Windowing and Binning

There are various methods used to window and bin the LRIS red and blue channel CCDs. Windowing and Binning are used to decrease the CCD readout times on both the Red and Blue channels. Below, we describe these methods and their benefits. At the bottom are speed test results for different windowing and binning setups.

The CCD... subwidgets available from the XPOSE GUIS on the red and blue side have functions that alter the windowing and binning.
- Binning 1x2 - bins only in the spectral direction, decreases the readout time.
- Binning 2x2 - bins in both spectral and spatial directions, decreases the readout time, but adds an image artifact.
- Window full frame mosaic (1x1) - resets to binning 1x1 and full frame mosaic
- Two amp readout on blue CCD0 (left) - selects only the left most CCD
- Two amp readout on blue CCD1 (right) - selects only the right most CCD
- Window CCD 2048x2048 (1x1)
- Window CCD longslit (1x1), decreases the readout time.
Blue side windowing: windowb is a script that may be used to window the blue side. The script is called windowb and takes four arguments (x, y, w, h). The x and y are the starting pixel and the w and h are the width and height of the desired readout region. You cannot window across the two chips, and the script will complain if the desired window spans both chips.
Red Side Windowing: There are three methods for windowing the red side. The best is to use the windowr script.
```
 windowr xstart ystart width height 
```
To window the red side using the win option on XLRIS or by setting the keyword, first find the region you want to window. Determine a starting x_start, y_start, width, and height. If you use either the lris red keyword window or the WIN option on the XPOSE GUI you must use use 2048-y_start as the y starting value.
As an example, to window a 400x400 region around pixel 500, 500 the x_start, y_start, width, and height values would be:
```
 300 848 400 400 
```
where:
x_start = 500 - 400/2
y_start = 2048 - 500 - 400/2
width = 400
height = 400
The WIN... subwidgets available from the XPOSE GUIs on the red and blue side allow manipulation of the CCD readout window and binning. The blue side may not be windowed, unless the readout is for a single chip. To bin data on the blue side, "Two amp readout" on the left or right must first be selected. If you try to sub_window the blue side or you try binning without setting "Two amp readout," the blue side crate will be lost after the first readout and will require rebooting. See XPOSE for more information.

Speed Test Results

Wall clock times for a 1 sec exposure. Wall clock times include time needed to erase, expose, and redout the CCDs.

54s - Window Full Frame & 1x1 binning.
54s - Windowed in the spatial direction only (100 pix width) & 1x1 binning
36s - Window Full Frame & 1x2 binning
23s - Window 1/2 Frame & 1x2 binning

Note that time is not saved by windowing in the spatial direction (across columns) on the Blue channel CCD. To save time by windowing, the number of rows read must be reduced.

72s - Full Frame mosiac (readout time only).
42s - Long Slit mode (readout time only).