VLA-A Array AL218 Texas Survey Source Snapshots ("AL218")

The VLA-A Texas Survey consists of a sample of objects extracted in 1989 by Ray Lucas and Kenneth Chambers from the earlier Texas Interferometer 365 MHz Survey of radio sources covering a strip of sky from approximately -35.5 degrees declination to +71.5 degrees declination, and complete to flux densities of 0.25 Jy, with positional accuracies of ~1 arcsecond in RA and DEC. The Lucas and Chambers sample from 1989, which comprises the VLA-A Texas Survey is a subset of 71 sources drawn from the area of one optical Schmidt sky survey plate (covering ~6.5x6.5 degrees), Region S861, centered at approximately RA=190.640822109, DEC=-0.273834224277 (J2000), from the UK Schmidt SRC-J Survey. The Region S861 was initially chosen because it represented the combination of the deepest UK Schmidt plate material (the best optical survey material available at the time of our sample definition in 1989) and the highest galactic latitude, thereby emphasizing the extragalactic nature of the survey and also maximizing the likelihood of having more optical detections. Much more recently, the area of this plate has been covered by a number of important sky surveys including 2MASS, NRAO VLA FIRST, and the Sloan Digital Sky Survey (SDSS). In particular, the availability of the Sloan Survey data provides 5-band ugriz color information at optical wavelengths, to a depth of g,r=22.2.

VLA-A Texas巡天（VLA-A Texas Survey）由雷·卢卡斯（Ray Lucas）与肯尼斯·钱伯斯（Kenneth Chambers）于1989年从早期德州干涉仪365MHz射电源巡天中提取的天体样本构成。该早期射电源巡天覆盖赤纬约-35.5°至+71.5°的天区，流量密度完备性达0.25央斯基（Jy），赤经（RA）与赤纬（DEC）的位置精度约为1角秒。
构成VLA-A Texas巡天的1989年卢卡斯与钱伯斯样本，是从英国施密特SRC-J巡天（UK Schmidt SRC-J Survey）的一块光学施密特巡天底片（覆盖约6.5°×6.5°天区，编号S861天区）中选取的71个射电源子集，该天区中心坐标为J2000坐标系下RA=190.640822109、DEC=-0.273834224277。
选择S861天区的最初原因，在于其兼具1989年样本定义阶段可用的最深施密特光学巡天资料，以及最高的银纬；这一选择既突出了本次巡天的河外天体属性，同时也最大化了获得更多光学探测信号的可能性。
近年来，该底片覆盖的天区已被多项重要天区巡天覆盖，包括2微米全天巡天（2MASS）、美国国家射电天文台VLA FIRST巡天（NRAO VLA FIRST）以及斯隆数字巡天（SDSS）。其中斯隆巡天的数据提供了光学波段的5波段ugriz测光颜色信息，g、r波段的探测深度可达22.2星等。