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Focusing the telescope

Obtain an image of a bright star with binning 4x4: To focus the telescope point the telescope to a bright star (V$\approx$6 mag). Go to the main camera control buttons of the main camera section, select a binning of 4x4 and an exposure time of 5 sec (Fig. 11). Press exposure to obtain an image. The large binning is selected only to speed up read-out of the CCD.

Figure 6: The camera operating in ``focusing mode''.
\includegraphics [width=8cm]{focus.ps}

Set the camera into ``focusing'' mode: Select now a binning 1x1. Move the mouse pointer to the center of the star in the main camera display, press the right mouse button, and select fit frame with the left mouse button. Press the button misc and select focusing with the left mouse button (Figf. 11 and 13).

The camera now makes continuously exposures and displays a subwindow (the fit frame) centered on the star in the main camera display (Fig. 6). A two-dimensional Gauss-Fit is applied to the brightness distribution of the star and is displayed as ellipse in the main camera display. The Gauss-Fit parameters are displayed in a Gauss fit parameter display at the top of the main camera display. Given are background and amplitude, x-position and full width half maximum (FWHM) in x-direction in arcsec, y-position and y-FWHM in arcsec, and offsets from the center position of the first exposure in arcsec.

Set a starting focus value: The focus is set in the control panel of the Telescope section. The focus is changed by moving the sliding button in the set focus section (Fig. 2) with the left mouse button. While the focus motor is operating the background of the slider turns to red. The values are in arbitrary units. Start with focus 235.

Make a focus imaging sequence: Change the focus stepwise, while the camera is in ``focusing mode'' and record the FWHMs displayed. The best focus is the one where the FWHMs are minimized. You may choose step sizes of four. A reasonable FWHM for a well focused telescope is $<$3'' depending on the seeing. With good seeing, you may now fine-tune the focus with step sizes of two.


next up previous contents
Next: Focusing the guiding camera Up: Focusing telescope and guiding Previous: Focusing telescope and guiding   Contents
Dieter Engels
2003-01-15