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PISCO Redshifts: Simulations. Will High Harvard University December 2006. The LPPC Contingent. Chris Stubbs (Harvard Physics, Astronomy) Tony Stark (SAO) James Battat (Harvard Astronomy) Passbands Will High (Harvard Physics) [email protected] Photometric redshifts.

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Pisco redshifts simulations

PISCO Redshifts:Simulations

Will High

Harvard University

December 2006


The lppc contingent
The LPPC Contingent

Chris Stubbs

(Harvard Physics, Astronomy)

Tony Stark

(SAO)

James Battat

(Harvard Astronomy)

Passbands

Will High

(Harvard Physics)

[email protected]

Photometric redshifts


The problem
The Problem

  • Measure the distance to as many galaxy clusters as possible, as efficiently as possible

    • Redshift == distance given knowledge (read: assumptions) of geometry of universe

  • Solution already exists:

    • The photometric redshift principle: Use

      • broadband, differential photometry

      • presumed knowledge of convenient source spectra

      • and knowledge of filter transmission curves

    • … to arrive at redshifts z to |dz| < 0.1

    • Spectra of Luminous Red Galaxies increase ~monotonically in well understood way… 4000A break

  • Our problem: we know our filters’ bandwidths and central wavelengths change as a function of field position! How does this affect redshift recovery?

  • Our tool: simulated observations of LRGs using expected filter curves




Dichroic curves

Dichroic Curves Glass

Solid line = transmit

Dashed line = reflect


Rule of thumb: Glass

Dichroics pass red




Pisco bandpasses at the focal planes

PISCO Bandpasses Glassat the Focal Planes

Multiply those curves appropriately


With ccd quantum efficiency
With CCD Quantum Efficiency Glass

  • Not yet accounted for:

  • Glass & other optics

  • Atmosphere

  • Possible y band at 1 micron

Multiply by MIT/LL high resistivity CCD transmission curves

Transmission

r

i

g

z

Wavelength (Angstrom)


Dichroic info
Dichroic Info Glass

  • Company: Barr Associates

  • Cost:

    • BS I $12k

    • BS II $11k

    • BS III $11k

  • Delivery: 12-14 weeks (including substrate procurement time of 8-10 weeks)

  • Barr can make effective index of refraction as high as 2.0 to minimize the effect of angle of incidence on the transmission/reflection properties (see next slide)


CWL = cut-on wavelength Glass

Ratio of cwl at some angle  to cwl at 0 degrees.

Increasing n (the effective index of refraction) decreases the effect of AOI (shallower slope)


Position dependence
Position Dependence Glass

  • Dichroics in highly collimated beam

    • Angle at dichroic → position in focal plane

    • But different angles have different cut-on wavelengths! Therefore…

  • Bandpasses depend on position in focal plane

    • Effective wavelength λeff changes

    • Bandwidth changes

    • Both ill behaved across field

  • Study how well we recover redshift assuming

    • Naïve bandpasses = transmission at center of field, assumed to hold everywhere

    • Correct bandpasses = true transmission, which changes with field position


How we measure redshift
How We Measure Redshift Glass

  • Observe redshifted Luminous Red Galaxy (LRG) spectrum with correct bandpasses

  • Estimate AB mags using naïve and correct bandpasses

  • Fit colors: LRG color ↔ LRG redshift

    • kcorrect

    • Use both naïve and correct bandpasses

true LRG

observed

Photon Flux Density

r

i

g

z

Wavelength (Angstrom)


