11-Jun-04 1 Joseph Hora & the IRAC instrument team Harvard-Smithsonian Center for Astrophysics The Infrared Array Camera (IRAC) on the Spitzer Space Telescope

11-Jun-04 2 Outline overall behavior of detectors relative to pre- flight predictions, plus: a.) short wavelength response/qe issue; b.) cosmic ray effects; c.) latent images and their mitigation d.) implications of operation without a shutter; e.) effects when detectors are first turned on f.) status of absolute calibration

11-Jun-04 3 IRAC Description Infrared Array Camera Top-Level Requirements (Actual Performance) (Predicted) ChannelCenterTotalAngularField ofRelativeBroadband No.WavelengthBandwidthPixel SizeViewPhotometricSensitivity (microns)(%)(arcsec.)(arcmin.)Accuracy(5 sigma/200sec.) (max error %)(  Jy) 1 (InSb)3.6 (3.55) 20 (21) 1.2 (1.210)5.12 x 5.12 (5.17 x 5.17) 2.0 (<2.0) 4.6 (2.5) (2.0) 2 (InSb)4.5 (4.52) 23 (23) 1.2 (1.207) 5.12 x 5.12 (5.15 x 5.15) 2.0 (<2.0) 6.1 (4.5) (4.2) 3 (Si:As)5.8 (5.70) 25 (26) 1.2 (1.213) 5.12 x 5.12 (5.18 x 5.18) 2.0 (<2.0) 30 (15.5) (27.5) 4 (Si:As)8.0 (7.92) 38 (37) 1.2 (1.209) 5.12 x 5.12 (5.16 x 5.16) 2.0 (<2.0) 45 (25.0) (34.5)  IRAC is a simple 4-channel camera with fixed broad-band filters centered at 3.6, 4.5, 5.8 and 8.0  m  Four 256 x 256 pixel detector arrays (2 InSb, 2 Si:As). – simultaneous readout of all four arrays  Two nearly adjacent fields of view (5.2 x 5.2 arcmin), viewed in pairs (3.6, 5.8 μm and 4.5, 5.8 μm).

11-Jun-04 4 IRAC DETECTOR CHARACTERISTICS IRAC ARRAY PERFORMANCE IS EXCELLENT. ARRAYS BUILT BY RAYTHEON/SBRC. (37) (55)

11-Jun-04 5 QE Issue ISA was entered on 9/23/2003 based on results obtained during the first few campaigns in IOC. IRAC throughput was measured in campaigns B, C, D, and E using six calibrator stars. The in-band fluxes of these stars, all of which are K-giants, were estimated for the IRAC bands using existing spectra of K-giants scaled by ground based optical/near-IR photometry of the calibrator stars. Although the measured throughputs for IRAC channels 1 and 2 were consistent with pre-launch measurements, the values for channel 3 and 4 were consistently only 45% and 61% of the pre-launch predicted throughputs, respectively.

11-Jun-04 6 Si:As QE Ch3 Ch4

11-Jun-04 7 IRAC PSFs

11-Jun-04 8 Array Droop

11-Jun-04 9 Flux Scattering Out of Aperture Graph of the Ratio of Total Flux in all Bands to Total Source Flux versus wavelength. The total flux in all other pixels (not shown in graph) is 130% of the Ratio of Band to Source at all wavelengths

11-Jun In-flight Array Response (Flats)

11-Jun Internal reflections in Si:As Detetors

11-Jun Stray light in Channels 1 & 2

11-Jun FPA Cover

11-Jun IRAC ARTIFACTS Multiplexer bleed, banding and column pulldown

11-Jun CR Statistics

11-Jun Normal Cosmic Rays

11-Jun Cosmic Ray Transients

11-Jun Cosmic Ray Scattering

11-Jun Particle Showers

11-Jun Cosmic Ray Comets

11-Jun Cosmic Ray Removal – 16 frames

11-Jun Cosmic Ray Removal – 2 frames

11-Jun Residual Images Normal residuals – after exposure to bright source, next image it is <0.5% of bright source, decays exponentially

11-Jun NEW: Persistent images: long-lived During IOC, we learned that very bright sources leave persistent images that can last >7 hrs in channels 1 and 4 Lab tests confirmed channel 1 persistence due to known array defect; channel 4 under study Preventive mitigation: –Every 12 hours, anneal ch1&4 –Anneals remove latents; implemented 1 st campaign –NEW: Move observations containing K<3 stars before anneal; implemented 3 rd campaign (by hand) Channel 4 “dark” hours after bright star

11-Jun Ch1 Downlink Residuals Frames after an anneal showing residuals

11-Jun Ch1 Downlink Residuals First (left) and last (right) exposure of Separated by 1.5 hours. The actual latent image. This is the derived image of whatever the array was staring at.

11-Jun Residual Removal through anneals Ch1 Ch4

11-Jun Routine Anneals Cernox sensors saturated, T~30K

11-Jun Shutterless Operation Basic result is that instrument is stable, but calibrations take significantly more time Dark frame stability is extremely good –No variations in dark pattern seen –Short-term variations from residuals present –Long-term drifts seen in baseline level – zodi background changes? Calibration relies on standard sources, no quick pixel-wise check possible Flat fields measured on sky – no quick measurement possible Linearity measurement in flight difficult for Ch. 1 and 2

11-Jun Dark Frame Variations

11-Jun Calibration Stability The scatter between standards is consistent with the 2%-3% uncertainty expected in the stellar models. The relative calibration stability for a particular standard star over the six campaigns is in the 1%-2% range for all channels.

11-Jun Effects of Anneals on Arrays Without anneal With anneal

11-Jun Dark levels after turn-on

11-Jun Standard star after turn-on

11-Jun Conclusions: overall behavior of detectors relative to pre- flight predictions – no changes, QE lower than expected, Point/extended source issue, but could have known a.) short wavelength response/qe issue – scattering inside array, b.) cosmic ray effects – 4-8 CR pixels/sec c.) latent images and their mitigation –Unexpected downlink and long-term Ch4 latents, but are removed with anneals

11-Jun Conclusions (2): d.) implications of operation without a shutter –Arrays stable, ground-based calibration valid, –In-flight calibrations more time consuming or not practical e.) effects when detectors are first turned on –Dark current instability in first ~30 min –Stellar calibration constant f.) status of absolute calibration –Variations between stars 2-3% in all channels

11-Jun IRAC IMAGE QUALITY 30″ 5′5′ FWHM (″)

11-Jun IRAC ARTIFACTS Stray light (point sources and diffuse light)