Tracking and focus

In astrophotography long exposures require high precision tracking over longer periods of time. Because of small inaccuracies (periodic variations in the gay, we, small inaccuracies of the telescope mount adjustment) tracking has to be continuously corrected. This is accomplished by a so-called auto-guiding system, which consists of a small computer and a camera that is attached toa a guiding scope. The camera and the computer ensure that a guide star in the field of view of the guiding scope is continuously kept at a fixed location with high precision. To this end, the computer sends short-term correction signals to both stepping motors of the tracking system.

Left: guiding system of the telescope on the Skywatcher EQ6 SynScan mount for both axes (right ascension, declination). On the left, the hand control for the basic tracking is shown, further down on the right (in front of the power supply) the small control box for the auto-guiding camera MGEN Lacerta is shown.

Canon EOS 1100Da (left), finder scope (D = 50 mm, f = 208 mm, middle), guiding scope (D = 60 mm, f = 240 mm) with Lacerta autoguiding camera (right).

Focusing is achieved with a motor. This implies that one need not touch the telescope while focusing, thereby avoiding inevitable oscillations of the telescope. Together with the use of a Bahtinov mask, which is attached in front of the aperture during focusing, optimum focus is achieved within a short time.

Above: Bahtinov mask. Right: Defocused (top) and in-focus (bottom) diffraction image of a star produced by the Bahtinov mask (Liveview on the display of the Canon 1100Da).

Motorised focus at the Maksutov-Newton telescope. The hand control allows fast and fine adjustment of the focus in both directions.

Autoguiding is accomplished by a special camera, which is attached to a guiding scope. This ensures that the telescope precisely tracks the target in the rotating sky. Deviations in tracking result from mechanical inaccuracies (e.g. deviations in the slope of the worms give rise to periodic oscillations). The MGEN Lacerta system keeps a guiding star in the centre of the window with high precision (ca. 1/30 Pixel, or 0.2"). The corrections in Right Ascension and Declination are applied with a defined time interval. At the same time, the system also controls the exposures taken with the Canon EOS 1100Da.

MGEN guide star

Profile of focussed guide star

Drifts during autoguiding

Top: RA

Bottom: DEC

Meanwhile, two Canon EOS cameras can be controlled simultaneously with the MGEN Lacerta. One of them is attached to the Maksutov/Newton, the other one to the ED refractor. The picture shows the Y-bifurcation of the control cable.