1. Requirements for a bolometer prototype at the 30m telescope
S.Leclercq
23/04/2009

2. The IRAM 30m telescope (MRT, Pico Veleta)
In the Sierra Nevada (Spain), at 2900m.
4 atmospheric windows available: 3, 2, 1, 0.9 mm.
Primary mirror diameter = 30m, secondary = 2m.
F=f/D ~ 10  diffraction beam ~ 10”. FOV ~ 4’.
Cassegrain with Nasmyth focus (beam along elevation axis).
Current bolometer instrument: MAMBO 2: 117 pixels (feedhorns), FOV=3.5’, NEFD ~ 40 mJy·s1/2.

3. Bands available at the 30m
ATM opacity model at Pico Veleta, for winter (260K) and summer (300K) with good weather (1mm of water vapour) and bad weather (7mm)
Centre of the bands for a maximal width, and corresponding size of the FWHM diffraction pattern
l (mm)
n (GHz)
Airy HPBW
3.2 94
22.6"
2.05
146
14.5"
1.25
240
8.8"
0.87
345
6.2"

4. Optical chain efficiency and real beam
Definitions and efficiency measurements
r = (/2) (/D) = diffraction space natural radius
I = relative intensity
example: Airy diffraction pattern
L = relative power
Aperture efficiency = relative flux losses from the optical chain: ea = Ae /A = Pcollected(0)/Pincident
Beam efficiency = relative power at the main beam radius (1st dark ring of the Airy beam): Beff = L(rmb)
Forward efficiency = relative power from the 2p steradian plane in front of the telescope: Feff = L(r2p)
Surface deformations on the main dish alter the diffraction pattern.
Parameters: steepness factor (R), aperture efficiency at long wavelength (e0), RMS deformations height (sh=55m), correlation lengths (3 components: de= [ ] m). Ruze law : ea(l)=e0 exp(-S(sh 4pR/l)2)
Components of the aperture efficiency from measures conducted in 2007 [C.Thum]: e0 = ohmic losses (total all mirrors 89%) * blockage (98%) * 13dB taper spillover (92% (ground emissivity = 30%)) * 13dB taper illumination (87%) * alignment & leakage (97%) * 86GHz (95%) = 65 %
Other efficiencies (for simulations): cryostat filters tf  70%, detector efficiency and others: to = 85%

5. Optical chain efficiency and real beam
Graphics
Relative powers
Beams
Dash lines = Empirical Gaussians
Solid lines = Antenna Tolerance Theory
q = radius in powers of ten times a 10dB edge taper main beam
q = radius in units of a 10dB edge taper main beam
Legend of the curves:
Airy diffraction pattern
Real beam l=3.4mm
Real beam l=2.0mm
Real beam l=1.3mm
Real beam l=0.86mm
Feff= % 92 90 86 75
ea= 61 45 35 16 %
Beff= 73 54 42 19 %

6. Simulations for an optimal bolometer array
Bands
Pixel types
Central n:
Bandwidth:
0.5Fl square bare multimodes pixels in a filled array
2Fl round 10dB edge monomode feedhorns in a compact array
Global efficiency
Extended source
Point source
< 50 %
~ ea/4
< 65 %
~ ea
540
1400
3600
7300
2400
5700
16000
32000 40 95
260
530
170
420
1100
2300
Number of pixels for 2 fields of view
Square grid:
Hexagonal grid:
FOV = (4.8' 10')

7. Simulations for an optimal bolometer array
Matrices below: columns = weather condition: good (1mmwv) / bad (7mmwv) ; lines = bands: 3mm / 2mm / 1mm / .9mm
Collected power
Background sources: atmosphere, ground, telescope, cryostat.
Benchmark sources: Jupiter, 1KRJ extended, 1mJy point (Jy = W/(m2Hz)
0.5 Fl bare pixel
2 Fl feedhorn
Noise Equivalent Power
Convenient noise unit: nu = W/Hz1/2
Summing Nb pixels RN=NEPNb/NEP1
Shot noise:
Bunching noise:
Best pixel noise:
Total:
0.5 Fl bare pixel
NEPbkgTb / 6
NEPpix ~<1nu
2 Fl feedhorn
NEPbkgTh / 6
NEPpix ~ few nu

