The 2MASS arrays imaged the sky while the telescope scanned continuously in declination at a rate of approximately one arcminute declination per second of time.  The 2MASS data "tiles" are 6 degrees long in the declination direction and one camera frame (8.5 arcminutes) wide in the east-west direction. While the entire telescope scanned in the declination direction the telescope's articulated secondary mirror tilted opposite the scan direction to momentarily freeze the focal plane image. At the end of a 1.3 second exposure the secondary moved quickly back to its start position to freeze a new piece of sky slightly displaced from the previous frame. The dead time between frames used for secondary and array reset is less than 1/10 of a second. 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 sky mapping technique was suggested by Frank Low.

 Click on this icon to see a movie of a scan through M92.      

The camera field-of-view shifted by approximately 1/6 of a frame in declination from frame-to-frame. This figure 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 seconds. The scan rate (and thus the frame-to-frame declination offset) and array orientation were set so that each of the six apparations of a given star occur at a different location relative to a pixel center. This sub-pixel "dithering" improves the ultimate spatial resolution of the final co-added images relative to a single undersampled image taken with 2.0" pixels. Before coadding,  the raw images were repixilated into 1 arc-second pixels using an algorithm suggested by Martin Weinberg. The image below compares a single survey frame with the final co-added image product.

Unprocessed raw single frame image.	Coadded repixilated image.

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