HARP / ACSIS A B-Band Survey “Camera” (Sub)Millimetre Observing Techniques Russell O. Redman
James Clerk Maxwell Telescope JCMT • Mauna Kea, Hawaii • High, dry site – Above tropical inversion layer • Good access to – Communications – Transportation – Support facilities
HARP / ACSIS Partly Commissioned • Spectral Imager – Like visible-light IFU – Datacubes instead of images
Observing Bands & Atmospheric Opacity • • • Frequency Bands – A : Rx. A 3 (211 -279 GHz) – B : HARP-B (325 -375 GHz) – C: N/A – D : Rx. W-D (620 -710 GHz) – E: N/A Weather Bands – 1 : cso_tau ≤ 0. 05 – 2 : 0. 05 < cso_tau ≤ 0. 08 – 3 : 0. 08 < cso_tau ≤ 0. 12 – 4 : 0. 12 < cso_tau ≤ 0. 20 – 5 0. 02 < cso_tau : B-Band – Available 2/3 of the time – Most productive band – Equivalent to 850 m – Rx. B 3 needs replacement B C D E A Atmospheric transmission calculated (using the IRAM ATM routine; see text) as a function of frequency in the submillimetre window for three different water vapour pressures (1 mm pwv is a `good' night, 0. 5 mm pwv `exceptional', and 5 mm pwv is `rather nasty'). Useful observations are possible only in the 230 GHz region in the latter case. http: //docs. jach. hawaii. edu/JCMT/OVERVIEW/tel_overview/
How to build a better receiver Detectors: better or more? • Power Temperature • Historically, reduce TRX – TRX ≈ (1 - ) TAMB – Close to photon-detecting • Options – Space-based telescopes – More detectors • Challenges – Tight financial limitations – Hard to make uniform sets of detectors – HARP: 4 x 4 array of detectors
How to build a better receiver: Single or double sideband? • • • Signal (usually) in one sideband – Signal and Image sidebands Noise from BOTH sidebands – DSB: sky adds TAMB in both SB – SSB: Direct image SB to TSSB – (Rx. B 3 could do both!) HARP : SSB – Use polarizing Mach-Zehnder interferometer as a SSB filter – C 2 F is fixed, C 2 M moves (both curved!) • Motion required for DSB too large • – TSSB ~ 20 K ALWAYS check for strong lines in the image sideband!
How to build a better receiver: Optical design • Curvature of projected FOV – Unexpectedly bad for SCUBA • 3 -bolometer photometry – Design goal for HARP • Models show good performance • Measured patterns in lab were poor for bottom row – Alignment of the internal optics was far off center – Fixed, but not re-measured – Commissioning measurements are incomplete • Stay tuned…
How to build a better receiver Beam size and separation • Diffraction-limited Optics – Planck function @ 4 K • Dewar is dark @ 10 m • Dewar is bright @ 850 m! – Detectors have horns to direct beams out aperture – At design frequency • CO (3– 2) 345. 7959899 GHz • Separation = 2 FWHM
Beam Size and Separation Jiggle Maps • Nyquist sampled image – Every 1/2 BW – 4 4 grid of samples • (5 5 at 370 GHz) • Switching – Chop (with subreflector) – Position (move telescope) – Frequency (move LO) • Under-Sampled Images – 5 -point – 2 or 3 -detector chopping – 2 x 2 grid (FBW sampling) – 1 1 grid (Stare mode)
Beam Size and Separation Scan Maps • Large-Area Mapping Mode – Sweep telescope – Sample regularly – K-mirror rotates FOV to match grid axes (optional)
How to build a better receiver Rapid, Automated Tuning • Range 324 -376 GHz • Automated tuning – Fast Program Changes – Spectra Line Surveys • Speed – Goal 30 sec – Actual < 40 sec – Similar specs to Rx. A 3, Rx. B 3 – (Rx. W > 30 minutes!) • Requires – Reproducible tuning positions – Versatile software
Auto. Correlator Spectrometer and Imaging System • Radio “cameras” have 2 parts – Frontend : HARP – Backend : ACSIS • Built at DRAO • etc…
First Light • • • A very important moment for every instrument-building team Points to note – Clean spectrum – Sharp features – Flat baseline – Plausible calibration Caveats – Not pointed – Not focused – Not central position of W 75 N • No central detector
REAL “First Light” • New hardware/software – HARP – ACSIS – K-Mirror – IF-system – OCS software • Pointing • Focus • Observing modes • • – Real-time display – Data reduction software – etc… All had to work together for the first time Almost like re-commissioning the whole telescope
Promise of things to come • Orion CO (3 -2) TA* dv • Orion red and blue wings • Orion Mean velocity • NGC 1333 CO (3 -2) TA* dv
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