8. Simulations for an optimal bolometer array
Noise Equivalent Temperature
Noise Equivalent Flux Density
(Nb = number of pixels, obs = observing mode efficiency : OTF =1.6, OnOff =2.1)
(no pixels efficiency hNb in P1KRJ)
(pixels efficiency hNb inside P1mJy)
4  0.5 Fl bare multimodes pixels
2 Fl monomode feedhorn
Integration time to detect a source at S/N=s: t = 0.5(shobsNEP/P)2 = (sNET/T)2 = (sNEFD/F)2
Extended source T=100K:
Point source (Ps<< background):
Mapping speed comparison:
Filling ratio bare square grid vs feed hexagon grid: Nb/Nh=13.9
4 bare pixels vs 1 feedhorn
Comparison with Griffin's: with shot noise only my results ~1.3x more favorable to feedhorns (assumptions on throughput, efficiencies, filters, geometry) ; including bunching noise my results ~2-3x more favorable to bare pixels (multimode vs monomode) !
Time & speed simulations in this presentation assume no sky noise & no confusion

9. Expectations for the future science grade instrument
At least 2 colors (bands / channels)
Current preferred colors: l = [1.25 ; 2.05] mm (n = [146 ; 240] GHz)
Total efficiency per pixel > 40% ?
Background limited instrument : NEPpix<NEPbkg/6 (in previous slide NEPbkg given for hpix=90%, if pixel less efficient NEPbkg lower, hence factor 6 rather than 3)
Sensitivity: ~0.5mK/Hz1/2 & 1mmwv, and stay <1mK/Hz1/2 & <10mJy/Hz1/2 in a large dynamic range ( KRJ background)
Preference for fully sampling (0.5Fl) pixels (advantage for mapping) ?
Preference for filled array (best to fight anomalous refraction in sky noise)
Field Of View  6'
Preference for multiplexing since FOV>6'  100s s pixels
Negligible sensitivity to stray-lights
Cost < 6M€ including (5M€ as dedicated time  <1M€ cash)

10. Requirements to test a prototype at the 30m
Working array with at least 32 pixels in a single attached block or area.
Array fully characterized with lab tests: pixels + multiplexing.
Agreement by collaborators on the procedures to measure pixel noise performance and sensitivity in lab (noise spectra, black body response, etc.).
Sensitivity for useful tests and first light science: pix0.5? & NEPinst1F<10­16W/Hz1/2.
Translation of lab to on site performance must be worked out (NEEL & IRAM), my rough estimate for summer time (5mmwv): (tot_ext~25%, tot_pix~10%) & /c~30%  good weather: NET~0.5mK/Hz1/2 , NEFD~8mJy/Hz1/2 , ~ few seconds.
Preliminary frequency range of optimization is 1-20 Hz, noise spectra will be taken for a statistically significant number of pixels.
Optical measurements showing that the internal optics is working according to the design goals: valuable illumination of the telescope and no stray-light (optical filters ready, XY maps with chopper, secondary lobes).
Instrument control & mapping software OK to avoid down time during telescope tests.
Only hardware successfully tested in laboratory can be employed at the telescope.
List of sources for observations prepared and agreed in advance.

11. Constraints for a prototype at the 30m
The prototype components must fit in the available space in the receiver cabin.
The instrument must fit on the anti-vibration table, which can't be moved for such tests.
The only structures than can be removed are the MAMBO 2 elements on the anti-vibration table, in particular the M5-M6 tower must stay in place.

12. Constraints for a prototype at the 30m
No interference with telescope observation during time not allocated to the prototype.
For communication between instrument and control room use the 1Gb shared ethernet link, (the availability of a separate twisted pair cable that can run at 100Mb/s is not warranted yet).
Use a special process to request "real time " position of the antenna via ethernet ; more complete information can be written in FITS files every minute.
A maximum of 8 external persons at a time can be lodged at the telescope.
Cryogen needs must be known several weeks in advance.

13. Schedule and expectations for the summer 2009 prototype test
June: lab test at Neel in collaboration with IRAM (MR/SL/KS), for a potential green light at the end of the month (see requirements).
July: IRAM deliver M7 & M8 (HDPE lenses ?) to Neel. Optical tests.
July: agreement on the list of sources for the observation with the prototype.
August 4-25 or August 11-31: tests at the 30m
Week 0: all hardware shipped to the telescope.
Week 1: mounting and test on site without the telescope beam (the prototype can be mounted in the receiver cabin only the 3 last days of this week).
Week 2: day time use of the telescope beam.
Week 3: night time use of the telescope beam.
Week 4: dismount the prototype.
Expectations:
For green light to week 3: at the end of week 2 observation of selected sources must be successful.
Objective: observation of ~10mK / ~100mJy sources in few seconds...