Enhancing the Science Return of the Spitzer Warm Mission

Planning is underway for the post-cryogenic ("warm") operation of the Spitzer Space Telescope which will start around April 2009 after all of the liquid helium has been depleted. Only channels 1 and 2 (3.6 and 4.5 microns) of Spitzer's Infrared Array Camera (IRAC) will be operational at full sensitivity at that time -providing an unmatched sensitivity from 3 to 5 microns until the James Webb Space Telescope is launched. The other channels of all remaining instruments will not operate at the elevated temperatures (25-30K) of Spitzer will experience during its warm mission phase. Last year at AMOS 2006, I showed how the recorded flux of bright point sources observed with IRAC Ch1 is corrupted by lossy detectors which have large intrapixel quantum efficiency variations. During the past year, I have worked closely with members of Spitzer's IRAC Instrument Team to demonstrate that my NASA-funded MATPHOT algorithm for precision stellar photometry and astrometry can yield an improvement in the precision of stellar photometry obtained from IRAC Ch1 observations of bright stars of more than 100% over the best results obtained with aperture photometry corrected with the radial correction recommended in the IRAC Data Handbook. I describe some results of an ongoing effort to develop new calibration procedures for IRAC Ch1 and Ch2 which have the potential of significantly improving the precision of IRAC bright point-source photometry. This timely research effort is intended to not only enhance the science return of existing IRAC Ch1 observations in the Spitzer data archive but also those that will be made during the Spitzer Warm Mission. 1. SPITZER SPACE TELESCOPE’S WARM MISSION The Spitzer Space Telescope (SST) was launched into an Earth-trailing orbit from the Kennedy Space Flight Center on 2003 August 25 UT (see Fig. 1) [1]. The 85-cm cryogenically cooled beryllium Ritchey-Chretien telescope system operates at temperatures as low as 5.5 K. Planning is underway for the post-cryogenic ("warm") operation of the Spitzer Space Telescope which will start around April 2009 after all of the liquid helium has been depleted. Only channels 1 and 2 (3.6 and 4.5 microns) of Spitzer's Infrared Array Camera (IRAC; [2]) will be operational at full sensitivity at that time -providing an unmatched sensitivity from 3 to 5 microns until the James Webb Space Telescope is launched. The other channels of all remaining instruments will not operate at the elevated temperatures (25-30K) of Spitzer will experience during its warm mission phase. This article describe some results of an ongoing effort to develop new calibration procedures for IRAC Ch1 and Ch2 which have the potential of significantly improving the precision of IRAC bright point-source photometry. This timely research effort is intended to not only enhance the science return of existing IRAC Ch1 and Ch2 observations in the Spitzer data archive but also those that will be made during the Spitzer Warm Mission. 1 This work is based on archival data obtained with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by an award issued by JPL/Caltech. Fig. 1. This artist rendition shows an external view of the Spitzer Space Telescope in its Earthtrailing solar orbit. The insert image shows the launch of the SST from the Kennedy Space Flight Center on 2003 August 25 UT [1]. 2. OBSERVATIONS AND PHOTOMETRIC REDUCTIONS Sixteen short (0.4 s) calibration observations of the K0 star PPM 9412 were obtained on 2003 October 8 UT with Channel 1 (3.6 μm) of the Infrared Array Camera onboard the Spitzer Space Telescope (see Table 1). Table 1. IRAC Ch1 Observations of PPM 9412. The IRAC basic calibrated data (BCD) images were retrieved from the Spitzer data archive. These observations were analyzed with the imexamine task of NOAO’s Image Reduction and Analysis Facility (IRAF; [3–4]) package and a new experimental version of the MATPHOT [5] photometric reduction package, called MPDZ, which uses the following relative intrapixel quantum efficiency (QE) variation map [6] for IRAC Channel 1 (Ch1),