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 mirror
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 to 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.
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.
Last modified: Tue May 19 12:52:39 PDT 2009