFC superbias reference images approaches pre-2007 levels. The striping is not well estimated by the limited WFC overscan regions alone, complicating stripe removal in non-bias frames. A WFC superdark reference image (average of ~24 darks) is effectively stripe-free, but science targets rarely comprise so many exposures.
WFC images obtained before SM4 showed intermittent bias variations of a few tenths of a DN during readout. Bias frames occasionally exhibited horizontal bias jumps in one or more quadrants that lasted for several hundreds of rows (Figure 4.6
). The probable cause of these jumps was electronic interference from other scientific instruments and/or spacecraft activities. There is no automatic detection of these bias jumps within the calibration pipeline. Bias jumps at the sub-DN level are not important for most science applications, but users should be aware of their possible existence in their calacs
The DSI mode (Dual-slope Integrator, implemented in the replacement CCD electronics during SM4) of WFC operation induces a signal-dependent bias shift, the cause of which is closely related to that of the bias gradient described in Section 4.2.1
. The DC level of the DSI mode is sensitive to changes in the CCD output voltage in such a way that the pixel bias level is shifted positively by 0.02%–0.30% (depending on the amplifier) of the signal from the previously integrated pixel. This phenomenon is well characterized for ACS/WFC full frame images and can be analytically removed using a parametric algorithm described in ACS-ISR 2012-02
. The calacs
pipeline now performs this correction, only for ACS/WFC post-SM4 full frame images, during the doBlev
Unfortunately, this convenient use of full frame reference images became unsuitable for post-SM4 subarray science images because the two-dimensional bias gradients imposed by the DSI (see Section 4.2.1
) are dependent on the timing patterns used to read out the CCD. The bias gradients seen in the standard 512 x
512 and 1024 x
1024 subarray images are significantly different from the gradients seen in the full-quadrant and full frame readout modes.
In May 2016, partway through HST
Cycle 23, the ACS flight software was changed to introduce a new set of WFC subarray modes that make obsolete the modes used during post-SM4 Cycles 17–23 (ACS ISR 2017-03
). These subarray modes were designed to have identical readout timing pattern as the WFC full-frame mode, except with readout from only one amplifier (always 2048 columns plus pre-scan) and potentially with fewer rows (512 or 1024 rows; 2048-row subarray also available). On-orbit tests in November 2015 indicated that these revised subarray modes successfully reproduce the full-frame bias gradient structure. An initial, pedestal-like offset was found in the new, calacs
-processed subarray images between amplifiers A and C and amplifiers B and D. The offset was due to an incorrect mapping of the overscan regions in the OSCNTAB reference calibration file for amplifiers B and D. An updated version of the OSCNTAB reference file was released in February 2017 to remove the pedestal-like offset (ACS ISR 2017-06
is designed to perform bias subtraction of both pre-SM4 and post-SM4 WFC subarray images; no special directions for the calibration pipeline are needed. The pipeline initially performs a search for contemporary superbias images with the appropriate subarray dimensions and, if unsuccessful, reverts to the pre-SM4 procedure of extracting the corresponding region from a contemporary full frame superbias.