5.3 WFC3 CCD Readout Formats

5.3.1 Full-frame Readout

The WFC3 UVIS channel contains two CCD chips, each of which has two readout amplifiers. The amplifiers on chip 1 are designated A and B, and those on chip 2 as C and D. Although a chip (or part of a chip) may be read out through a single amplifier, the default and fastest readout mode employs all four amplifiers simultaneously, such that each amplifier reads out half of a chip.

A full-frame UVIS exposure produces a single FITS file in which the data from each of the two chips are stored in separate image extensions, along with associated error and data quality arrays for each image. For consistency with ACS, the image data from CCD chip 2 are stored in SCI array 1 (FITS extension 1) and the image data from CCD chip 1 are stored in SCI array 2 (FITS extension 4). Table 5.2 lists the chips, the associated amplifiers, and the FITS extensions of the science image data.

Each CCD chip contains 2051 × 4096 active pixels, but the raw images returned by the WFC3 electronics contain a larger number of pixels. This is due to the detector overscan: portions of the detector that are not exposed to light. Overscan regions are useful for characterizing detector electronics performance, and especially for measuring the bias level contained within an image. Serial overscan corresponds to a fixed number of unexposed pixels at each end of each serial shift register. Conversely, parallel overscan is generated by additional parallel shifting before or after all of the exposed rows of the detector have been read out. In principal, both serial and parallel overscan can be implemented either as physical overscan or virtual overscan. The physical overscan is a characteristic of the detector hardware, whereas virtual overscan is a software function, and the number of rows and columns of virtual overscan generated for an image is controllable via the readout timing pattern.

The WFC3 CCD overscan regions are described in more detail in Section 6.7, and dimensions for unbinned and binned exposures are specified in Table 6.10.

Table 5.2: WFC3 CCD Naming Conventions.

CCD Chip


Science Image FITS File Extension

Error Array FITS File Extension

Data Quality Array FITS File Extension


A, B





C, D




5.3.2 Subarrays

The default CCD readout mode is to read all pixels of both chips, including all available overscan regions. It is also possible to restrict the readout to rectangular subarray regions. Only data from the area within the subarray are stored in buffer memory, and the rest of the image is discarded. The subarray can be chosen from several pre-defined configurations.

UVIS subarray images contain no virtual overscan data and serial physical overscan is present only if the defined subarray boundaries overlap the physical overscan columns on either end of the chips. Thus all corner subarrays contain physical overscan data, while centered subarrays do not (Table 6.1).

Subarrays are discussed in greater detail in Section 6.4.4.

5.3.3 On-Chip Binning

The UVIS CCDs also provide an on-chip binning capability, in which several adjacent pixels may be read out as a single pixel. The available choices are 2 × 2 and 3 × 3 on-chip binning. On-chip binning and subarrays can not be used simultaneously. See Section 6.4.4 for details on the use of on-chip binning in WFC3/UVIS observations. If on-chip binning is used, the overscan geometry is complicated by the need to truncate “odd” pixels, and each half of a row must be considered separately. As a result, depending on the binning mode, some science pixels adjacent to the overscan region may be binned together with overscan data. Details are given at the end of Section 6.7.2.