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IRAC Calibration Trending

The IRAC Instrument Support Team tracks the stability of IRAC. Three different tracking metrics are listed below. The first is the stability of the response as seen through the calibration stars. The second is the bias level as observed in 12s skydarks. The third is the number of pixels affected per second by cosmic rays. Any changes from the expected values are investigated thoroughly. We update this page frequently to include data from the most recent calibration observations. As can be seen in these plots, IRAC calibration is remarkably constant in time.

1. Photometric Stability

2. Bias Offsets from Skydarks

3. Radiation Hits

1. Photometric Stability

The plots below show the 3.6 and 4.5 micron calibration star normalized flux densities as a function of time from 2013 to the present with the goal of monitoring the calibration of IRAC by periodically checking that the aperture photometry of the calibration stars is the same throughout the mission. The plots include 21 primary and secondary calibrators. Primary calibrators are located in the continuous viewing zone so the entire set of primary stars can be observed every two weeks. The set of secondary calibrators have positions near the ecliptic plane and therefore only one is visible at any given time. A secondary calibrator is observed roughly daily for a few weeks before switching to the next star in the set. The primary calibrators are shown in blue, red, black, and green colors; the secondary calibrators are shown with a multitude of repeating colors as they cycle through the year. The primary calibrator data points are mostly not visible behind the more frequently observed secondary calibrator data points (but we will see them if they move away from the linear trend). The flux densities have been measured using aperture photometry corrected for both the location of the center of the star with respect to the pixel (pixel phase effect) and the location on the array (array location dependence). The flux densities for each star are normalized to the median value for that star. Error bars are calculated as the sigma clipped standard deviation in the bins divided by the square root of the number of data points in each bin. The black line at normalized flux density = 1.0 is shown to aid the eye in determining what a flat line would look like. All data have been processed with pipeline version S19.2. These 21 stars include full array and subarray observations, as well as many different exposure times and well depths. These plots are updated weekly as new observations are made. The gap at December 2015 is from an anomaly, with no obvious effect on the calibration after the instrument was turned back on.

Over the course of the entire mission, we do see a decrease in sensitivity of IRAC photometry of order 0.1% per year in channel 1 and 0.05% per year in channel 2. For more information on those trends please see the IRAC photometric stability page.


Channel 1 normalized calibration star fluxes. 
See paragraph above for meaning of various colors.

Channel 1 normalized calibration star fluxes. See paragraph above for meaning of various colors.



Channel 2 normalized calibration star fluxes.
See paragraph above for meaning of various colors.

Channel 2 normalized calibration star fluxes. See paragraph above for meaning of various colors.


2. Bias Offsets from Skydarks

Instead of using a shutter, dark current and bias offsets are calibrated by observing a dark region of the sky ("skydark") near the north ecliptic pole every seven days. The resulting image of the minimal uniform sky background contains both the bias and dark current. When subtracted from the routine science data, this eliminates both of these instrumental signatures. To monitor for any changes in the bias, we examine the median value of the skydarks after they have been pipeline-processed.

The plots below display the median value of the 12s frametime skydarks over the course of the warm mission. The two vertical lines denote two anomalies, which occurred on 2014-08-02 and 2015-11-28. The first one caused the arrays to reset with less than the usual applied reverse voltage across the detectors. After normal operation was restored, the dark bias levels settled back to level 3.2% higher. The second event caused IRAC to turn off, or "safe". After recovery, the 3.6 micron array quickly settled back to the same higher bias, however the temperature of the 4.5 micron array was below normal for over a week while off. Once its normal temperature was restored, the dark bias quickly settled to level slightly lower than it was between the anomalies. These trends are seen in all frametimes, so only one frametime is used for monitoring. The 12s frame time was chosen for its balance between read noise and background noise so that changes in the bias level can be seen most clearly. The red line traces out the predicted zodiacal variation for each wavelength band.

As the increased bias is observed in both the science and skydark frames, it is subtracted out and does not cause any change in the calibration of the instrument.


Channel 1 (above) and channel 2 (below) skydark bias levels.

Channel 1 (above) and channel 2 (below) skydark bias levels.


3. Radiation Hits

Extra-solar high energy particles and cosmic rays are the primary space weather effects on IRAC. During nominal observing, and in the absence of solar flares, the number of affected pixels is tracked using the calibration observations taken with IRAC once a week. During the mission, the statistical baseline of the average number of affected pixels per second has been established as approximately 4-6 pixels affected per second.

The IRAC Instrument Science Team (IST) actively monitors solar coronal mass ejection (CME) events that can sometimes raise the number of affected pixels by a factor of 10-15 due to the large number of high energy particles produced. While IRAC images are always constantly monitored, the IST performs even more careful checks of image quality for data taken during several days around the predicted impact time of a CME event on the spacecraft to determine whether any observations need to be retaken.


Channel 1 and 2 radiation hit rates during warm mission.

Channel 1 and 2 radiation hit rates during warm mission.