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Detector Binning and Windowing

Effective 10 August 2010, we can offer binning in all 4 WFC chips and windowing in chip 4 only.


WFC binning can be set with the following command:

SYS > bin 1 1 (1x1 pixels, no binnig, by default)
SYS > bin 2 2 (2x2 pixels, pixel scale 0.66")
SYS > bin 3 3 (3x3 pixels, pixel scale 1.0 ")

Following some bias tests to probe the noise in binning modes 2x2 and 3x3, we found some differences between no binning and binning in the sense that chips 1+4 have similar noise (binning versus not binning), while chips 2+3 present higher noise used in binning modes (about 2-5 times higher) than used without binning. Probably this effect is due to electronics (WFC uses two amplifiers) which we could not investigate yet.

Regarding reading time for the whole 4 CCD mosaic measured in slow and fast readout modes, we measured the following times:

bin 1x1 - 48 sec slow mode (28 sec fast mode)
bin 2x2 - 25 sec slow mode (15 sec fast mode)
bin 3x3 - 15 sec slow mode ( 8 sec fast mode)


WFC windowing is possible only in chip 4. To set binning window [X1:X2,Y1:Y2] (with X and Y coordinates to cover the necessary region (CCD coordinates of chip #4) around the object, type:

sys> window 4 '[X1:X2,Y1:Y2]'

where (X1,Y1) and (X2,Y2) are the physical corners of the window defined around the object.

to disable this window, type:

sys> window 4 disable

For example, to define a small central 1'x1' window in CCD4, one should use:

sys> window 4 '[934:1114,1960:2140]'

To define a small central 5'x5' window in CCD4, one should use:

sys> window 4 '[569:1479,1595:2505]'

While to define a central 10'x10' window in CCD4, one should use:

sys> window 4 '[168:1988,1190:3010]'

NOTE: Please do not attempt to use the entire CCD4 as a window! This will crash the system, plus there will be no gain in readout time (which is the same as using the entire 4 chip camera).

NOTE: By defining a central window, the overscan information is lost. In order to keep the overscan, one should define the window close to the edge of the CCD4 overscan region (BIASSEC='[10:2150,4105:4190]'). Warning: In 2018 some WFC ICS disconnections were reported (ex FR 22920), apparently caused by windowing close to the CCD overscan region. We recommend caution and test during daytime, and eventualy use of central windows (far from the CCD edge).

For example, to define a 1'x1' which includes the overscan, one should use:

sys> window 4 '[934:1114,3830:4190]'

(which include in Y both the 180 pixel window and the original overscan from Y=4105 to 4190).

Once again, CCD #4 is the only chip that can be windowed.

Moreover, one can define four different windows at the same time on CCD #4 (provided they do not overlap), namely window 1, window 2, window 3 and window 4. Remember, these are not related with the four chips.

Regarding reading time for different size windows, we measured the following times in slow mode:

Whole CCD4 (2154x4200 pix) - 46 sec (thus, no gain from whole WFC)
10'x10' FOV (1820x1820 pix) - 18 sec
5'x5' FOV (910x910 pix) - 6 sec
2'x2' FOV (360x360 pix) - 4 sec
1'x1' FOV (180x180 pix) - 3 sec

NOTE: A strange windowing behaviour was observed by Raine Karjalainen in 2013, affecting the window sky orientation, depending on the chosen window corners. Apparently, windowing up to Y an odd number produces an image rotated with respect to the image obtained by windowing up to an Y even number. Please be aware of this effect and carefully check your windowed field and its sky orientation before starting to observe your field. We hope to address this problem, although the INT and WFC resources are more limited.

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Contact:  (WFC Instrument Specialist)
Last modified: 04 June 2018