HST Calibration Workshop– 23 June 2010 SJC - 1

Some Thoughts on Cross Calibration of Space Based Infrared Telescopes

Sean CareySpitzer Science Center

HST Calibration Workshop– 23 June 2010 SJC - 2

Outline

• Introduction• Calibrating the speaker• State of infrared calibration

– A Holy Grail – An infrared zero point that can be agreed upon• That longer wavelength Great Observatory• Some cross calibration examples drawn from personal

experience– Spitzer instrument cross calibration– Cross calibration to help with instrument features

• What can we do next

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Relative versus Absolute Calibration and Cross Calibration

• Relative calibration provides correct flux ratios (colors)• Absolute calibration places a good relative calibration to

physical units, relative calibration referenced to a good photometric standard– Hard to do!– Calibrate to NIST standards

• Cross calibration is comparison between instruments• If instruments share a common absolute cal methodology then

cross calibration does not probe the absolute calibration– Have to understand photometric reference of each instrument– Literature can be incomplete / conflicting

• Bottom line is that current state of the art absolute calibration is good to no better than 2% in infrared (> 2 m) and worse at longer wavelengths (> 30 m)

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Does Science Depend on Absolute Calibration

• Most science can get away with an incomplete absolute calibration– Except for dark energy experiments– but Spitzer observations are routinely limited by abs cal same for JWST– ~1% error in flux compared to model will have little effect in derived

source temperatures, distances/luminosities• But biases between instruments need to be understood when

measuring colors, fitting SEDs, looking for excesses• Most transiting exoplanet science is immune as long as the

differential signal in a band is robust– Knowledge of host star limits estimation of planet temperatures, radii

and atmospheric composition– Spectral typing / radius measurement uncertainties should be considered

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Personal Background and Potential Biases about Calibration

• Worked on MSX data processing– Indoctrinated in the Price et al. (2004) methodology– MSX: 35 cm, 5 bands (4-21 m),

• Responsible for current state of IRAC calibration as Instrument Team lead– IRAC makes use of the Cohen spectral templates

• PI of Inner Galactic plane mapping with MIPS (MIPSGAL)– Worked closely with MIPS team at SSC and Rieke et al. (2008) methodology

• Career at > 2 m– Mostly and blissfully ignorant of calibration issues in optical

• If calibration is a religion, agnostic when it comes to infrared calibration– No preferred calibration standards

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Issues with Infrared Absolute Calibration

• Vega is a bad choice of primary flux standard– Infrared excess due to circumstellar disk at > 5 m– Pole on rapid rotator so also a bad fit to A0V in V-band

• Normalized “Vega” A0V Kurucz model used instead– Much confusion in the literature on how to extrapolate from V band to

infrared– Authors use conflicting “Vega” fluxes in infrared

• Rieke et al. (2008) find offsets between instrumental calibration relative to MIPS 24 m– Used A stars and solar analogs– Most of each offset can be attributed to different zero points– 2MASS 2% low, IRAC 1.5% low, IRAS 25 m 2% high

• Conflicts with Price et al. (2004) result which included emissive spheres and same model Vega

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More Issues with Absolute Infrared Calibration

• Uncertainty in truth of standards used to transfer photometric zero point

• A0V standards – Can be bright (good for previous telescopes/spectroscopy)– Use some form of Kurucz model– Can have circumstellar disks in IR (10-15% of field stars)– Assume most are better behaved than Vega

• KIII standards– Molecular absorption features in mid-IR (CO, SiO) complicate analysis

• Solar analogs– ~2% variation between models

• White Dwarfs– Haven’t been used in IR and are faint

• Best current strategy is to use ensemble of calibrator types

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Hope for the Future

• Use of better models particularly replacing Engelke functions for > 20 m– Castelli and Kurucz (2004)– MARCS (Decin & Eriksson 2007)

• Better templates through improved IR spectra (Engelke et al. 2006)– Reconciles 3.6 m KIII derived flux conversion with AV derived flux

conversion for IRAC– 7% discrepancy reduced to 1.5%

• More observations of more standards– Joint HST/Spitzer calibration campaign (see later talks)

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Status of Spitzer

• First cycle of warm observations is almost complete• IRAC has been on for 345 days without incident• Current warm calibration accuracy is 3%• Flux conversions and various other calibrations varied slightly

and as mostly as expected– Intra-pixel responsivity variations more significant– Have identified function form of responsivity variation in warm data

and are now applying it to cryogenic data– Arrays are more non-linear

• MIPS final reprocessing has finished• Final calibration for cryogenic IRAC is winding up• IRS closeout is progressing

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Warm / Cryogenic IRAC Cross Calibration

Warm 3.6 m (top)Cryo 3.6 m (bottom)Same functional form Different amplitude

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Engelke / Cohen Comparison

• IRTF SpecX data of NPM1p68.0422 (K2III) calibrator for IRAC

• Red is ratio of spectra / Cohen template• Blue is ratio of spectra / Engelke template

m

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IRAC AV / KIII Calibration Offset

