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Spitzer-IRAC/Akari-IRC Cross-Calibration PowerPoint PPT Presentation

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Spitzer-IRAC/Akari-IRC Cross-Calibration. Jason Surace April 19, 2007. With figures by Ohyama, Wada, and Tanabe.

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Spitzer-IRAC/Akari-IRC Cross-Calibration

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Jason Surace

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.

Our Interest in Akari (formerly Astro-F, formerly IRIS)

Brief Akari Overview

Much like Spitzer:

68 cm telescope, this satellite is not small! 4m long, 960kg!

Imaging and Spectroscopy

Sun-synchronous Earth orbit

68cm telescope

3.7m, 960kg

170l He, 1.8 yrs


Primarily all-sky survey

Earth-trailing Solar orbit

86cm telescope

4.4m, 950 kg

350l He, 5.5 yrs


Primarily pointed observations


10x10’ imaging


Low-res slit and slitless spectroscopy over entire wavelength range


5x5’ imaging

~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

single1-min observation

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.

Some Difficulties

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.

Near-IR Prism

2MASS 1757132

KF06T2 - K-star

Note - calibration is set by this object at short end.

Black is Cohen template, blue is IRC data.

Near-IR Grism

BP +66 1073 - K-star

HD 42525





<NP>K vs. A(in mJy*um^2 unit)







<NG>K vs. A(in mJy*um^2 unit)




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!

<SG1>K vs. A (in mJy*um^2 unit)




Mid-IR Short Grism 1

BP +66 1073


HD 42525



NPM 1p67 536


Mid-IR Short Grism 2

BP +66 1073


HD 42525



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.

Are We Learning Anything?

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.

Akari Calibration Locations

“Deep” Component

Central 5x5’

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.

“Shallow” Component

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.

Early Results from Cross-Cal Field Encouraging

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