Submillimeter Astronomy

ZEUS

An echelle grating spectrometer for the 10.4 m Caltech Submillimeter Observatory (CSO) and 12 m Atacama Pathfinder EXperiment (APEX)
R ~ 1000 in three telluric bands: 350, 450, and 620 µm (5th, 4th, and 3rd order of the echelle)

Primary Design Driver: Redshifted [CII]

Arguably the most interesting epoch for which to trace the [CII] line emission from galaxies is that between redshifts of 1 and 2 (7.5-10 Gyr ago), during which the star formation activity of the Universe peaked
At z ~ 1 to 2, the [CII] line is transmitted throught the 350 and 450 µm submilimeter telluric windows with about 40% coverage
At z ~ 1 to 2, galaxies are essentially the point sources to current submillimeter telescopes a 5 to 9" beam at 350 µm corresponds to about 60 kpc at z = 1
Therefore, we desire the best possible point source sensitivity --> we select a (spectrally multiplexing) grating spectrometer

Orders in a Grating

Resonant condition: D=N*h=mλ/2
m (= nxN) is an integer, and equal to the resolving power of the grating at λ
Therefore, at any angle, there can be more than one wavelength that is at resonance
We picked the grating rulings so that it would be in resonance in (n =) 5^th order at 539 µm
Therefore, it also resonates at 5/4*359 = 449 µm in 4^th order, 5/3*359 = 598 µm in 3^rd order

Design Driver: Sensitivity

Desire R = λ/Δλ ~ 1000 optimized for detection of extragalactic lines
Operate at diffraction limit: - Maximizes sensitivity to point sources - Minimizes grating size for a given R
Long slit desirable - Spatial multiplexing - Correlated noise removal for point sources
Operatate in 4^th and 5^th orders covering the 450 and 350 µm windows respectively

Optical Path

There is a series of a scatter, quartz, 2 long λ pass, and a bandpass filter in series to achieve dark performance (P. Ade)
Total optical efficiency: ~ 30%, or 15% including bolometer DQE

Optics

Detector sensitivity (NEP ~ 2x10^-17 W/Hz^-1/2) requires a dual stage ³He refrigerator (T ~230 mK)
Spectral tuning is easy - turn the grating drive chain
Operate at 350 and 450 µm simultaneously with bandpass filters on array
Array is 1 x 32 pixels - 1 spatial position - Spectrum sampled at 1 resolution element/pixel to maximizing both sensitivity and spectral coverage

Grating

Littrow Mode Grating: 35 cm long
R2 Echelle (Blaze Angle: 63.43º)
Grid Constant: 992 mm
Orders: ! ; - 5th @ 359 µm - 4th @ 449 µm
Tilt Range: 57º to 73º
Resolving Power: ~ 800 to 1500
With a 32 pixel array, in principle each submm window is covered with 3 to 4 "tunings" of the grating for line searches

Simple Bolometer

Photon comes in, is absorbed, and deposits its energy as heat in the substrate: C_v ∝ T³ - want low T and very small mass to make big ΔT
Thermometer: silicon doped for high resistance and large temperature coeffient of resistance: R(T) ~ T^x
Measure V(t) across bolometer <--> T(t) <--> T₁(t) <--> P₁(t)

Detector Array

ZEUS operates with a 1 x 32 pixel thermister sensed array from GSFC (SHARC-2/HAWC prototype)
Achieves sensitivity within a factor of 1.5 of the background limit
At present, the array is split by filters at the detector to deliver 1 x 15 element spectrum at 350 and 450 µm simultaneously