Date: Tue, 17 Aug 1999 09:46:28 -0700
From: Russell E Owen
Subject: focus, followup
In my last message about focus I mentioned moving the secondary (by
adjusting the vanes) to optimize focus and also mentioned open loop
focus correction, but I did not discuss the connection.
With open loop focus correction it becomes easy to measure the
difference in focus between the different instruments. The difference
is approximately a constant. If the values prove very different then
it may be worth adjusting one or more instruments to bring them
closer in line (it is probably fairly easy to change the engineering
camera).
Having an estimate for focus vs temperature (and altitude, if
possible!) and for the difference between different instruments will
give us a much better idea of where to place the secondary to
accomodate focus for all instruments. (Of course it will also tend to
make focus much more stable over time, simplifying operations, which
is the main advantage to predicting focus.)
-- Russell