III. 2MASS Facilities and Operations


2. Data Acquisition

b. Survey Strategy

i. Tile Definitions

The sky was divided into 59,650 unique "tiles" for 2MASS operations (59,731 tiles appear in the scan database because 81 "gap-fill" tiles were defined to recover missed sky due to bad pointings). The right ascension width of a tile was 504, set by the intersection of the sky coverage of each of the three slightly offset arrays. Tiles were defined in 15 declination bands between the equator and each pole (0° - 6°, 6° - 12°, 12° - 18°, etc., and similarly in the Southern Hemisphere). The declination length of each tile was 6 plus one full frame (8.5) to provide overlap between tiles in adjacent declination "bands". Tiles in the Northern celestial hemisphere had this declination overlap located on the southern end of the tile, while tiles in the Southern celestial hemisphere were observed to include overlap on the northern end of the tile. This arrangement resulted in a double overlap at the celestial equator. Right ascension tile overlaps were slightly larger than 10% of a frame width to account for possible pointing errors, precession, and the requirement that a source be a few arcseconds onto the chip to obtain a valid flux.

The cameras acquired 273 images of each survey tile in overlapping frames (see Sec IV.4.b Figure 1) with a declination step size between frames 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 different cameras in each system and the need to obtain precise sub-pixel offsets in each of the six apparitions of a star. All arrays were rotated slightly from the scan direction, so that a star would cross one or two columns in its six apparitions, providing, once again, precise sub-pixel positioning of each star image. The 273/4 frames includes additional frames at the beginning and end of each tile needed to obtain a full 6 frames deep sky coverage within the actual tile boundary, plus 8.5´ of six-frame-deep overlap with adjacent tiles in the declination direction.

The measured rotation angle and platescales of each array are given below. North1 refers to the period before H-band array replacement in the northern camera in 1999 August:

Array rotation angles
(degrees relative to North-South)
J
H
Ks
North1 0.372+/-.020 0.440+/-.0220.326+/-.019
North2 -0.303+/-.015 -0.420+/-.0160.265+/-.015
South 0.225+/-.006 0.252+/-.0050.175+/-.005

Array platescales
(arcseconds per pixel)
J
H
Ks
North1 and 2 1.997 2.006 1.984
South 1.989 1.992 1.980

The rotations, platescales, and step sizes combined to produce optimal sub-pixel sampling for each wavelength.

ii. 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 RA=0h00m00s, DEC=+0°, tiles are numbered with increasing RA and then with increasing DEC, up to tile 29824 at RA=23h52m55".9, DEC=+84°. Tiles in the negative declination bands are numbered similarly, starting with tile 100000 at RA=0h00m00s, DEC=-0°, and ending with tile 129824 at RA=23h52m55.9s, DEC=-84°.

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.
Tile Numbers by Dec Band
Dec
band
North
Min #
North
Max #
Max
Dec
Min
Dec
South
Min #
South
Max #
Tiles
in Band
Index
First Tile
1 29514 29824 84 90 229514 229824 311 1
2 28897 29513 78 84 228897 229513 617 312
3 27980 28896 72 78 227980 228896 917 929
4 26773 27979 66 72 226773 227979 1207 1846
5 25289 26772 60 66 225289 226772 1484 3053
6 23545 25288 54 60 223545 225288 1744 4537
7 21559 23544 48 54 221559 223544 1986 6281
8 19354 21558 42 48 219354 221558 2205 8267
9 16953 19353 36 42 216953 219353 2401 10472
10 14383 16952 30 36 214383 216952 2570 12873
11 11672 14382 24 30 211672 214382 2711 15443
12 8850 11671 18 24 208850 211671 2822 18154
13 5947 8849 12 18 205947 208849 2903 20976
14 2989 5946 6 12 202989 205946 2958 23879
15 0 2988 0 6 200000 202988 2989 26837
16 100000 102988 -6 0 300000 302988 2989 29826
17 102989 105946 -12 -6 302989 305946 2958 32815
18 105947 108849 -18 -12 305947 308849 2903 35773
19 108850 111671 -24 -18 308850 311671 2822 38676
20 111672 114382 -30 -24 311672 314382 2711 41498
21 114383 116952 -36 -30 314383 316952 2570 44209
22 116953 119353 -42 -36 316953 319353 2401 46779
23 119354 121558 -48 -42 319354 321558 2205 49180
24 121559 123544 -54 -48 321559 323544 1986 51385
25 123545 125288 -60 -54 323545 325288 1744 53371
26 125289 126772 -66 -60 325289 326772 1484 55115
27 126773 127979 -72 -66 326773 327979 1207 56599
28 127980 128896 -78 -72 327980 328896 917 57806
29 128897 129513 -84 -78 328897 329513 617 58723
30 129514 129824 -90 -84 329514 329824 311 59340

iii. Sky Coverage Boundaries

The northern survey began observations of the +12 declination band and initially worked northward toward the pole. The southern survey began observations of the -0 declination band and initially worked southward. The tiles between declination +0° and +12° were enabled for observation only toward the end of survey observations. The final declination boundary between coverage from the two observatories was determined by optimal dynamic scheduling of tiles in the last months of Survey observations, with the final result illustrated in Figure 1. Figure 2 is a "movie," showing the Survey's coverage week by week. At intervals coverage appears to "regress," when previously observed tiles were queued for re-observation, due to their inadequate quality.

Figure 1Figure 2

iv. Time Requirements

The integration time for a frame included: two 51 ms resets (one occurred during the secondary flyback period, the second started as the secondary began scanning), one 51 ms "Read_1" (R1) integration, and one 1.3 s "Read_2" (R2) integration. An additional 5 ms of delay was added to allow for overhead and settling. The total dwell time on the sky for a single frame is thus 1.455 s.

A 273 frame survey scan required approximately 7 minutes (including overhead). Calibration observations consisted of six repeated scans of 42 consecutive frames requiring one minute each. Each set of six calibration tiles required approximately 8 minutes of observing time, including slewing and overhead.

[Last Update: 2003 March 5, by M. Skrutskie]


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