• In Reach et al. (2005) difference between Predicted/Observed between AV and KIII calibrators was 7.3%, 6.5%, 3.6% and 2.1% for 3.6, 4.5, 5.8 and 8.0 m

• Revised templates improve discrepancy

• 4.5, 5.8 and 8.0 m analysis in works

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Spitzer Instrument Cross Calibration

• Each instrument used different calibration methodology– No cross calibration requirement

• IRAC used 4 AV stars as primary calibrators (Reach et al. 2005) and Cohen et al. templates and zero points using the “Vega” template verified by Price et al. (2004)– Zero points of 280.9, 179.7, 115.0 and 64.13 Jy. – A 3% absolute accuracy is quoted.

• MIPS used 22 A stars, Ks – [24] = 0 and a “Vega” zero point (Engelbracht et al. 2007)– Zero point of 7.17 Jy– 4% absolute accuracy is quoted

• IRS calibration is based on MARCS models of Decin et al. (2004). HR 7341 (K1III) is used as the primary standard.– 5% absolute accuracy is quoted in Instrument Handbook

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Spitzer Cross Instrument Calibration

• Gizis et al. (in prep) compared IRAC to IRS and IRS to MIPS • 8 m photometry of the IRS calibrators, HR 7341, HR 2194

(A0V) and HR 6606 (G9III), to IRAC magnitudes inferred from IRS spectra and IRAC response function– Combination of SL1 and SL2 orders for IRS– The photometry for all three sources agrees to better than 1%

• 24 m photometry was compared to IRS synthesized photometry from the LL module for HR 2194 and HR 6348 (K1III)– IRS 2.2%±1.0% fainter than MIPS 24 m– In agreement with Rieke et al. (2008) comparison of IRAC and MIPS

HST Calibration Workshop– 23 June 2010 SJC - 15

MIPSGAL Point Source Check

• Color excess between predicted and observed for 20 AV and 7 KIII

• 3 A stars have 24 m excess

• 24 m 3% brighter• Similar result noted

by SAGE legacy team

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Archival Search for Non-linearities

• To support the observations of JWST calibrators it is important to verify if possible that there are Spitzer observations are linear at low well depth– Difficult measurement to make

• No indication that Spitzer arrays (InSb, Si:As, Si:Sb) exhibit count-rate non-linearity

• Some indication from FEPS team (Carpenter et al. 2008) that IRAC and MIPS photometry is a function of frametime– Not monotonic in IRAC frametimes– Not repeated in IST tests of IRAC relative calibration with frametime

HST Calibration Workshop– 23 June 2010 SJC - 17

IRAC Si:As Extended Source Calibration

• The 5.8 and 8.0 m arrays exhibit significant internal scattering (and droop) resulting in extended sources having larger measured fluxes than they should

• Cohen et al. (2007) and the IRAC IST independent measured this effect– Using HII regions and comparing 8.0 m to MSX 8.3 m– Extrapolating MSX 8.3 m to IRAC 5.8 m assuming a PDR like SED– Using elliptical galaxies and extrapolating 2MASS Ks to IRAC

wavelengths• Measured effect is significant ~30% at 5.8 and 8.0 m

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IRAC Internal Scattering

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MIPSGAL to MSX Extended Source

• Compared MSX 21 m to MIPSGAL 24 m surface brightness for M16

• Data smoothed to 20 arcsec MSX resolution

• Color corrected using model spectra for star forming ISM (Flagey 2007)

• Correlation goes as the expected 1.5 for dust emission due to SFR

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Calibration at Longer Wavelengths

• Harder to do as stars are faint at > 30 m– Either need very sensitive telescope (JWST) – Or use diffuse emission (much larger uncertainty)

• For MIPSGAL 70 m needed to correct for significant non-linearity and gain offset

• Used transformation from IRIS 60 m data to MIPSGAL 70 m – IRIS version of IRAS data tied to DIRBE (Miville-Deschênes & Lagache

2005)– Applied color correction using model of dust emission and 60/100 color

• MIPSGAL 70 m used as sanity check on PACS 70 m maps from HiGal

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MIPS 70 m Galactic plane

No gain correction

Per stim-flash correction

Global / IRIS correction

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MIPSGAL 70 m / HiGal 70 m

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What’s Next

• Take more cross calibration data– Some IRAC observations of HST calibrators finished– Cryogenic IRAC for AKARI standards need analysis– AKARI spectra for IRAC calibrators need analysis– Warm IRAC observations planned of subset of DIRBE calibrators– HST observations of infrared KIII calibrators

• Observations of standards tied directly to NIST standards– ACCESS and SNDICE for example

• Need the calibration community to come up with a unified zero point concept– Possibly a calibration summit to resolve current differences