Meter-class telescopes, such as ESA's own Optical Ground Station in Tenerife, are usually capable of observing objects as faint as magnitude 22 or so. This is the typical limiting magnitude that can be reached in less than an hour of exposure time, under dark and clear conditions.

However, it is of course possible to push the capabilities of these systems to much fainter magnitudes, if we can devote sufficient time to a single observation. This is what we recently did with the 0.8-metre Schmidt telescope at Calar Alto, when we tried to observe an object that was predicted to have a visual magnitude of approximately 23.5.

We obtained about 200 images of the asteroid's location, 90 seconds each, from the time it rose above an altitude of 30° in the evening, to when it set below the same limit in the morning. We then combined them together in a stack, using the known motion of the target asteroid.

The result was a clear, albeit faint, detection of the object. When the detected source was measured photometrically, we discovered that the object was actually fainter, with a measured visual magnitude of about 24.2. This observation was also obtained under poorer-than-average seeing conditions, approximately 2.5". On a night with a better seeing it should be easily possible to reach an even fainter limiting magnitude.

This unusual mode of operation for a small-class telescope could offer interesting follow-up opportunities for faint objects, that are typically only observed with larger professional facilities.

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Caption: Plain stack of the images (left), and the same stack processed to enhance the target and remove background sources (right). The object is visible at the centre of the red box. Stacks produced using Tycho. Credits: ESA NEOCC