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LaTeX
LaTeX2HTML
latitude
Lattice
leap second
Lempel-Ziv compression
Lempel-Ziv Welch compression
Linux
lobe rotator
local mean solar time
local oscillator
local sidereal time
local standard of rest
locked file
log
longitude
lookup table
loop gain
LSR
LST
luminance
Leslie Lamport's document preparation system based on TeX. It adds commands to TeX to simplify typesetting, so that the user can concentrate on the structure of the text rather than on formatting. LaTeX was developed at SRI International's Artificial Intelligence Laboratory. See LaTeX, A Document Preparation System, Leslie Lamport, Addison-Wesley 1986, ISBN 0-201-15790-X. A Guide to LaTeX, H. Kopka & P. Daly, Addison-Wesley 1993, ISBN 0-201-56889-6, or NASA's on-line help at http://molscat.giss.nasa.gov/LaTeX/.
The 1995-1996 revision and update known as LaTeX2e is documented on-line at http://www.math.upenn.edu:80/latex/latex2e.html.
A freely-distributed Perl script, originated by Nikos Drakos at the University of Leeds, that co-operates with LaTeX, GhostScript and other public-domain utilities available on many computer systems, to convert LaTeX-mastered documents into HTML format for display on the World-Wide Web.
Source code is available from http://www-dsed.llnl.gov/files/programs/unix/latex2html/sources/.
An on-line users' manual is available from http://www-dsed.llnl.gov/files/programs/unix/latex2html/manual/.
An authors' guide is available at http://www.cv.nrao.edu/~abridle/l2h4aips/l2h4aips.html.
In a planeto-centric co-ordinate system, the angle measured at the center of the planet from the equator to a point on the planet's surface, towards one of the poles.
Also used, by analogy, as one co-ordinate in the ecliptic and galactic systems.
1. In AIPS++, an abstract base class which presents an interface to a finite-volume, linear, rectangular, or hyper-rectangular structure. The Lattice is fundamentally described by its shape.
2. In domain theory, a partially ordered set in which all finite subsets have a least upper bound and greatest lower bound.
A one-second correction that may be applied to atomic time to align it more closely with Universal Time, compensating for changes in the Earth's rotation. Since the first correction was made in 1972, all leap seconds have been positive, reflecting the general slowing of the Earth's rotation due to tidal braking. See also UTC.
For more detailed information, including a list of all leap second corrections made to date, see http://tycho.usno.navy.mil/leapsec.html.
Substitutional compression schemes proposed by Jakob Ziv and Abraham Lempel in 1977 and 1978. The schemes work by entering phrases into a dictionary. When a reoccurrence of a particular phrase is found, they output the dictionary index instead of the phrase. The algorithms differ mainly in how they manage the dictionary.
LZ77 compression tracks the last n bytes of data, and when it encounters a previously-seen phrase, it outputs a pair of values corresponding to the position of the phrase in the previously-seen buffer of data, and the length of the phrase. In effect the compressor moves a fixed-size "window" over the data (generally referred to as a "sliding window"), with the position part of the (position, length) pair referring to the position of the phrase in the window.
See J. Ziv and A. Lempel, A Universal Algorithm for Sequential Data Compression, IEEE Transactions on Information Theory, IT-23, No. 3, (1977), 337-343.
The popular archivers arj, lha, zip, and zoo, and the GNU compressor gzip, are variations on LZ77. Lempel-Ziv Welch compression is a variant of LZ78.
(LZW): The algorithm used by the Unix compress command to reduce the size of files, e.g. for archival storage or transmission. LZW was designed by Terry Welch in 1984 for use in hardware for high-performance disk controllers. It is a variant of LZ78, one of the two Lempel-Ziv compression schemes.
The LZW algorithm relies on reoccurrence of byte sequences (strings) in its input. It maintains a table mapping input strings to their associated output codes. The table initially contains mappings for all possible strings of length one. Input is taken one byte at a time to find the longest initial string present in the table. The code for that string is output and then the string is extended with one more input byte, b. A new entry is added to the table mapping the extended string to the next unused code (obtained by incrementing a counter). The process repeats, starting from byte b. The number of bits in an output code, and hence the maximum number of entries in the table is usually fixed and once this limit is reached, no more entries are added.
See A Technique for High Performance Data Compression, Terry A. Welch, IEEE Computer, 17(6) (1984), 8-19.
An open-source operating system originally written by Linus Torvalds, but now the product of an Internet-based collaborative development headed by Torvalds.
Same as fringe rotator.
Time as measured by Universal Time plus the East longitude of the observer..
Practical radio astronomy receivers incorporate frequency conversion because it is technically more convenient to amplify, filter, delay and cross-correlate signals at a fixed intermediate frequency (i.f.) than at their original, changeable (and often higher) radio frequencies. The frequency conversion is done in one or more mixers, in which a non-linear element (usually a diode) forms signals at the sum and difference frequencies between the astronomical signal and a tunable local oscillator (l.o.) signal.
(LST): Time as measured by the right ascension of the hour circle that is currently at an observer's meridian. See also sidereal day.
(LSR): The sun has a systematic motion relative to nearby stars, the mean depending on the spectral type of the stars used for comparison. The standard solar motion is defined to be the average velocity of spectral types A through G as found in general catalogs of radial velocity, regardless of luminosity class. This motion is 19.5 km/s toward 18 hrs right ascension and 30° declination for epoch 1900.0 (galactic co-ordinates l=56°, b=23°). Basic solar motion is the most probable velocity of stars in the solar neighborhood, so it is weighted more heavily by the radial velocities of stars of the most common spectral types (A, gK, dM) in the solar vicinity. In this system, the sun moves at 15.4 km/s toward l=51°, b=23°.
The conventional local standard of rest used for galactic studies is essentially based on the standard solar motion. It assumes the sun to move at the rounded velocity of 20.0 km/s toward 18 hrs right ascension and 30° declination for epoch 1900.0. This choice presumes that the earlier spectral types involved in determining the standard solar motion, being younger, more closely represent the velocity of the interstellar gas.
A computer file on which one or more of the usual types of manipulation, e.g., adding, deleting, moving or renaming, cannot be performed.
In AIPS++, a sequence of informational messages with a priority and a type (both of which may be filtered on). The log is typically attached both to the data being modified and to the user interface (for a history of all operations). Ideally programs may be rerun directly from information contained in the log.
In a planeto-centric co-ordinate system, the angle around the equator measured between a fiducial meridian circle (for the Earth, conventionally that of Greenwich) and that of a point on the planet's surface.
Also used, by analogy, as one co-ordinate in the ecliptic and galactic systems.
(LUT): In computer graphics, the color options in a graphics system, arranged by an index number. Also known as a color map. The lookup table may be allocated either by hardware or by software.
In the Högbom CLEAN algorithm, the fraction m of the largest residual which is used in determining the amplitude (flux density) of a CLEAN component. Convergence can be achieved with m in the range (0,2) but generally a small value, say m = 0.1, is recommended, especially when dealing with extended sources.
See local sidereal time.
See intensity.
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abridle@nrao.edu, 26 August 1996, 15:45 EDT