What are ACS subarrays, and how can I use them?

Dark frames and flat fields will be extracted from full-frame images. Bias frames will also be extracted from full-frame images, unless the subarray extends into the adjacent amplifier quadrant. Tests have shown that this does not degrade the quality of the dark, flat-field or bias corrections, as compared to full-frame data. It is strongly advised that subarrays not be defined such that they span amplifier quadrants. In this case, users must obtain their own subarray bias images (with one-amp readout), otherwise the bias level difference between adjacent amp quadrants would be applied inappropriately.

A request for use of unsupported subarrays or for obtaining bias images requires prior approval. This should be requested from the Program Coordinator (PC), who will relay the request to the relevant ACS Instrument Scientist.

The subarrays may include physical overscan areas and there is some advantage in doing so to obtain bias data. On the WFC, a subarray may only be specified in conjunction with aperture WFC1 or WFC2. They may not be used with any polarizer filters or the filter F892N which are automatically assigned fixed subarray apertures.

The following discussion indicates how the size and position of a general subarray may be specified by the four values SIZEAXIS1, SIZEAXIS2, CENTERAXIS1, and CENTERAXIS2. All values are in pixels, but for subarray definition the two chips are treated as a single array. Physical overscan pixels are included in the count, so AXIS1 ranges from 1 to 4144, while AXIS2 runs from 1 to 4096. Supplying values for the CENTERAXIS parameters is optional. If not specified the center will be at the reference position of the aperture on which the subarray lies.

There is a 1-pixel border around each chip that is not accessible to subarrays which would make the maximum value for SIZEAXIS1 4142 and SIZEAXIS2 2046. SIZEAXIS1 AND SIZEAXIS2 have to be even numbers and the left hand edge of the subarray must start at an odd numbered pixel. These requirements further restrict the AXIS1 range in which a subarray may lie to be between 3 and 4142 and the maximum size of SIZEAXIS1 to 4140. In effect there is a two pixel segment at each AXIS1 edge that is not accessible to subarrays. The two rows 2048 and 2049 which are part of the chip borders must also be avoided. The minimum allowed value for each SIZEAXIS is 16.

As stated above SIZEAXIS1 and SIZEAXIS2 are even numbers. Defining the central pixel of an even numbered range leads to some ambiguity; the pixel must lie one side or the other of the true center. For subarrays the CENTERAXIS pixel is defined to fall on the high AXIS1 side and the low AXIS2 side of the exact centerlines. For instance, a 200 by 200 area would have its center pixel at a relative position of (101,100). In the diagram the subarray centerlines and the position of the CENTERAXIS point have been indicated. The exact extent of the subarray in the AXIS1 direction is then from CENTERAXIS1-SIZEAXIS1/2 to CENTERAXIS1+SIZEAXIS/2-1. In the AXIS2 direction the range is from CENTERAXIS2-SIZEAXIS2/2+1 to CENTERAXIS2+SIZEAXIS2/2. In the AXIS1 direction, the subarray starts at CENTERAXIS1-SIZEAXIS1/2 which, from the above conditions, must be an odd number. Such details are only of concern when trying to place subarrays exactly at the edge and when you wish to know how many overscan pixels are incorporated.

SIZEAXIS1/2 +3 <= CENTERAXIS1 <= 4143-SIZEAXIS1/2 

CENTERAXIS1 - SIZEAXIS1/2 must be an odd number.

SIZEAXIS2/2 +2049 <= CENTERAXIS2 <= 4095-SIZEAXIS2/2 

SIZEAXIS2/2 +1 <= CENTERAXIS2 <= 2047-SIZEAXIS2/2 

The ground system software takes care of all these restrictions, including placing the subarray on the proper chip and restricting it to lie wholly on a single chip. When a phase 2 proposal is being submitted, the Astronomers Proposal Tool (APT) will make any necessary adjustments and report as warnings the changes it has made.

For the HRC, AXIS1 ranges from 1 to 1062 including 19 pixels of physical overscan at each edge. AXIS2 runs from 1 to 1024. The AXIS1 and AXIS2 sizes are even and range from 16 to 1058 and 1022 respectively. Again the CENTERAXIS positions must be chosen so as not to cause the array to overlap the edge pixels and CENTERAXIS1-SIZEAXIS1/2 must be odd. Only 17 overscan pixels at each edge may be included. The CENTERAXIS positions become the reference position for the observation. The HRC limits are:

SIZEAXIS1/2 + 3 <= CENTERAXIS1 <= 1061 - SIZEAXIS1/2 

  and

SIZEAXIS2/2 + 1 <= CENTERAXIS2 <= 1023 - SIZEAXIS2/2 

Supported subarrays

All these subarrays may be invoked by selecting the matching aperture name. Details of the size and center positions will be handled in software.

Examples of resulting readout times

  Readout time from detector to buffer = 4 x 135 s = 540 s
  T_dump for 4 dumps = 4 x (311s + 38s overhead)   = 1392 s
  Total = 1932 s

  Readout time from detector to buffer = 4 x 36s = 144 s
  T_dump  =  (311s + 38s overhead) = 348s
  Total   =  492 s