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A Multiband Imager for Magellan

This conceptual design proposes a multiband imager for the Magellan telescope, capable of capturing images in multiple passbands for various scientific opportunities including supernova follow-up, cosmological probes, and cluster detection. The design includes a doublet field flattener, rapid readout, and high efficiency, with the potential for complementarity with the Dark Energy Camera.

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A Multiband Imager for Magellan

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  1. A Multiband Imager for Magellan • Christopher Stubbs • Department of Physics Department of Astronomy • Harvard University • cstubbs@fas.harvard.edu

  2. Conceptual Design r g Pass l > 5500 A Pass l > 8500 A z Pass l > 7000 A Common shutter i

  3. Existing “standard” passbands U B V R I

  4. Pan-Starrs bands 4000 A break at z of 0.2 0.5 0.8 1.0 1.25 1.5 g r i z y

  5. Collaborators • Christopher Stubbs, CfA & Harvard Physics • Melissa Franklin, Harvard Physics Dept • Tony Stark, CfA • John Geary, CfA • South Pole Telescope/Dark Energy Survey collaboration: J. Carlstrom, J. Mohr... Alan Uomoto, OCIW, has been very supportive

  6. Science Opportunities • Supernova followup observations • Type Ia and type II Sne as cosmological probes • Requires multiband images, multiple epochs • Photometric redshifts of clusters • 4 band imaging over modest field • Transient followup • Evolution of SED for GRBs • Microlensing light curves • Planetary occultations • Multiband data useful for discrimination

  7. Cosmology from SZ Clusters • Multiple projects now funded and under way to use SZ effect to detect galaxy clusters, down to ~2 x 1014 Msolar • Expectation is ~ 7 clusters per sq degree • South Pole Telescope will map 4000 sq deg; 29,000 clusters • SZ signal strength z-independent • Optical observations needed for z • Spectroscopy – SALT • Wide field multiband survey – DEC 2009 • Targeted moderate field multiband observations

  8. South Pole Telescope (SPT) Survey • Bolometric focal plane, 1000 elements, and 10m aperture telescope. • Will map Southern extragalactic sky • RA from 20 hrs to 7 hrs • DEC from –30 to –75 degrees • Currently funded by NSF polar programs, under construction • Early 2007 first light.

  9. Anticipated Cluster z-distribution 90% within z<1.2 50% within z<0.5 Carlstrom, Holder and Reese Ann Rev Astron Astrophys, 2002

  10. Photometric Redshift for Clusters • Photo-z’s for individual galaxies tend to have scatter of sz/(1+z)~0.03, but with a few “catastrophic” outliers. • Combination of morphology, magnitude, color and location can be used to establish cluster’s redshift. • Robust statistics can be used to eliminate “outliers”.

  11. Photometric Redshifts in SDSS bands Blanton et al, astro-ph/0205243

  12. Early-type galaxies in SDSS bands Bernardi et al, astro-ph/031629 g-r 0.7 r-i 0.3 r-z 0.8 g 20.7 r 20.0 i 19.7 z 19.2

  13. Relevant Magnitudes are 18<r<24 Brodwin et al, astro-ph/0310038

  14. Time (sec) to reach SNR=10 Extended source, ABmag in 2.2 arcsec aperture Dark time, 0.8 arcsec, airmass=1.2, scaled to Magellan and high-r http://rpm.cfht.hawaii.edu/~megacam/diet/DIET.rpm Galaxy colors roughly follow contours of constant integration Shaded boxes are DEC target 10s magnitudes. We should get ½ the clusters (those with z<0.5) in 60 sec

  15. Clusters per unit telescope time

  16. Return on Magellan Time Investment Could get 200 clusters out to z~1, or 100 clusters out to z of 1.5, in 1 night. Fifteen dark nights on Magellan could produce photo-z’s for 3000 clusters out to z=1, about 10% of the total expected from SPT.

