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2009 Aug 20 — SAC update WFOS/MOBIE 1
WFOS/Multi-Object Broadband Imaging Echellette
MOBIE Team, to date:
PI / optical designer: Rebecca Bernstein
Project Manager: Bruce Bigelow
Project Scientist: Chuck Steidel
Science Team:• Bob Abraham (U. Toronto)• Jarle Brinchmann (Leiden)• Rebecca Bernstein (UCSC)• Judy Cohen (Caltech)• Sandy Faber (UCSC)• Raja Guhathakurta (UCSC)• Jason Kalirai (STScI)• Jason (Xavier) Prochaska (UCSC)• Connie Rockosi (UCSC)• Alice Shapley (UCLA)• Chuck Steidel (Caltech)
• 3 options (1 low blaze, 2 high-blaze angle)• R = / ∆ = 1000, 3000, 5000, or 8000
• Refracting cameras • Diameter < 400mm lenses• Blue: all fused silica, CaF2 (perfect internal transmission)• Red: fused silica, glass, CaF2, 400 mm diameter elements max.• RTV-bonded multiplets • 22-24o field of view• F/1.9• High transmission (~40%)
• Rrms < 0.1arcsec
MOBIE summary: a multi-slit red/blue echellette.
Slit mask
collimator
dichroic
Fold
mirrorRed
grating Blue
grating
(dummy
Surface)
2009 Aug 20 — SAC update WFOS/MOBIE 4
MOBIE overview: 3 Spectroscopic Modes
• Low: R~1000 Only dispersion elements change• Medium: R~ 2,500 and/or 5000 Each grating is fixed.• High: R ~ 8,000
TMT focal plane.
(mask not visible) collimator
grating
Fold mirror (red side)
Dichroic (blue side)
2009 Aug 20 — SAC update WFOS/MOBIE 5
MOBIE overview: 3 Spectroscopic Modes
• Low: R~1000 Only dispersion elements change.• Medium: R~ 2,500 and/or 5000 Each grating is fixed.• High: R ~ 8,000
TMT focal plane.
(mask not visible) collimator
grating
Fold mirror (red side)
Dichroic (blue side)
2009 Aug 20 — SAC update WFOS/MOBIE 6
MOBIE overview: 3 Spectroscopic Modes
• Low: R~1000 Only dispersion elements change.• Medium: R~ 2,500 and/or 5000 Each grating is fixed.• High: R ~ 8,000
TMT focal plane.
(mask not visible) collimator
grating
Fold mirror (red side)
Dichroic (blue side)
2009 Aug 20 — SAC update WFOS/MOBIE 7
MOBIE overview: Example Configuration Options
7
• R~1000 (single order)
• R~2500 (3 orders)
• R~5000 (5 orders)
• R~8000 (7-8 orders)
• Prism cross-dispersion
sets the slit length: 2” – 4”
(for multiple orders)
MOBIE OCDD
2009 Aug 20 — SAC update WFOS/MOBIE 8
MOBIE overview: Echellette spectroscopy
• Echelle advantages:• Always “on blaze”• Standard ruled reflection grating• No grating tilt or camera articulation required• High resolution with modest beam size (300 mm on TMT)
2009 Aug 20 — SAC update WFOS/MOBIE 9
MOBIE overview: field of view
Field of view is similar to DEIMOS if placed on Keck.
• Field dimensions: • Slit length: 9.6 arcmin (max)
• Area: ~40.3 sq. arcmin
• Off-axis • Required with reflective collimator
• Achromatic, high throughput
• Contiguous field:• Simple, cheap, better for surveys.
• (Empirically, multiple fields work badly.)
2009 Aug 20 — SAC update WFOS/MOBIE 10
MOBIE overview: field of view
2009 Aug 20 — SAC update WFOS/MOBIE 11
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2009 Aug 20 — SAC update WFOS/MOBIE 12
CDP upcoming: Background Subtraction
• Some sort of beam-switching is mandatory — short slit pairs or within a longer slit.• minimal systematics if cadence is faster than variability timescale
(2-30 sec for rms sky variance < 0.1%)• ideally no fitting/interpolating required
(measure sky / target at same time, same slit)• Short exposures or nod+shuffle both possible.