What is drift scanning?

SPACEWATCH® pioneered the use of "Drift-Scanning" with CCD's as a way to maximize the amount of data they could collect with their telescope. Stars and asteroids, just like the Sun and Moon, are affected by the Earth's rotation. They rise in the East, move overhead, and set in the West over the course of a day. Most telescopes will move to whatever target they wish to observe, use the telescope's motors to follow the target, take a picture with a CCD and then read the picture out so it can be examined. The problem with this method was that for most CCD's in the 20th century the read time was longer than the exposure time and so only half (or even less) of the wall clock time was spent actually observing the night sky.

SPACEWATCH®, under Tom Gehrels, pioneered an alternate way to use a CCD camera that avoids this problem. Instead of going to the target of interest you lead it a little bit on the sky then you shut the telescope drive off. The stars start to move ("drift") across the CCD. If you exposed the CCD and then read it off they would appear trailed along the direction of the sky's motion since you didn't use motors to track them. However, Tom lined up the CCD with the sky and then started the CCD reading at exactly the rate at which the sky was moving. The stars therefore appeared round and the resulting picture was really long without any read time wasted. You didn't get something for nothing, however, since the first bit of the data (the "ramp frame") was hard to use and the exposure time was fixed for the instrument. Drift scanning was a solution to SPACEWATCH®'s survey needs between 1984 and 2011.  Nowadays our CCD imagers read out rapidly and we no longer need to use drift scanning.