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Some Thoughts on Cross Calibration of Space Based Infrared Telescopes. Sean Carey Spitzer Science Center. 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

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some thoughts on cross calibration of space based infrared telescopes

Some Thoughts on Cross Calibration of Space Based Infrared Telescopes

Sean Carey

Spitzer Science Center

  • 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
relative versus absolute calibration and cross calibration
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 mm) and worse at longer wavelengths (> 30 mm)
does science depend on absolute calibration
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
personal background and potential biases about calibration
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 mm),
  • 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 l > 2 mm
    • 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
issues with infrared absolute calibration
Issues with Infrared Absolute Calibration
  • Vega is a bad choice of primary flux standard
    • Infrared excess due to circumstellar disk at > 5 mm
    • 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 mm
    • 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 mm 2% high
  • Conflicts with Price et al. (2004) result which included emissive spheres and same model Vega
more issues with absolute infrared calibration
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
hope for the future
Hope for the Future
  • Use of better models particularly replacing Engelke functions for l > 20 mm
    • Castelli and Kurucz (2004)
    • MARCS (Decin & Eriksson 2007)
  • Better templates through improved IR spectra (Engelke et al. 2006)
    • Reconciles 3.6 mm 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)
status of spitzer
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
warm cryogenic irac cross calibration
Warm / Cryogenic IRAC Cross Calibration

Warm 3.6 mm (top)

Cryo 3.6 mm (bottom)

Same functional form

Different amplitude

engelke cohen comparison
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


irac av kiii calibration offset
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 mm
  • Revised templates improve discrepancy
  • 4.5, 5.8 and 8.0 mm analysis in works
spitzer instrument cross calibration
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
spitzer cross instrument calibration
Spitzer Cross Instrument Calibration
  • Gizis et al. (in prep) compared IRAC to IRS and IRS to MIPS
  • 8 mm 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 mm 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 mm
    • In agreement with Rieke et al. (2008) comparison of IRAC and MIPS
mipsgal point source check
MIPSGAL Point Source Check
  • Color excess between predicted and observed for 20 AV and 7 KIII
  • 3 A stars have 24 mm excess
  • 24 mm 3% brighter
  • Similar result noted by SAGE legacy team
archival search for non linearities
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
irac si as extended source calibration
IRAC Si:As Extended Source Calibration
  • The 5.8 and 8.0 mm 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 mm to MSX 8.3 mm
    • Extrapolating MSX 8.3 mm to IRAC 5.8 mm 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 mm
mipsgal to msx extended source
MIPSGAL to MSX Extended Source
  • Compared MSX 21 mm to MIPSGAL 24 mm 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 l1.5 for dust emission due to SFR
calibration at longer wavelengths
Calibration at Longer Wavelengths
  • Harder to do as stars are faint at l > 30 mm
    • Either need very sensitive telescope (JWST)
    • Or use diffuse emission (much larger uncertainty)
  • For MIPSGAL 70 mm needed to correct for significant non-linearity and gain offset
  • Used transformation from IRIS 60 mm data to MIPSGAL 70 mm
    • 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 mm used as sanity check on PACS 70 mm maps from HiGal
mips 70 m m galactic plane
MIPS 70 mm Galactic plane

No gain correction

Per stim-flash correction

Global / IRIS correction

what s next
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