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 1||Figure 2|
[Last Updated: 2003 March 4, by M. Skrutskie.]