III. 2MASS Facilities and Operations

2. Data Acquisition

b. Survey Strategy

Tile Definitions

The sky has been divided into 59,650 unique "tiles" for 2MASS operations. The right ascension width of a tile is 504", set by the intersection of the sky coverage of each of the three arrays. The declination length of each tile is 6° plus one full frame (8.5') to provide overlap between tiles in adjacent declination "bands". Tiles observed from the northern observatory have this declination overlap located on the southern end of the tile while tiles observed from the southern observatory include overlap on the northern end of the tile. This results in a double overlap at the boundary between tiles observed from the northern site and those observed from the southern site.

Tile (RA) overlap includes: a 51".2 overlap as required by galaxy coverage, a 10" pointing error compensation, a 6" compensation for processing requirements, and a variable precession compensation (up to 5".6 for the 0° declination band, 1" for the 6° declination band, and 0" for all other declination bands).

Each survey tile consists of 273 frames with a step size of 82".6 at the northern site and 274 frames with a step size of 82".3 at the southern site. The slight difference in step size results from the very small plate scale differences between the two optical systems. The number of frames includes the overhead needed at each end to get a full 6 frames deep sky coverage of 6° in declination plus 1 frame of overlap with adjacent tiles in the declination direction.

The rotation and platescale of each array are given below:

North South
BandPlate ScaleRotationPlate ScaleRotation
The optimal rotation and resulting step sizes result in a well-spaced sub-pixel sampling for each wavelength.

Tile Numbering

Tiles are described by their declination band (as defined by the equatorward edge of the tile ignoring tile overlaps, e.g. +0, +6, +12,... and -0, -6, -12, ...) and then by the RA of their western equatorward corner. Tiles observed by the northern observatory are numbered in the following fashion: starting with tile 0 which is located at 0h00m00s RA, +0 DEC, tiles are numbered with increasing RA and then with increasing DEC up to tile 29824 at 23h52m55".9 RA, +84 DEC. Tiles in the negative declination bands are numbered similarly starting with tile 100000 at 0h00m00s, -0 and ending with tile 129824 at 23h52m55.9s RA, -84 DEC.

Tiles observed from the southern observatory are numbered in the same way as those observed from the north, but have a value of 200000 added to the tile number. This numbering scheme is designed to allow the same region of sky to be observed by both the northern and southern observatories as needed without conflict.

To determine in which tile a given coordinate falls, use the on-line utility CoordSearch.

Sky Coverage Boundaries

The northern survey has begun observations of the +12 declination band and north. The southern survey has begun observations of the -0 declination band and south. The declination boundary between observations from the two observatories will be determined closer to the end of the survey.

Time Requirements

The integration time for a frame includes: two 50 ms resets (one occurs during the secondary flyback period, the second starts as the secondary starts scanning), one 51 ms R1 integration, one 1.3 sec R2 integration. An additional 5 ms of padding is added to allow for the trigger pulse and to center the integration period on the scanning ramp. The total dwell time on the sky for a frame is thus 1.455 sec.

A 273 frame survey scan takes 6.97 min (including overhead). Allowing for telescope slew time, an average survey tile takes 7.05 min. A full calibration observation consisting of six calibration tiles takes 10 min (including overhead and telescope slew time).

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