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Advances in Nod & Shuffle Techniques for Effective Spectroscopy in Astronomical Surveys

This update discusses advancements in slit spectroscopy using the 'Nod & Shuffle' technique, which addresses common limitations such as sky subtraction issues, slit irregularities, flat-field errors, and read noise penalties. The method enhances the efficiency of CCDs by effectively managing charge storage and enabling rapid switching for minimized noise. Highlighting techniques employed in major projects, including the Las Campanas IR Survey, the goal is to improve redshift measurement and our understanding of early galaxy evolution through detailed analysis with high resistivity CCDs.

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Advances in Nod & Shuffle Techniques for Effective Spectroscopy in Astronomical Surveys

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  1. Nod & Shuffle at Magellan LCIR Survey Update GDDS Preview October 18 2002

  2. Conventional Slit Spectroscopy • Sky subtraction is primary limitation • Slit irregularities • Flat-field errors • Residual Fringing • Geometric distortions • Low slit density on sky • Beam switching ? • Variable sky spectrum • Read noise penalty • High read-out overhead • The solution: ‘nod & shuffle’

  3. First Exposure Obscured Charge Storage Area Active slit area Obscured Charge Storage Area

  4. Now nod telescope and shuffle charge “B” position “A” position

  5. “B” position “A” position Nod & shuffle the other way

  6. Repeat N times and then readout

  7. Difference of two positions

  8. Finally shift and add both

  9. LBL High Resistivity CCDs

  10. LBL High Resistivity CCDs No fringing, but high CR rates

  11. LBL High Resistivity CCDs Straight average - 2 hours Nod & Shuffle

  12. LBL High Resistivity CCDs +/- 200 DN rejection

  13. A B A-B Sky cancellation: ‘nod and shuffle’ Storage of ‘sky’ image next to object image via ‘charge shuffling’Zero extra noise introduced, rapid switching (60s) Typically A=60s/15 cy: 1800s exposure10-3 subtraction

  14. Another example

  15. GMOS N&S Sky residuals SUMMED along long slit (1.8 arcmin) Cycle:A=60sB=60s + 25s o/head Raw Sky/20 Subtracted sky (i.e. ~10-3 level is enough for 200,000 sec pointed obs.)

  16. GMOS Nod&Shuffle Multislit

  17. GMOS Nod&Shuffle Multislit

  18. Maximum Slit Utilization

  19. Nod & Shuffle on IMACS

  20. Nod & Shuffle on IMACS 2’’ slits 2’’ gaps

  21. Micro-Shuffling on IMACS 2” slits 2” gaps 4000A per spectrum

  22. Micro-Shuffling on IMACS

  23. Macro-Shuffling on IMACS High Slit Density or IFU mode

  24. Macro-Shuffling on IMACS High Slit Density or IFU mode

  25. Macro-Shuffling on IMACS High Slit Density or IFU mode

  26. Technical and Practical Considerations • Telescope, Guider and CCD controller must be well synchronized • Active Optics must work with short dwell time • Overheads must be minimized • Mask making software needs special capabilities • Reduction software (done! -Abraham &Glazebrook) • Order blocking filters?

  27. Las Campanas IR Survey McCarthy, Persson, Martini, Koviak (OCIW) Chen (MIT), Marzke(SFSU), Carlberg, Abraham(UT) Ellis (Caltech) Evolved Galaxies at 1 < z < 2

  28. Las Campanas IR Survey • Goal: Empirical understanding of early galaxy evolution • Target: 1 square degree to K = 21 • Pilot survey in 2000/2001: VRIH to H=20.5 • Six fields around the equator (2 in south!) • 1 square degree in BVRIz’H • 0.5 square degrees in J & K to K = 20.8 • 200+ redshifts with LDSS2 • ~ 50 redshifts with GMOS & LRIS

  29. Color-Magnitude Diagram Stars 0.0 < z < 1.0 1.0 < z < 1.5 1.5 < z < 2.0 500 sq. arcmin

  30. Color-Color Diagrams • Stars form distinct sequence • Z > 1 galaxies appear at K ~ 19 • Z > 1.5 galaxies at K > 20.5

  31. Color-Color Diagrams • Stars form distinct sequence • Z < 1 galaxies well sampled at K ~ 19

  32. Color-Color Diagrams • Stars form distinct sequence • Z > 1 galaxies appear at K ~ 19

  33. Color-Color Diagrams • Stars form distinct sequence • Z > 1 galaxies appear at K ~ 19 • Z > 1.5 galaxies at K > 20

  34. Color-Color Diagrams • Stars form distinct sequence • Z > 1 galaxies appear at K ~ 19 • Z > 1.5 galaxies at K > 20 • Reddest galaxies follow minimal evolution track

  35. Color-Redshift Diagrams

  36. Photometric Redshifts from LCIR

  37. Photometric Redshifts from LCIR

  38. Clustering of Red Galaxies

  39. Evolving Luminosity Functions • LFs derived from photo-z’s with modified likelihood approach • LF at intermediate z agrees well with CNOC2 • Very little apparent evolution in L* to z ~ 1.2

  40. Gemini Deep Deep Survey GDDS Team: Karl Glazebrook (JHU), Bob Abraham (Toronto), Pat McCarthy (OCIW),Rick Murowinski (DAO), Ray Carlberg (Toronto), Ron Marzke (SDSU), Sandra Savaglio (JHU), H-W Chen (OCIW) David Crampton (DAO), Isobel Hook (Oxford), Inger Jørgensen & Kathy Roth (Gemini) Goal: Deep 100,000 sec MOS exposures on Las Campanas IR Survey fields to get redshifts of a complete K<22.4 I<25 sample covering 1<z<2

  41. Goals: • First Complete sample 1<z<2 • use photo-z’s to weed out low-z galaxies (BVRIzJHK) • Determine luminosity and mass functions • Can we see the assembly of mass? • Massive galaxies at z=2 would severely trouble CDM • Mass(z) more robust than SFR(z) • Relate to galaxy morphology (ACS) • Identify Ell/Sp/Irr over 1<z<2 • Track low-z behavior to high-z • E.g. can we see mass assembly of giant Ellipticals? • Can we track the dynamical evolution of spiral disks • Track SFH over 1<z<2: • Age of galaxies, metallicities of population

  42. GDDS history • Sep 2001: start of GDDS evil planning • Jan 2002: team approached Gemini observatory with nod & shuffle proposal • Feb 2002, obtained Gemini go-ahead. • Feb-May 2002. Implementation of N&S at DAO (~$10K cost) • May 2002: first N&S engineering observations on 8m • July 2002: N&S commissioned on sky • Aug 2002: First 4 nights of GDDS - Science Verification for N&S - success!! • Sep-Dec 2002: Band I queue time, 50 hrs

  43. Gemini + GMOS Gemini GMOS spectrograph Tel.+instr. efficiency GMOSLRISLDSS1 GMOS represents the best possible option for a red sensitive MOS. Ideal system for nod & shuffle

  44. GDDS sample LCIRS4 fields BVRIzJHKs2626Limits:B<26.0 V<26.5R<26.8 I<25.8z<24.7 J<22.5H<22.5Ks<22.4 Use photo-z’s to weed out z<0.7 foreground I<25 typical model n(z):

  45. GDDS mask 84 objects - 2 tiers with150 l/mm grating

  46. GDDS Spectra 77 objects 40,000 secs

  47. GDDS Nod&Shuffle Mask

  48. GDDS Nod&Shuffle Mask

  49. [OII] Redshifts from GDDS 23.7 < I(AB) < 24.2

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