IRAC Instrument Handbook - IRSA - California Institute of Technology
IRAC Instrument Handbook - IRSA - California Institute of Technology
IRAC Instrument Handbook - IRSA - California Institute of Technology
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
<strong>IRAC</strong> <strong>Instrument</strong> <strong>Handbook</strong><br />
array and many times throughout the mission to monitor any changes that may have occurred. Calibration<br />
stars with measured spectral types and accurate absolutely calibrated fluxes in the <strong>IRAC</strong> bands have been<br />
determined. These absolute calibration stars were in the continuous viewing zone (CVZ) so that they<br />
could be observed at any time necessary and could be monitored throughout the mission.<br />
Four stars were observed in the CVZ at the beginning and end <strong>of</strong> each instrument campaign. These<br />
standards remained the same throughout the mission, and provide the absolute flux reference for <strong>IRAC</strong>.<br />
Additionally, a calibrator near the ecliptic plane (which was different for each campaign) was observed<br />
every twelve hours. Its placement in the ecliptic plane minimized telescope slews. This calibrator was<br />
used to monitor any short-term variation in the photometric stability.<br />
Analysis <strong>of</strong> the flux calibrator data indicates that absolute flux calibration is accurate to 3% (reflecting<br />
mostly the uncertainty in the models). Repeatability <strong>of</strong> measurements <strong>of</strong> individual stars is better than<br />
1.5% (dispersion), and can be as good as 0.01% with very careful observation design (e.g., Charbonneau<br />
et al. 2005, [6]). The absolute calibration is derived taking several systematic effects into account. The<br />
steps are described in detail by Reach et al. (2005, [22]). If this methodology is not applied, then point<br />
source photometry from the Level 1 products (BCDs) can be in error by up to 10%.<br />
<strong>IRAC</strong> is calibrated using both so-called primary and secondary calibrator stars. The primary stars are used<br />
to monitor long-term variations in the absolute calibration. They number 11 stars, are located in the<br />
continuous viewing zone (CVZ), and were thus observable year-round. They were observed once at the<br />
beginning, and once at the end <strong>of</strong> each campaign, i.e., about every 10 days whenever the instrument was<br />
switched on. The primary calibrators (in decreasing brightness) are (J2000; with flux densities in mJy in<br />
channels 1−4, respectively):<br />
NPM1+67.0536 = SAO 17718 = 2MASS J17585466+6747368 (K2III, Ks=6.4); 843.6, 482.3, 320.0,<br />
185.3<br />
HD 165459 = 2MASS J18023073+5837381 (A1V, Ks=6.6); 647.7, 421.3, 268.6, 148.1<br />
NPM1+68.0422 = BD+68 1022 = 2MASS J18471980+6816448 (K2III, Ks=6.8); 580.4, 335.5, 223.2,<br />
128.9<br />
KF09T1 = GSC 04212-01074 = 2MASS J17592304+6602561 (K0III, Ks=8.1); 169.9, 104.7, 67.03,<br />
38.75<br />
NPM1+66.0578 = GSC 04229-01455 = 2MASS J19253220+6647381 (K1III, Ks=8.3); 140.9, 82.37,<br />
54.54, 29.72<br />
NPM1+64.0581 = HD 180609 = 2MASS J19124720+6410373 (A0V, Ks=9.1); 63.00, 41.02, 26.18,<br />
14.40<br />
NPM1+60.0581 = BD+60 1753 = 2MASS J17245227+6025508 (A1V, Ks=9.6); 38.21, 24.74, 15.74,<br />
8.699<br />
KF06T1 = 2MASS J17575849+6652293 (K1.5III, Ks=11.0); 13.92, 7.801, 5.339, 3.089<br />
KF08T3 = 2MASS J17551622+6610116 (K0.5III, Ks=11.1); 11.77, 7.247, 4.642, 2.691<br />
KF06T2 = 2MASS J17583798+6646522 (K1.5III, Ks=11.3); 10.53, 5.989, 4.050, 2.339<br />
2MASS J18120956+6329423 (A3V, Ks=11.6) ; 8.681, 5.662, 3.620, 2.000<br />
Calibration 35 Photometric Calibration
Change language