  17. Conceptual Optical Design Exists Identical triplets (all spherical) Doublet field flattener (2 aspheric surfaces) Field stop T. Stark, CfA

  18. Optical Performance Plate scale is 0.062 arcsec per 15 mm pixel FOV is 4.1 x 4.1 arcminutes (~ 700 kpc at z=0.3) 80% encircled energy in ~0.15 arcsec:

  19. Tightly coupled software/observing Take Image 1 30 sec Analyze Image: flatten, WCS, sextractor Galactic reddening corr. Produce z, sz OK? Offset Take Image 2 30 sec Offset if appropriate More images Slew to next target

  20. Rapid Readout and High Efficiency Integrated SW reduces wasted time Binning 2x2 gives • 0.124 arcsec/pix, • Single amplifier output per 2K x 4K chip 8 sec readout @ 250Kpix/sec Effective telescope time multiplier: • For “balanced” exposures, sequential images take Tseries = 4(texp+treadout)x Nframes • Parallel imager takes (with 0.8 throughput degradation) Tparallel = (texp/0.8 + treadout) x Nframes For texp>>treadout total time is reduced by a factor of ~3

  21. Controlling Systematics • Our design is readily baffled • Can use both field stop and pupil stop • Suppresses stray and scattered light • Better flatfielding • Single common shutter near pupil • Reduced shutter artifacts • Flux ratios with a single pointing and 2-3 exposures, under all conditions! • Even with patchy cloud cover, get Poisson-limited colors.

  22. Complementarity with Dark Energy Camera Multiband Camera: 16 arcmin2, 6.5m aperture: - Get 100 clusters to z~1 in 4 hours. - Can be on the sky at start of SPT survey Dark Energy Camera: 3 sq deg, single band, 4maperture: - Survey approach on CTIO 4m gets 3 sq deg x 7 clusters/sq deg = 21 clusters per hour, or 84 clusters in 4 hours. - Delivers other science as well: weak lensing, SN detection... Multiband imager’s cluster hunting advantage is 100/84 = 1.2 • Does not take into account seeing advantage on Magellan • Does not take into account cloud-immunity of multiband imager • If SPT survey falls short of flux goals, multiband advantage increases Multiband camera better suited to chasing z>1 clusters.

  23. Two-stage plan • 2007 – 2009: SPT in operation Use multiband camera to image ~2000 clusters Initial cluster count vs. z results Use camera for SN followup as well... • Post-2009: Dark Energy Camera on Blanco 4m (if $) Deep wide DEC survey will find SNe Follow SN light curves with multiband camera Chase higher-z clusters with multiband camera

  24. Status • Rough conceptual design done • Proof-of-concept optical design done • Detectors are in hand • Lincoln labs 2K x 4K, 15 mm pixels • Some epitaxial, some high-resistivity • Readout electronics require replication • MegaCam architecture and board set • Machine shop capacity available

  25. Task list • Modeling • Slew vs. expose tradeoffs vs. redshift and richness • Photo-z determination • Hardware • Finish design work, order optics • Mechanical design, fabrication • Electronics and detector optimization • System integration, testing... • Software • Scripts that connect Sextractor to photoz codes • Test with SDSS and IMACS data • Integrate with instrument • Proposals • Plan is to submit NSF ATI proposal Nov 1.

  26. Summary • A multiband imager for Magellan makes sense: • Multiple science drivers • Unique capability • Quality photometry • Conceptual design completed • Detectors on hand, optics straightforward • Design needs to be refined, and optimal observing strategy devised • Software is a key aspect. • Your comments & participation welcome!

  27. Backup Slides and residual clutter

  28. Cluster luminosity function DM= 38.2 implies L* ~ 17.1 AB r mag at z = 0.1 F(M) Christlein and Zabludoff, astro-ph/0304031

  29. Solid: w= - 1 Dotted: w= - 0.6 Shortdash: w= - 0.2 Longdash: open CDM

  30. Limiting Magnitude • ½ L* cluster galaxies at redshift 4000A break leaving blue filter • g,r,i,z = 22.8,23.4,24.0,23.3 • Complete cluster catalog • Galaxy catalog completeness • g,r,i,z = 22.8,23.4,24.0,23.6 • Simple selection function • Blue galaxy photo-z at faint mags • g,r,i,z = 24.0,24.0,24.0,23.6 • Photo-z for angular power spectra and weak lensing AB Mag of ½ L*

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