Recovering of comet Catalina with the Schmidt telescope
CAHA/DICYT Solar System objects like asteroids and comets appear like moving targets in the stellar fields observed with telescopes. Observations spread over various nights allow us to compute accurately their orbits. Comets, in particular, can present elongated (elliptic), periodic orbits, like the famous comet Halley, passing close to the Earth every 76 years or so, and visible to the naked eye (the last time in 1986, but it was probably observed since prehistoric times).
Comets are usually no more detectable, even with the largest telescopes, when they are located close to their aphelion, the farthest point of their orbit from the Sun. However, they can be recovered once approaching again from Earth, using previously calculated ephemerids, if accurate enough.
Using the Calar Alto Schmidt telescope in remote, Erwin Schwab, an amateur astronomer expert in Solar System objects, was able to recover a previously known comet, P/2011 CR42 Catalina. Remarkably, this small comet was discovered in 2011 from another Schmidt telescope, based in California (UA- and NASA-funded Catalina Sky Survey). But, after passing close from Earth in 2011/early 2012, it “vanished” from our view while reaching distances nearly as far as Jupiter. With an orbital period of only 6.6 years, it was expected to be visible again in 2018.
Indeed, after observing on the nights of April 17th and 18th the area of the sky where Catalina was expected to lie (according to the NASA-JPL ephemerides), Erwin Schwab detected a faint moving object in the border of the Schmidt field of view, about 6 arcminutes (a fifth of the Moon apparent diameter) from the expected position. Very likely, it was the comet!
However, since the moving object was very faint (21st magnitude) and showed no coma or tail, typical of comets, the astronomers (E. Schwab, D. Koschny and M. Micheli) had to recalculate a new orbit determination, performing accurate astrometry with the Astrometrica software, to confirm the recovery of P/2011 CR42 Catalina. This step is required before sending the data to the official body of asteroids and comets, the Minor Planet Center of the International Astronomical Union, which acknowledged the recovery by releasing a circular.
Catalina will have its closest approach to the Sun on 22 June 2018, and may then show some sign of activity, before disappearing again from our view for the next five years or so.
Note that many other asteroids/comets were recovered by Erwin Schwab using the CAHA Schmidt telescope since 2016 (after an agreement with ESA), as listed in his website.