2009 aug 20 — sac update wfos/mobie1 wfos/multi-object broadband imaging echellette mobie team, to...

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)


• Minimize complexity & moving parts — Maximize coverage, resolution, field, transmission.

MOBIE overview: Design Principles


• ADC – always used, fused silica prisms• Slit-mask fed• Mirror collimator• Dichroic split at ~550nm • 300mm pupils • Prism cross-dispersion• Reflection gratings

• 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)


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)



• 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.


grating




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


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)


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.)


MOBIE overview: field of view


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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.

• limit set by overhead+detector noise.

AA

B

B

~4”~2”

~2”


• ADC and collimated field of view

CDP progress: Fused silica, linear ADC

2009 aug 20 — sac update wfos/mobie1 wfos/multi-object broadband imaging echellette mobie team, to...

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