HARP / ACSIS A B-Band Survey “Camera”

(Sub)Millimetre Observing TechniquesRussell 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 / ACSISPartly Commissioned

• Spectral Imager– Like visible-light IFU– Datacubes instead of images

Observing Bands & Atmospheric Opacity

• Frequency Bands – A : RxA3 (211-279 GHz)– B : HARP-B (325-375 GHz)– C: N/A– D : RxW-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– 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.

http://docs.jach.hawaii.edu/JCMT/OVERVIEW/tel_overview/

A

B C D E

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: 4x4 array of detectors

€

P = G ⋅(TRX + TRAD )

= G ⋅(TRX + (1−η )TAMB +ηTSKY )

TSKY = TSYS ⋅(PON − POFF ) /POFF

TSYS =TRX + (1−η ) ⋅TAMB

η

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

– (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

– 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 receiverBeam 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 SeparationJiggle Maps

• Nyquist sampled image– Every 1/2 BW– 44 grid of samples

• (55 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)– 11 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 receiverRapid, Automated Tuning

• Range 324-376 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

Gunn Tuner

0

500

1000

1500

2000

2500

320 330 340 350 360 370 380

F_LO (GHz)

Tuner (microns)

T(1)Fit

Modulator Bias versus F_LO

-1.5

-1

-0.5

0

0.5

1

1.5

320 325 330 335 340 345 350 355 360 365 370 375

F_LO (GHz)

Bias (V)

Bias

F

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)

TA* dv

• Orion

Mean

velocity

• Orion

red and

blue wings

• NGC1333

CO (3-2) TA* dv