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 : RxA3 ( GHz) –B : HARP-B ( GHz) –C: N/A –D : RxW-D ( 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 –RxB3 needs replacement 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 (1mm pwv is a `good' night, 0.5mm pwv `exceptional', and 5mm pwv is `rather nasty'). Useful observations are possible only in the 230 GHz region in the latter case. A BCDE

How to build a better receiver Detectors: better or more? Power  Temperature Historically, reduce T RX –T RX ≈ (1-  ) T AMB –Close to photon-detecting Options –Space-based telescopes –More detectors Challenges –Tight financial limitations –Hard to make uniform sets of detectors –HARP: 4x4 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  T AMB in both SB –SSB: Direct image SB to T SSB –(RxB3 could do both!) HARP : SSB –Use polarizing Mach-Zehnder interferometer as a SSB filter –C2F is fixed, C2M moves (both curved!) Motion required for DSB too large –T SSB ~ 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 4 K Dewar is 10  m Dewar is 850  m! –Detectors have horns to direct beams out aperture –At design frequency CO (3–2) 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 –2x2 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 GHz Automated tuning –Fast Program Changes –Spectra Line Surveys Speed –Goal 30 sec –Actual < 40 sec –Similar specs to RxA3, RxB3 –(RxW > 30 minutes!) Requires –Reproducible tuning positions –Versatile software

AutoCorrelator 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 W75N 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)  T A * dv Orion Mean velocity Orion red and blue wings NGC1333 CO (3-2)  T A * dv