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false color
Faraday rotation
fast Fourier transform
FDDI
feed
filled table
FITS format
floating-point
floating-point accelerator
flux density
form-based interface
frequency switching
fringe
fringe frequency
fringe rotator
front end
FTP
full-synthesis image
FWHM
In digital imagery, a false color display is one which is generated by
using a number n (>1) of real-valued functions
f
(x,y),.....,f
(x,y)
to control the
proportions, at each pixel coordinate
(x,y) of an additive mixture of three primary hues.
In practical terms, the user of a digital display system supplies
f,.....,f, and twists knobs that control the mapping
R->R
that sends the n pixel values
at each (x,y) into the proper image chromaticity
and intensity.
Compare pseudo-color display.
A so-called true color display is obtained with n = 3 and with transfer functions chosen such that the color assignment corresponds in an approximate way to the actual coloration of a scene (as in a color photograph).
Rotation of the plane of polarization of linearly polarized radiation on passage through a plasma containing a magnetic field with a component along the propagation direction.
(FFT): A fast algorithm for the computation of the
discrete Fourier transform
(DFT) 
of a sequence of n complex numbers 
,
typically requiring only O(nlogn) arithmetic
operations---or a multi-dimensional generalization thereof. By
contrast, straightforward, or naïve evaluation of the DFT requires
O(n
) operations.
The fast Fourier
transform algorithms which currently are the most popular are the
Cooley-Tukey (1965) algorithms, for the case of n highly
composite. For n a power of two, the (radix-2) Cooley-Tukey
FFT requires about
2nlogn
real multiplications and
3nlogn
real additions.
More generally, the Cooley-Tukey algorithms require a few times ns(n) complex arithmetic operations, where s(n) is the sum of the prime factors of n, counting their multiplicities. S. Winograd has produced FFT algorithms which are more efficient than those of Cooley and Tukey, typically requiring about the same number of additions, but only about 20% the number of multiplications. (Computation of the required complex exponentials---or sines and cosines---is not counted, since these generally are either pre-computed and stored in compact tables, or generated recursively.)
A further advantage of the FFT algorithms is their avoidance of round-off error, which can build up severely when the DFT is evaluated by brute-force. There are related, fast algorithms for the convolution of sequences of real numbers, for the discrete cosine transform, etc. Algorithmic details may be found in H. J. Nussbaumer, Fast Fourier Transform and Convolution Algorithms, Springer-Verlag, Berlin, 1982. The computational complexity of the DFT is discussed by L. Auslander and R. Tolimieri (Is computing with the finite Fourier transform pure or applied mathematics?, Bull. (New Series) Amer. Math. Soc., 1 (1979) 847-897.
Fiber Distributed Data Interface: A high-speed networking standard. The underlying medium is fiber optics, and the topology is a dual-attached, counter-rotating token ring.
The device at the focus of a receiving antenna which converts the incident radio wave to a voltage at the input to the receiver(s). It may comprise a group of (usually two) receptors, which have the same phase center and are tuned to the same frequency, but have different polarizations. Many antennas have only one feed, but those of some single dish radio telescopes are equipped with multi-feed arrays.
In AIPS++, a Table object which stages data values to and from a secondary storage device. See also virtual table, and StorageManager.
Flexible Image Transport System: a magnetic tape data format well-tailored for the transport of image and u-v data among observatories and now the standard data interchange format in astronomy. FITS was introduced by D. C. Wells, E. W. Greisen, and R. H. Harten, FITS: A flexible image transport system, Astron. Astrophys. Suppl. Ser., 44 (1981) 363-370. For up-to-date details on FITS and other scientific data formats, visit the FITS Home Page at http://fits.cv.nrao.edu/FITS.html.
A number representation consisting of a mantissa, M, an exponent, E and an (assumed) radix (or "base") . The number represented is M*R^E where R is the radix - usually ten but sometimes 2.
Many different representations are used for the mantissa and exponent. The IEEE specify a standard representation that is used by many hardware floating-point systems.
(FPA): Hardware to perform functions on floating-point numbers such as addition, multiplication, logarithms, exponentials, trigonometric functions and various kinds of rounding and error detection especially rapidly. A floating-point accelerator often functions as a coprocessor to the CPU.
The term "floating-point accelerator" suggests a physically large system, often a circuit board, whereas a "floating-point unit" (FPU) may be one chip or part of a chip.
In radio astronomy, a measure of the power incident on a detector per unit surface area of the detector per unit bandwidth of the radiation. It thus measures both a spatial (surface) density and a spectral density of the received flux, or radiative power.
A type of user interface used, for example, on the World-Wide-Web, to organize questions or options for the user so that they resemble a traditional paper form to be filled out by pointing to the fields and typing text, or by choosing from a list.
A mode of radio astronomical spectroscopy wherein the frequency of the local oscillator signal is rapidly changed to move features in and out of the i.f. bandpass of the receiver and the difference signals are analyzed. As the noise power from the front end depends on frequency, this method is used only when the range of interesting frequencies is a small fraction of the bandpass of the front end. Symmetrical frequency switching, in which the comparison spectrum is taken alternately above and below the "signal" spectrum in frequency, eliminates the effect on the spectral baseline of the first three terms (through quadratic) in the Taylor series expansion of the bandpass noise spectrum; asymmetrical frequency switching eliminates only two.
1. In optics generally, the successive light and dark patterns caused by the interference of waves that are in and out of phase at different positions, as in a Young's double slit experiment.
2. In radio astronomy, the oscillation in time of the output of a correlation-type interferometer before such oscillations are stopped (see fringe rotator).
The relative phase of the signals at two elements of an interferometer changes
by 2
when the geometrical delay changes by one wavelength, i.e. when w, the delay measured in wavelengths,
changes by 1.
For a source at declination 
being observed at an hour angle h with the instrumental delay held constant, the fringe frequency is
= dw/dt = -
u cos
where = dh/dt is the angular velocity of the earth. Note that the fringe frequency goes to zero on the
v-axis of the u-v plane.
In a correlating-type radio interferometer, a mechanism to introduce a time-varying phase shift into the local oscillator signal of a receiver, in order to reduce the frequency of the oscillations of the correlator output. Fringe rotation allows the correlator output (whose amplitude is proportional to visibility amplitude) to be sampled at a lower rate. The fringe rotation is chosen so that the fringe frequency for a point source located at the so-called fringe stopping center would be reduced to zero, or at least very close to zero. Usually the fringe stopping center and the delay tracking center coincide; both then are called the visibility phase tracking center.
A modular package containing a radio-frequency (r.f.) amplifier and any associated cryogenic systems, routers and converters (mixers), whose input is the voltage from a receptor and whose output is an intermediate-frequency (i.f.) signal. In some antenna systems, a feed and associated receptors are also packaged with a front end module, in an integrated front end box. See also local oscillator.
File Transfer Protocol: A client-server protocol which allows a user on one computer to transfer files to and from another computer over a TCP/IP network. Also the client program the user executes to transfer files. It is defined in STD 9, RFC 959.
In earth-rotation aperture synthesis, with stationary interferometer elements, an image derived from an observation which is of such lengthy duration that the fullest possible u-v coverage is obtained (i.e., from an observation extending from "horizon to horizon). Compare snapshot.
The full width between half-maximum values of any function, e.g. the width of a spectral line (in frequency or velocity) between its half-maxima in intensity, or the width of an antenna power pattern (in angle) between its half-power points (synonymous in this case with the HPBW).
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Copyright © 1995,1999,2000 Associated Universities Inc., Washington, D.C.
abridle@nrao.edu, 19 July 1996, 11:08 EDT