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The NAIC-NRAO School on Single-Dish Radio Astronomy Techniques and Applications -- July 2005

The Measurement Process with Fully-Filled Apertures
Chris Salter (NAIC)

Accuracy in Radio Astronomy
· Pulsar Periods: 1 part in 1012 · HI Redshifts: For z 0.1, about 1 part in 104 · Source Intensities and Polarization Parameters: Typically ±10%

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Basic Definitions
A: Intensit y/Surface Brightness:

B: Brightness Temperature:

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C: Flux Densit y

D: Distance Dependencies

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The Basic Radio Receiver
Celest ial radio emissio n can be subdivided into two main spectral categories: 1. Broadband or Continuum Emissio n. Typical examples: (a) planetary surfaces, (b) HII regions, (c) the Cosmic Microwave Background & (d) those Galact ic and extragalact ic sources that emit via the synchrotron process.

2. Line emissio n due to low-energy transit io ns within atoms and mo lecules in space.

Terzian & Lewis, (u npu blished)

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The Super-Heterodyne Receiver

Recorder

Mixer: Why frequency change rather than amplify at RF all the wa y down the receiver chain? 1. Long cables are less lossy at lower frequencies. (Nowada ys less important due to use of optical fibers. 2. Fear of positive feedback causing receiver oscillations. 3. If LO can be varied over a wide range, a standard, fixed frequency, IF chain can service a wide range of receiver front-ends. Detector:

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Signal-to-Noise Considerations
System No ise:

Cascaded Amplifiers:

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Lossy Co mponents

The System Temperature
There are extra noise-power contributions from such things as; a) Emission from the neutral atmosphere. b) Ground radiation in the far sidelobes. c) Celestial noise contributions from; i) the background radiation of the Milky Wa y. ii) the 2.7 K radiation of the Cosmic Microwave Background. iii) possibl y a contribution from the target source itself!

Where the sum of all contributions from outside the observing system is called the ANTENNA TEMPERATURE, TA.

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The Radiometer Equation
How weak a sour ce ca n we detect with a given r eceiver? For our total-power receiver, the RADIO METER EQ UATION gives the r ms noise ripple a gainst which we try to detect our tar get sour ce, In units of tempera ture;

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CONFUSION

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Single-Dish Imaging and Sampling

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How far apart should we place our scans (and how often should We sample alo ng a scan) in order to lose no informat ion which our telescope is capable of passing?

Let us consider a one-dimensional case;

Convo lut ion:
D = Te lescope D iameter = Wavelength

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TA Delta-fns

X

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