We measure photo z s na ve
We Measure Photo-z’s: GlassNaïve

z in = 0.4

z out = 0.438

z in = 0.8

z out = 0.835

Black = output spectrum

z in = 1.1

z out = 1.174

z in = 1.4

z out = 1.412

r

i

Gray = input spectrum

z

g


We measure photo z s correct
We Measure Photo-z’s: GlassCorrect

z in = 0.4

z out = 0.403

z in = 0.8

z out = 0.804

Black = output spectrum

z in = 1.1

z out = 1.128

z in = 1.4

z out = 1.408

r

i

Gray = input spectrum

z

g


Photo z errors na ve
Photo-z Errors: GlassNaïve

Underestimated

z in = 0.4

err = 3.9e-2

z in = 0.8

err = 3.7e-2

Overestimated

z in = 1.1

err = 1.3e-1

z in = 1.4

err = 1.1e-1


Photo z errors correct
Photo-z Errors: GlassCorrect

Underestimated

z in = 0.4

err = 3.1e-3

z in = 0.8

err = 6.2e-3

Overestimated

z in = 1.1

err = 5.0e-2

z in = 1.4

err = 9.5e-2


Worst photo z errors
Worst Photo-z Errors Glass

i → z

g → r

r → i

z → …

Naive passband

photo-z error (max absolute deviation)

Correct passbands

z in


G ab mag errors na ve
g AB Mag Errors: GlassNaïve

Brighter

z in = 0.4

err = 8.3e-2

z in = 0.8

err = 1.3e-1

Dimmer

z in = 1.1

err = 1.5e-1

z in = 1.4

err = 8.4e-2


G ab mag errors correct
g AB Mag Errors: GlassCorrect

Brighter

z in = 0.4

err = 4.4e-2

z in = 0.8

err = 8.8e-2

Dimmer

z in = 1.1

err = 1.0e-1

z in = 1.4

err = 4.0e-2


Worst g ab mag errors
Worst g AB Mag Errors Glass

i → z

g → r

r → i

z → …

g AB mag error (max absolute deviation)

Naive passband

Correct passbands

z in


G r color errors na ve
g – r Color Errors: GlassNaïve

Bluer

z in = 0.4

err = 3.7e-2

z in = 0.8

err = 8.9e-2

Redder

z in = 1.1

err = 5.8e-2

z in = 1.4

err = 5.7e-2


G r color errors correct
g – r Color Errors: GlassCorrect

Bluer

z in = 0.4

err = 1.2e-2

z in = 0.8

err = 4.9e-2

Redder

z in = 1.1

err = 2.8e-2

z in = 1.4

err = 8.8e-2


Worst g r color errors
Worst g – r Color Errors Glass

i → z

g → r

r → i

z → …

g – r error (max absolute deviation)

Naive passband

Correct passbands

z in


G i color errors na ve
g – i Color Errors: GlassNaïve

Bluer

z in = 0.4

err = 1.3e-1

z in = 0.8

err = 1.4e-1

Redder

z in = 1.1

err = 1.8e-1

z in = 1.4

err = 9.3e-2


G i color errors correct
g – i Color Errors: GlassCorrect

Bluer

z in = 0.4

err = 3.8e-2

z in = 0.8

err = 4.9e-2

Redder

z in = 1.1

err = 9.4e-2

z in = 1.4

err = 3.6e-2


Worst g i color errors
Worst g – i Color Errors Glass

i → z

g → r

r → i

z → …

g – i error (max absolute deviation)

Naive passband

Correct passbands

z in


Worst color errors
Worst Color Errors Glass

NOTE THE SCATTER

Naive passband

g – r error

Correct passband

Wake et al, 2006

astro-ph/0607629

z in


What s next
What’s Next? Glass

  • Add y band? 1 micron, a la Pan-STARRS

  • Increase sophistication of photo-z module

    • More templates

    • Priors


Summary
Summary Glass

  • PISCO achieves broad optical bandpasses in an original way that is more efficient

  • Downside: bandpasses become weakly position dependent

  • Outlook for photo-z’s:

    • Hurts us most at z < 1.0, but in a clean way

    • At z > 1.0, 4000A break leaving bands, situation messier

    • Incorporate these facts into analysis (= calibrate)

  • Contact Will at [email protected]


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