Focal Plane Simulator Results (run 07/16/2008)

1. Overview

Here's a brief description of how SDL's Focal Plane Simulator hardware and software work. The software reads in a file with a total of 36 Sample-Up-the-Ramp (SUR) images. The first 9 images correspond to band 1, second set to band 2, etc. Each image corresponds to a "ramp sample" and consists of 1024x1024 pixels with values in DN ranging from 0 to 16383. The software then loads these images into a memory buffer in the simulator hardware. The hardware has the smarts to synchronize itself with the FEB. When it's time to output an image, the pixel data is read out of the memory buffer and sent to a bunch of D/A converters which output an analog voltage simulating what a real focal plane would put out. This voltage is then converted back to DN by the A/D converters in the FEB and passed on to the DEB.

Both the digital-to-analog and the analog-to-digital conversions introduce noise. The other thing to consider is that there's an offset and gain when doing the D/A conversions. In other words, a DN of X in the pixel data will not result in a DN of (X+noise) at the output of the FEB, it will be more like (a1*X+a0+noise). This has to do with matching the output voltage range of the DACs in the simulator and those of the ADCs in the FEB.

The 9 SUR images per band were generated by F. Masci by decomposing simulated WISE images of the galactic center into "ramp" samples. These were then loaded into the Focal Plane Simulator and run through the Engineering Model FEB and Payload DEB. The outputs from the DEB are images in native WISE "slope" units. 20 images were collected from each band, each processed using the same digital settings (see below). This number will enable us to average over the noise introduced by the focal plane simulator and the FEB digitization.

The SUR coefficients, number of LSBs truncated from the outputs, and DEB offsets are as follows:

   Band              Coefficients(ci)             Truncation(T)     Offset(O)
     1        {0, -7, -5, -3, -1, 1, 3, 5, 7}          3               0
     2        {0, -7, -5, -3, -1, 1, 3, 5, 7}          3               0
     3        {-4, -3, -2, -1, 0, 1, 2, 3, 4}          2               0
     4        {-4, -3, -2, -1, 0, 1, 2, 3, 4}          2               0

These parameters were used to convert the SUR data from the focal planes (FEB) into WISE native units (DEB units). In a nutshell, the signal S for a pixel in DEB units is computed from:

S = [O + ∑_ic_ip_i] / 2^T,

where p_i are the SUR values in DN and all other parameters were defined above. For more details, see the WISE Digital Electronics Box Data Processing Description. Also, saturation detection and its encoding was disabled in this run.

2. Analysis and Results

Each set of 20 DEB-output images (per band) were averaged to create effective simulated images from the DEB. These images are referred to as "SIM". To facilitate a comparison with what we expected a priori, we also generated "truth" images from each of the initial 9-SUR image-sets. These were made using the same algorithm above. We shall refer to these as predicted images or "PRED" for short. Below we compare the SIM and PRED images. Only bands 1 and 3 are shown. Bands 2 and 4 will be shown once we understand things better.

A metric we used to quantify PRED to SIM differences is the percentage change in the SIM pixel value relative to its PRED value:

∆S (%) = 100 * [SIM - PRED] / PRED.

This metric is shown in Figures 2 and 4 (for bands 1 and 3 respectively).

Figure 1 - Left: Band 1 truth or "PRED" image. Right: "SIM" image from Focal Plane Simulator created by averaging 20 simulation runs. Note the signatures from the 16 readout-amplifiers (columns). Both images are displayed at the same stretch.

Figure 2 - Band 1 "SIM - PRED" percentage difference image as defined above. Values around zero (or green) imply an agreement. Click on panel to enlarge.

Figure 3 - Left: Band 3 truth or "PRED" image. Right: "SIM" image from Focal Plane Simulator created by averaging 20 simulation runs.

Figure 4 - Band 3 "SIM - PRED" percentage difference image as defined above. Values around zero (or green) imply an agreement. Note the signatures from the 4 readout-amplifiers (quadrants). Click on panel to enlarge.