ZEUS-2 is a grating spectrometer that we are currently building and that will be used on CSO, APEX, and (possibly) JCMT to observe emission lines in all submillimeter bands that are accessible from the ground. It is a new spectrometer that is based on our present instrument ZEUS, but differs significantly in several aspects that will greatly improve the sensitiviy and observing efficiency. ZEUS-2 will have: A picture gallery of the first assembly of ZEUS-2 is available here


Broad Wavelength Coverage
  • » ZEUS-2 uses the grating of ZEUS, but now utilizes it from 3rd to 9th order (Fig. 1)
  • » Enables simultaneous imaging in up to 5 lines for extended sources (see Fig. 5)
  • » Enables observations of redshifted fine-structure lines over wide redshift range
    • - [CII]: z ~ 0.2 to 3
    • - [OIII]: z ~ 1.2 to > 5
    • - [OI]: z ~ 2.5 to > 5
Wavelength coverage
Figure 1 - ZEUS-2 Frequency bands (gray bars) overlaid on atmospheric transmission from various sites (ATM 2002 transmission model from Pardo et al).


Figure 2 - ZEUS-2 cryostat in its instrument cart with our Cryomech PT-407 and the multi-channel electronics (MCE) from UBC mounted on the top plate of the dewar.
Figure 3 - Janis two-stage ADR.
Dewar & Cyrogenics
  • » Light-weight aluminum dewar
    • - Fabricated locally by PulseRay
    • - 6061-T6 aluminum
    • - 21.5" OD x 26"
    • - vacuum shell weight < 40 lbs
  • » Pulse Tube Cooler
    • - Cyromech PT-407
    • - Dual stage
    • - 18 W @ 45 K
    • - 200 mW @ 3.5 K
  • » ADR
    • - Janis dual stage
    • - 13 µW @ 1 K (16 hr)
    • - 400 nW @ 100 mK (16 hr)


The grating from ZEUS will be transfered to ZEUS-2. All other optical elements are redesigned to maximize the usable field of view (FoV) within the limited instrument envelopes dictated by the CSO and APEX telescopes. The final optics are very similar in concept to those of ZEUS, but we increased the usable FoV from 83 arcsec in the original design to 155 arcsec (CSO values), we eliminated one mirror, and we enabled a larger tilt range for the grating. Figure 4 illustrates our design. The f/12 beam from the telescope enters the dewar window (made of HDPE), reaching a focus just inside the 3 K stage. It is then reflected off of flat mirror M1 onto collimating mirror M2 and through a Lyot stop to M3. The M2/M3 pair change the f/# of the system from 12 to 2.75, appropriate for the detector array. M3 focuses the light onto the entrance slit of the spectrometer, from which it then expands to fill the right half of the collimating mirror, M4. M4 creates a 10 cm collimated beam, which illuminates the 38 cm long grating. The dispersed light returns to the left side of M4, and then is reflected up to flat mirror M5 which sends the beam to the focal plane array.
  • » Grating Properties:
    • - R2 Echelle
    • - L ~ 35 cm
    • - 9th order 200 mm
    • - 8th order 230 mm
    • - 5th order 350 mm
    • - 4th order 450 mm
    • - 3th order 610 mm
Figure 4 - Optical ray trace of ZEUS-2. Green and blue beams are spatial limits of Array 1. Yellow and red beams are spatial limits for Array 2/3.


Focal plane
Figure 5 - Focal plane layout. Array 1 (right) has the top half in 5th order, bottom half in 4th order of the echelle. Array 2/3 (left) operates in 9th (top), and 3rd (bottom) orders. The five bright lines illustrated are observed simultaneouly in extended sources.

Focal Plane Array
  • » (3) NIST 2-d TES bolometer arrays
  • » Back short tuned
  • » 5 lines in 4 bands simultaneously
    • - 215 µm (1.5 THz)
    • - 350 µm (850 GHz)
    • - 450 µm (650 GHz)
    • - 625 µm (475 GHz)
  • » Imaging capability (9-10 beams)

Array frame
Figure 6 - Structural mock-up of Array 2/3 (top) and Array 1 (bottom).


Imaging Capabilities
  • » Astrophysics
    • - ZEUS-2 will observe both [CI] lines at 370 and 609 µm. The line ratio provides a strong constrain on the gas temperature.
    • - ZEUS-2 can observe the optically thin 13CO (J=6-5) line in many ULIRGs and nearby galaxies, thus constraining the CO opacity. This is very important for modeling molecular clouds using the 12CO transitions, which are usually optically thick.
    • - ZEUS-2 on APEX will observe the [NII] 205 µm line: This line is an important coolant of the ionized gas and in addition reveals the fraction of [CII] emitted from ionized media.
  • » Mapping advantages
    • - "Perfect" spatial registration of the lines due to the long slit and multi-line capability.
    • - Simultaneous observation of emission lines in several telluric bands couples the corrections for telluric transmission and thus simplifies the absolute calibration in each band.

ZEUS-2 slit footprint on M83
Figure 7 - Illustration of the ZEUS-2 slit foot-print on M83. The top half of the slit yields 9 beams in the CO(7-6), [CI] 370 µm, and 13CO(6-5) lines, while the bottom half yields 10 and 5 beams on the sky respectively in the [NII] and [CI] 609 µm lines.


Redshift coverage
Figure 8 - The co-moving star formation history of the Universe (Blain et al. 2002, Phys.Rep., 369, 111). The transparent bars mark redshift ranges accessible to ZEUS-2 in the 158 µm [CII] line.
Redshift Coverage
  • » ZEUS detects [CII] from redshifts z ~ 1.1 to 2.1 (350 and 450 µm windows) --> Covers the peak in the star-formation rate per co-moving volume
  • » ZEUS-2 expands the [CII] coverage to z ~ 0.25 to > 4 --> Straddles the peak, tracing the star formation history of the Universe from 11 Gyr ago to the current epoch
  • » Other redshifted lines are accessible as well:
    • - [OI] 63 µm: traces PDRs, density of PDRs, and UV field strength
    • - [OIII] 88 µm: probes the hardness of the UV field
    • - [NII] 122 and 205 µm: traces electron density and gives the fraction of [CII] emission from HII regions