III. 2MASS Facilities and Operations

2. Data Acquisition

a. Scanning Strategy

The 2MASS arrays imaged the sky while the telescope scanned smoothly in declination at a rate of 57 per second. The telescope scans were designed to cover pre-defined "Tiles" 6 long in the declination direction and one camera frame (8.5) wide in right ascension. While the entire telescope scanned in the declination direction at a constant right ascension, the telescope's secondary mirror tilted opposite the scan direction and momentarily froze the focal plane image. At the end of each Reset-Read-Read cycle described in Section III.1.b, the secondary flew back to its start position to freeze a new piece of sky displaced by about one-sixth of a frame from the previous frame. The dead-time between frames was less than 0.1 sec, and was used for secondary flyback and array reset. When accounting for this dead-time and the time to point the telescope and initiate a scan, the 2MASS observing system integrated on sky approximately 84% of each night. This movie shows several consecutive frames from a scan through the globular cluster M92.

The camera field-of-view shifted by approximately one-sixth of a frame in declination from frame-to-frame. Figure 1 illustrates the relationship between individual camera frames and survey Tiles. The camera imaged each point on the sky six times for a total integration time of 7.8 sec. The scan rate (and, thus, the frame-to-frame declination offset) and array orientation were set so that each of the six apparitions of a given star occured at a different location relative to a pixel center. This sub-pixel "dithering" improved the ultimate spatial resolution of the final coadded Atlas Images relative to a single undersampled image taken with 2.0 pixels. Figure 2 compares a single survey frame with the Atlas Image product.

At the end of a 6-long scan the telescope shifted position by 90% of a frame width in right ascension and began another scan. Thus, all 2MASS Tiles overlap by 10% in right ascension (approximately 50), and data from this overlap region are used to monitor the photometric consistency of the survey from scan to scan. 2MASS Tiles are slightly longer than 6, to provide for a full 8.5 frame overlap in the equatorward direction between declination bands. Since the 2MASS Tiles are located on a sphere the amount of overlap varied between the North and South ends of each 6° Tile. The telescope was always commanded to a position which would provide a minimum of 10% overlap at any point in a scan.

Figure 1Figure 2

[Last Updated: 2003 March 4, by M. Skrutskie.]

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