Some thoughts on cross calibration of space based infrared telescopes
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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

Outline Telescopes

  • 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 Telescopes 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 TelescopesAbsolute 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 TelescopesPotential 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 Telescopes 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 TelescopesCalibration

  • 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 Telescopes

  • 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 Telescopes

  • 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 Telescopes Cross Calibration

Warm 3.6 mm (top)

Cryo 3.6 mm (bottom)

Same functional form

Different amplitude

Engelke cohen comparison
Engelke / Cohen Comparison Telescopes

  • 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 Telescopes

  • 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 Telescopes

  • 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 Telescopes

  • 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 Telescopes

  • 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 Telescopes

  • 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 TelescopesCalibration

  • 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 Telescopes

  • 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 Telescopes

  • 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 Telescopesmm Galactic plane

No gain correction

Per stim-flash correction

Global / IRIS correction

Mipsgal 70 m m higal 70 m m
MIPSGAL 70 Telescopesmm / HiGal 70 mm

What s next
What’s Next Telescopes

  • 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