1. Overview

The loss of the IRAC shutter has only minor effects on the IRAC science thread. This is because the science thread presupposes the existence of calibration darks and flats, and the loss of the shutter primarily affects our ability to make these calibration data. The primary change is the addition of lab dark subtraction step in addition to the sky subtraction, and a partial reordering of some of the modules.

2. Detailed Description Of Thread

The following are the detailed steps to be taken in the thread. For each incoming DCE the following steps will be performed. They closely resemble the current processing, and are virtually identical to the new "sky dark" thread. I have shown in red modules that I did not code into my csh script in section 3, either because they did not manipulate pixels or they wouldn't run. I have also shown in red where in the thread the new modules should be placed.

1. TRANHEAD will process the headers and add keywords.
2. INSBPOSDOM will flip the sense of channels 1 and 2.
3. CVTI2R4 will convert to floating point.
4. IRACWRAPDET1 will detect wraparound.
5. IRACWRAPDET2 will detect doughnuts.
6. IRACWRAPCORR will fix the wraparound.
7. IRACNORM will correct for barrel-shift and fowler sampling.
8. EBWC will correct channels 3 and 4 bandwidth effects.
9. A "lab dark" will be subtracted from the data. A library of these will be supplied by the IST, and will be based on ground testing. They are needed to remove as best as possible the absolute bias level prior to linearization. The also eliminate the muxbleed from hot pixels.
10. First Frame Correction
11. MUXBLEEDCORR will correct muxbleed in channels 1 and 2.
12. FOWLINEARIZE will linearize the data. This linearized frame will be saved as input to the image combination step.
13. Optical Crosstalk Flagging
14. A "sky dark" will be subtracted from the data.
15. A flat-field will be divided into the data.
16. IMFLIPROT will flip channels 2 and 4. It is not included here because the ISDS includes a WCS and IMFLIPROT will not run on these images.
17. Banding Flagging
18. DNTOFLUX will apply the flux calibration.
19. Addition of background stored in sky dark header.

3. Demonstration of Prototype Thread

A prototype of this thread was created using the actual S6 pipeline modules and a CSH wrapper. This wrapper is called iracsci.csh. It performs the steps described above needed to manipulate the pixels. It does not do error propagation, or some of the other book-keeping steps the real pipeline will need to do.

Input data for this test was generated with the IRAC Science Data Simulator (ISDS). It consists of a simulation of the elliptical galaxy NGC 4589, based on 2MASS data. It also contains stars based on 2MASS and the stellar luminosity function. It does not include an extragalactic object model, which is why the source density is so much lower than the previous tests (particularly at 8 microns). This was reduced using the sky darks and flats generated in the previous two exercises.

Figure 1 shows the data as it flows through this pipeline. For clarity, some steps (e.g. CVTI2R4) have been omitted, because they have no effect on the pixel values. The data looks extremely good. One useful thing to ask is whether or not the data reduction improved the data. Did the background noise get better, or at least get no worse? The improvement in noise was 25%, 22%, 22%, and 1% in channels 1 through 4, respectively, as measured by examining the noise in a small, aparrently object-free box. The low number for channel 4 is because this channel is dominated by photometric noise due to the high background.

Figure 1	Ch.1	Ch.2	Ch.3	Ch.4
Raw Data
INSBPOSDOM
IRACWRAPCOR
IRACNORM
IRACEBWC
Lab Dark-Sub
MUXBLEEDCORR
FOWLINEARIZE
Sky-Subtracted
Flattened