submillimetre astronomy - helsinki

Submillimetre astronomy

Feb 17 11

Submillimetre continuum

observations


Heterodyne receivers conserve the phase information of the incoming radiation. Thus it is possible to observe the intensity of the incoming radiation as a function of frequency (spectrum). The bandwidth of the heterodyne receivers is typically 1 GHz to 8 GHz.

Incoherent detectors (bolometers) record the intensity of the incoming radiation in the detection band which is typically tens of GHz wide.


Bolometer observations are very sensitive to 1/f noise. Besides to the sky also the bolometer amplifiers cause 1/f noise.

In classical bolometers this problem has been solved using a chopper mirror (1 element bolometers) or using wobling secondary mirrors (array bolometers). Chopping (wobling) is done with about 2Hz which is faster than 1/f noise.

Submillimetre astronomy White noise

(zero mean, Gaussian distribution)

1/f noise


The observed Flux density as a function of time in a single bolometer (Laboca) channel. Only the variable part is shown and the total power offset is stored elsewhere. This is not white noise!

Submillimetre astronomy A chopper mirror is used when observing with a single

channel bolometer, wobling with the secondary with bolometer arrays. Wobling can be done at 2Hz, chopping faster.

Chopping or wobling allows to sample the high frequency spatial signal but the total power at the input is not modulated and is subject to 1/f noise.

Solution: Block the DC part of the signal (ie. the total power) and let only the AC part (the modulated signal) through (DC biased and AC coupled). In effect throw away the total power information!


Besides the noise from the sky bolometers are subject to thermal noise in the bolometer element. This is suppressed by cooling the detector using

● Liquid nitrogen (~70K)

● Liquid He2 (~1.5K in vacuum)

● He3 fridge (~0.3K)


NEP (Noise equivalent power) or NEFD (noise equivalent flux density) is the incident rms of signal power which produces signal to noise ratio (S/N) equal to 1. If the source temperature Ts is high enough that the RayleighJeans approximation holds the incident power is

where k is Boltzmans constant, is the band width and 2 appears because of two orthogonal polarizations.


The resulting S/N is

where is the integration time. Direct refers to direct (blometer) detection. The unit of NEP is

W/Hz½. In reality this equation is over simplification but it reflects correctly the actual situation. e.g. RJ approximation is certainly not valid at 10K20K source temperatures in the submillimetre region.

t


Point sources can be observed by keeping the telescope pointed at the source and chopping or wobling.

Extended sources are observed in the OTF mode. The scanning and chopping/wobling is done in azimuth. In the time line the source will appear first as a positive signal and later as a negative signal when it is in the offbeam. A special algorithm was developed to recover the source structure. Problem, possible artifacts.


A large map of the L183 cloud. IRAM MAMBO

1.2mm array.

(Kauffmann et al. 2008, A&A 487, 993).

Artifacts from using the wobler are clearly seen


Modern electronics allows AC biased, DC coupled bolometers. The signal is modulated with the AC bias. Need phase sensitive detection at the bias frequency!

The advantage is that the total power is conserved and maps contain all the structure.

Atmospheric variations cause power changes at the input of the amplifier. This must be compensated by an opposing voltage for each scan.

Submillimetre astronomy In practice AC bias moves the 1/f noise to lower

frequency. Integrating on LABOCA loads.

1/f noise starts at 0.1 Hz

Submillimetre astronomy Being able to record the total power does not

solve the problem that the atmosphere is a (strong) source of 1/f noise in the observed flux. However, when observing with large (n channels > 20) bolometer arrays it is possible to estimate the atmospheric emission statistically. The field of view of the array is small and at any moment all the bolometer elements see the same amount of emission from the sky. Correlating the bolometer elements with each other the emission, seen simultaneously by all the elements can be subtracted. Easy when observing point sources, more difficult when observing extended sources.

Submillimetre astronomy LABOCA time line for 9 channels (out of ~250).

The signal in separate elements is correlated!

Submillimetre astronomy Correlated noise removal:

The signal is sampled at a frequency >>1Hz in all n bolometer elements. At each time point correlate the signal in each bolometer element with the signal in surrounding elements and subtract the correlating part. If the source is compact one can correlate the signal in almost all the surrounding elements. If the source is extended the immediate surroundings of each element has to be masked. Noise removal can be aggressive (i.e. strong) or “soft”.

Submillimetre astronomy For extended sources the resulting map noise is

worse than for point sources.

Submillimetre astronomy MAMBO LABOCA 850 microns

1.2mm


Linear and spiral OTF scans can be used. The telescope must move quickly enough so that the array moves over the source in time which is less than the 1/f limit.

Spiral scan effective for point sources but linear scans are better for observations of large scale extended emission

Submillimetre astronomy A point source and an extended region observed

with LABOCA

at 0.85mm. The

LABOCA FOV

is ~11 arcmin.


Reduction of LABOCA data contains the steps● Flagging in time and space domain● Fourier filtering (blanking/tapering)● Despiking● Flat fielding● Opacity correction● Correlated noise removal

submillimetre astronomy - helsinki

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