Below are the results of recent measurements of the best focus values for
NSFCam as a function of inside dome temperature. The values were acquired
over 1.5 months (Jan - March 2000) when the tip/tilt secondary was mounted.
The relations below can be used to get a quick estimate of the focus and
as a starting point for a focus sequence. The parameter T
is the inside dome temperature in degrees C. These relations are applicable
only when the tip/tilt secondary is mounted.
0.15 arcsec plate scale: Focus value = -2.71 + 0.11*T
0.30 arcsec plate scale: Focus value = -3.05 + 0.11*T
To help observers estimate the focus as the temperature changes, there is a
GUI avalable that can be started up by typing focus.tcl . The default
values should put NSFCam fairly close to focus. Once the observer has
focussed, the values of the reference focus setting (-2.71 and -3.05 in the
relations given above) and the dome temperature can be
entered on the first 2 lines of the GUI. The focus change as a function of
temperature, df/dT, should not be changed from the default value of 0.11.
Any time the dome temperature changes significantly, the value can be
entered on the fourth line of the GUI (Current Temperature (deg C) - printed
in blue). Hitting a return will automatically display the Estimated
Current Focus (printed in red).
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