Spitzer-IRAC/Akari-IRC Cross-Calibration. Jason Surace April 19, 2007. With figures by Ohyama, Wada, and Tanabe.
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April 19, 2007
With figures by Ohyama, Wada, and Tanabe
It is in the overall interest of IRAC to ensure that the calibration of Akari is consistent with IRAC’s. This will allow direct comparison between datasets on both satellites, and along the way ensure we haven’t made any major errors.
IRC has capabilities Spitzer doesn’t. It has more broadband filters, and spectroscopic capabilities down to 1.5 µm.
Brief Akari Overview
Much like Spitzer:
68 cm telescope, this satellite is not small! 4m long, 960kg!
Imaging and Spectroscopy
Sun-synchronous Earth orbit
170l He, 1.8 yrs
Primarily all-sky survey
Earth-trailing Solar orbit
4.4m, 950 kg
350l He, 5.5 yrs
Primarily pointed observations
IRC and FIS
Low-res slit and slitless spectroscopy over entire wavelength range
IRAC, IRS, and MIPS
~4,5,6,8,24,70,160 µm, plus some minimal 16µm
Low and high-res slit spectroscopy over 5-40µm
Low-res spectroscopy 50-100µm
Spitzer: IRAC 3.6 and 4.5µm channels will continue working. Passive cooling.
Akari: IRC NIR channel, with 2, 3, 4µm filters plus spectroscopy will continue to work. Passive cooling plus cryo-coolers.
Equivalent of JPL+Goddard
Runs all space science missions (I.e. everything).
Development, assembly, operations, research, etc.
250 total people.
Akari looks to be operated by 10-20 people, total.
IRC Spectroscopic Capabilities
NIR N2 (1.7-2.7µm)
~10 min stack
NIR (InSb) NP Prism (2-6µm)
MIR-L Si:As LG2 Grism (17-25µm)
Akari Calibrator Stars
Akari calibration methodology identical to IRAC’s. Martin Cohen generates predicted fluxes based on instrument throughput and templates. Broad-band imaging and spectroscopy both calibrated in this fashion. Almost all Akari calibrators already observed by IRAC. Mostly A & K-stars. Deliberate overlap with IRAC to extent allowed by pointing constraints.
Difficult to use the slits due to pointing issues.
Advantages to using slitless (easier to handle slit loss), but there are confusion problems at short wavelengths.
Also, wavelength calibration not fixed and leads to problems near spectrum ends.
KF06T2 - K-star
Note - calibration is set by this object at short end.
Black is Cohen template, blue is IRC data.
BP +66 1073 - K-star
Unfortunately, this is confused by the fact that we don’t have the raw spectra, only the flux-calibrated ones which already have the standard star response in them.
But you can see the absorption trough in channel 2 easily!
Mid-IR Short Grism 1
BP +66 1073
NPM 1p67 536
Mid-IR Short Grism 2
BP +66 1073
For at least some K-stars, Martin’s templates have too deep troughs. But they work well for other stars. Inadequacy of optical spectral typing?
No obvious surprise features.
Still need to get raw data. Getting any data has been tricky.
Akari CVZ within 0.6 degrees of ecliptic pole (similar to WISE), vs. 4-5 for Spitzer.
All Akari cal stars located within their northern and southern CVZ.
Deep survey at ecliptic caps. Ideal for general cross-cal with IRAC.
IRAC dark field outside the Akari CVZ, although it is being observed in a fashion as part of a mission project (Egami, PI).
Observing Akari cal field as an IRAC cal activity.
Depth is 28x100 seconds
5-sigma = 0.6, 1.2, 7.5, and 9 µjy at 3.6, 4.5, 5.8, and 8 µm.
Confusion-limited at 3.6 and 4.5µm.
2 hours to execute.
Coverage of a 10x10’ area with all 4 arrays.
8x30 seconds = 2x SWIRE integration time
5-sigma = 2.5, 4.2, 28, and 32µJy at 3.6, 4.5, 5.8, 8 µm.
High Dynamic Range Mode
50 minutes to execute
Akari NEP Cal Field Observed by IRAC
Ch.1 Entire Field, all exposures
IRAC vs. IRC Broadband Photometry
Computed the expected slopes from Martin’s model calculations for specific stars.
IRC/IRAC numbers match to within 5%, the limit I could measure them to.
Somewhat circular, since Akari also calibrated to Cohen templates.
Indicates no significant issues with system throughput measurements.
Indicates that point source measurement and calibration methodology holds and agrees between missions.