CONTENTS
Default Form Expanded Form
Focus Telescope
The Focus Telescope widget controls the motion of the telescope's secondary mirror. The default form of the widget (above left) simply reports the current position of the secondary, and the status of its motor (locked or moving). The expanded form of the widget permits the observer to drive the mirror, move it to an absolute position, or move it by increments. Focus motion and telescope moves can be combined to partially automate the focusing procedure.

In the expanded form, the <<, < and >, >> buttons drive the mirror up or down, at a faster or slower rate. Entering a value into the Go to box and pushing the Go button drives the mirror to that position. Entering a value into the Change by box and pushing the adjacent < and > buttons drives the mirror up or down by that increment.

The lower part of the the expanded widget combines secondary and telescope moves. This is very useful when focusing for direct imaging, the usual method being to expose the science CCD for a short time on a star or field of stars, pause the exposure, change the focus by a small increment (typically 3-5 units), move the telescope a small distance (typically 20 arcseconds), resume the exposure, pause again, and repeat, hopefully passing through the best focus in doing so. The resulting CCD image -- a row or rows of stars at different focus settings, separated by the distance of the telescope move -- can then be measured to determine best focus.

To use this feature, enter the focus increment in the Increment Focus By box, select a direction for the telescope move from the N S E W radio buttons, and enter a size, in arcseconds, for the telescope in the Move Telescope by box. While paused during the exposure, press the Go button to move the secondary mirror and telescope simultaneously. Wait for the yellow "Moving to Target" panel on the Main Window to turn green and display "On Target", and for the secondary position to display "LOCKED" on the focus widget, before resuming the exposure.

Because of backlash in the secondary mirror mechanism, one should always set the focus from the same direction. Before beginning a focus run, drive the secondary to a position at least ten units below your start position, then move up to the start position. Always move monotonically upward when doing a focus run. When you have determined best focus, drive the secondary below that position and come back up to it.

N.B. To avoid confusion, mark the direction of the focusing run on the CCD by increasing the telescope motion (e.g. from 20 to 30 arcseconds) before the final move.