SCUBA-2 Imaging Fourier Transform Spectrometer David A. Naylor and Brad G. Gom University of Lethbridge, Canada On behalf of the Canadian SCUBA-2 Consortium

SCUBA-2 Imaging Fourier Transform Spectrometer

David A. Naylor and Brad G. GomUniversity of Lethbridge, Canada

On behalf of the Canadian SCUBA-2 Consortium

“SCUBA-2 Imaging Fourier Transform Spectrometer” SPIE San Diego Annual Meeting #48 [5159-10] August 6, 2003

SCUBA-2 The new Submillimetre Common User Bolometer Array camera for the JCMT

Key Features: Simultaneous imaging at 450 and 850 μm Large 8 x 8 arc minute field of view ~10000 pixels, 0.5fλ spacing Sky background limited performance Novel scanning modes to provide large

scale surveys Will have both imaging spectroscopic and

polarimetric capabilities


ISQUID

Amplifier

Vbias

R(T) TES

Resistance

RN

Temperature

RC

Tc

Bias point

Silicon quarter-

wave delay

Doped layer

absorber

Micro-machined

silicon walls

Detector chip

Multiplexer chip

Wire-bond pads

10m gap

0.5F = 1.135mm at 850m

Degenerately doped layer

Silicon walls(50m)

Silicon wafer(60m)

Bump bonds (2 per pixel)

MoCu bilayerTES Heater

SiN membrane(0.5m)

SCUBA-2 TES Detectors


SCUBA-2 on the JCMT

Mirror N1

Window

NasmythPlatform

Cryostat

Cabin optics

Elevation bearing(f/7 field image)

Tertiary mirror(f/12)

Detectors(f/2.7)


SCUBA-2 Feed Optics



Herschel/SPIRE(350m)25 arcsec : 16 arcmin2

(simulation by Hughes & Gaztanaga)

JCMT/SCUBA(850m)14 arcsec : 5 arcmin2

(HDF image from Hughes et al. 1998)

Compact ALMA(450m)0.01 arcsec : 0.02 arcmin2

JCMT/SCUBA-2(450m)7 arcsec resolution : 64 arcmin2 field-of-view(simulation by Governato et al.)

SCUBA-2 Field of View

•~1000x faster mapping than SCUBA•Pathfinder instrument for high resolution telescopes such as ALMA

•To reveal the underlying chemistry, we need spectral information at each pixel


•0.005 cm-1 resolution•Continuum is preserved

Naylor, D.A., Gom, B.G., Schofield, I., Tompkins, G., Davis, G.R., “Mach-Zehnder Fourier transform spectrometer for astronomical spectroscopy at submillimeter wavelengths”, Proc. SPIE, Millimeter and Submillimeter Detectors for Astronomy 4855, 540-551 (2003).


SCUBA-2 FTS Science Goalsthe FTS fills a niche between the SCUBA-2 continuum images and the higher spectral resolution, but limited size images, provided by heterodyne array receivers (e.g. HARP).

Interstellar Medium - offers both a rich spectrum, with continuum and line components, and a rich field. The IFTS will allow for the spectral index mapping of molecular clouds and in particular identify those sources where a significant contribution to the total band flux arises from line emission.Ultra-Luminous Infra-Red Galaxies (ULIRGs) – measure the Spectral Energy Distribution (SED) of the dust emission from ULIRGs. Planetary atmospheres - inventory molecular species and provide information on the physical and dynamical processes of the atmospheres (e.g. internal heat sources). Spectral mapping of the Jovian, Saturnian and Martian discs to study hemispheric, zonal and polar differences and transport effects. Super novae remnants - large scale mapping of super nova remnants and interaction with the interstellar medium.High red shift objects - initial estimates indicate that it may be possible to determine the red shift through careful measurements of the slope of the continuum across the 850 µm band.


SCUBA-2 FTS Features

Hyperspectral mapping - Combining the increased sensitivity of SCUBA-2 with the resolution of the FTS, will provide an unprecedented hyperspectral imaging ability in the submillimetre.Mach-Zehnder Design - This innovative FTS design provides high efficiency and access to all four ports of the interferometer. With this design, both ports could view the sky, or one could view a cold load.Dual wavelength operation - The SCUBA-2 FTS will take advantage of the unique simultaneous dual wavelength capability of the SCUBA-2 system.Variable spectral resolution - The resolving power of the FTS can be selected instantly within a range of 10 to 5000. Novel observing modes - The instantaneous, fully-sampled image plane in SCUBA-2 will provide better image fidelity. The potential exists for exploiting the DREAM observing mode to provide atmospheric correction for each frame in the interferogram.


Mach-Zehnder FTS Instrument Concept

•Access to all 4 ports•Factor of 4 optical path multiplication•Requires matched 50% beam splitters

Ade, P.A.R, Hamilton, P.A., and Naylor, D.A., “An Absolute Dual Beam Emission Spectrometer”, Optical Society of America, FTS topical meeting, poster FWE3, Santa Barbara, California, June 1999.


Intensity Beam Dividers

•4RT efficiency > 90%•Factor of 4 frequency range•2 metal meshes in an FP configuration•Allows HeNe laser alignment



FTS operating modes

SCUBA-2 frame rate is fixed at 200 Hz

Continuous scan mode•Table scanned at constant velocity•Frames read out at 200 Hz•Requires a non-uniform FT•Low res. scans in ~1 sec•High res. scans in ~30 sec•Dual inputs on the sky for nulling operation

Step-and-integrate mode•Table moved in discrete intervals•Requires double modulation of the signal for sky correction


SCUBA-2 DREAM Mode

•DC biased detectors – sky background variations are much larger than the signal•SMU moves through a jiggle pattern, so that each sky position is sampled by many bolometers•transform raw bolometer data into partially calibrated data on a regular spaced grid, thereby correcting for the effects of:

•The impulse response of the electronics.•The motion of the SMU.•Differences between bolometers (Piston correction)•The Earth's atmospheric emission.•Distortions in the bolometer positions.

•Corrected images are produced at ~1 Hz•High res. step and integrate scans would take ~1.5 hours!


DC band limited Nyquist sampling:

Shannon theorem for band limited signals:

Intentional ‘Aliasing’

If the detector is truly band limited, then we can use the Shannon theorem to reduce the number of samples in the interferogram


Nyquist = 160cm-1


Nyquist = 40cm-1


Nyquist = 20cm-1


Nyquist = 10cm-1


Nyquist = 5cm-1


Normal DC band limited Nyquist = 25 cm-1

‘Aliasing’ mode Nyquist = 5 cm-1


Expected Performance


Conceptual Design Review July 2003

Preliminary Design Review May 2004

Critical Design Review October 2004

Complete instrument testsOctober 2005

Delivery to telescope March 2006

Project Milestones

Documents

SCUBA-2 Imaging Fourier Transform Spectrometer David A. Naylor and Brad G. Gom University of Lethbridge, Canada On behalf of the Canadian SCUBA-2 Consortium