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framed on the imaging chip, just as you set it up before. With the software properly calibrated for
that orientation of the guide camera, it will often be able to accurately autoguide on the fainter
guide star, even though it was too faint for calibration.
However, if the guide star and the calibration star are at significantly different declinations, you
will have to compensate for this. This is because stars near the celestial equator move faster than
stars near the poles and therefore require larger correction movements. The autoguiding software
usually has a provision for this correction. There is a window in which you should enter the
declination of the star upon which you calibrate. When you are autoguide on a different star, you
simply change the entry to the declination of the actual guide star and the software will make the
appropriate adjustment to the magnitude of correction movements in each axis. This is simple
with the Compustar, which gives a continuous large read out of its declination. You can continue
to acquire new guide stars all over the sky and use the same calibration, provided you don‟t
change the orientation of the guiding camera (or off-axis guider) and you enter the declination of
each new guide star. This is particularly useful for separate guide scopes in which the orientation
of the guide scope and the guiding camera can certainly be kept the same between all imaging
targets. One could simply calibrate on a star on the celestial equator and enter the value of zero
for the declination, then enter the declination of each subsequent guide star. So long as the
orientation of the autoguider camera is never changed in the guide scope, the autoguider does not
need to be recalibrated. This approach is not practical with off-axis guiders, for which the
orientation is likely to be different with every new subject. However, because the goal is accurate
autoguiding, much can be said for re-calibrating on each new guide star for each new image even
with a separate guide scope. It only takes a few minutes.
Autoguider Adjustment Settings
You will have to adjust a number of settings for the actual autoguiding session. These are usually
listed on the camera control menu, or a separate menu. One of the most important is the
aggressiveness setting. This tells the software how aggressive to be when making corrections.
Typically, the aggressiveness is graded on a scale of 1-10. If you select 10, then 100% of the
guiding error will be corrected with each movement. While this sounds like a temptingly good
idea, it generally is not. Imagine if your house thermostat was made with 100% aggressiveness,
which also sounds like a good idea. If it was and you set the thermostat to 70 degrees, the
furnace would come on and heat the house to 70 degrees, at which point it would shut off.
However, as soon as the temperature in the house fell to 69.9 degrees, the furnace would be re-
activated, but only for a few seconds until the temperature was again raised to 70 degrees, at
which point it would shut off again. But as soon as the temperature dropped to 69.9, the furnace
would again be reactivated, and so on. Obviously, this would result in a rapid fire sequence of
the furnace being turned on and off. Fortunately, thermostats are not made with this degree of
aggressiveness. Rather, they are made with little bit of slop or error in the system. In other
words, the furnace will shut off when it reaches 70 degrees, but not be reactivated until the of 67,
before being reactivated. This results in much smoother operation of the system.
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