[asac][almanews] ALMA Memos 372 and 393 Released

[Stacy Oliver]

ALMA MEMO #372

An Amplitude Calibration Strategy for ALMA

R.Moreno (IRAM), S.Guilloteau (IRAM/ESO)

2002/05/10

Keywords: Calibration, Receiver

We propose an amplitude calibration strategy for ALMA which attempts to
reach the 1 % precision for absolute calibration. Key points and major
difficulties are identified. Polarization is ignored in this first step. We
first review the properties of possible calibration sources. Planets and
specially giant planets satellites are the preferred reference sources for
flux calibration. Asteroids are too complex for such a purpose, but are
useful as e.g. targets for relative pointing & focus determination. Stars
are too weak. Quasars have to be used for various intermediate steps:
bandpass, pointing, focus, and relative amplitude calibration. The amplitude
calibration precision may be divided into 3 major items: absolute flux
scale, relative amplitude calibration (as function of time and antenna), and
bandpass (dependence on frequency). Absolute amplitude depends on receiver
gain and atmospheric opacity correction, and on pointing & focus control.
The semi transparent vane is found to be 3 to 10 times less sensitive to
atmospheric parameters than the dual-load calibration system. Accurate
pointing & focus control can be done within reasonably short time (1 minute
or so). Relative amplitude calibration can be checked to 1 % accuracy at mm
wavelengths by observation of a nearby quasar, but integration time
considerations prevents reaching better than about 3 % at submm wavelengths.
Bandpass calibration, and specially the sideband ratio determination, is the
longest calibration procedure. Within 3 minutes, this can be calibrated to 1
% at mm wavelengths, but only to 3 % at submm frequencies. Based on these
considerations, we recommend 1) to further develop the semi-transparent vane
system, 2) to keep the 1 % accuracy goal at mm wavelengths, 3) to relax the
specification to 3 % in the submm domain, and 4) implement the coherent
signal from the subreflector for bandpass calibration purpose.

View a pdf version of ALMA Memo #372.
http://www.alma.nrao.edu/memos/html-memos/alma372/memo372.pdf

Download a postscript version of ALMA Memo #372.
http://www.alma.nrao.edu/memos/html-memos/alma372/memo372.ps


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ALMA MEMO #393

DSB versus SSB and Bandwith/Sensitivity tradeoff

S.Guilloteau (IRAM/ESO)

2002/05/14

Keywords: Receiver, Correlator


The ALMA performances will depend on the receiver design and performances.
Conflicting requirements on cost, reproducibility, maintainability,
bandwidth and noise temperature affect the receiver design, and the
capabilities of ALMA. This memo reviews the possible tradeoff between
receiver complexity and ALMA performances, considering DSB vs SSB and
Bandwidth vs Receiver noise tradeoffs. Realistic scientific applications are
considered. This memo reproduces the numerical results obtained in memo 304
using a different approach. However, the conclusion differ because I also
account for a scheduling strategy in which high frequency observations are
made only when the weather allows. Combined with a reasonable assumption on
the distribution of continuum (or dual-line) observations vs single spectral
line cases, this assumption favors DSB mixers at high frequencies. DSB
receivers may provide a 10 % advantage above 500 GHz, while SSB receivers
would provide a 10 % advantage below that frequency. Analysis of the
Bandwidth/Receiver noise trade-off indicate that reduction of a factor 2 in
detection bandwidth can be compensated by improvement by 25 -- 30 % in
Receiver noise.


View a pdf version of ALMA Memo #393.
http://www.alma.nrao.edu/memos/html-memos/alma393/memo393.pdf

Download a postscript version of ALMA Memo #393.
http://www.alma.nrao.edu/memos/html-memos/alma393/memo393.ps

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