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The Role of Time and Location Data in the Servo System
There seems to be an obsession among astronomers with accurate time and location data. Many of our customers use
Dimension4
™
or some other similar time server to keep their computer clock accurate to the millisecond. GPS units
are employed to precisely nail down time and location data to the second. However, for most of your mount's operation,
accurate time and location are really somewhat irrelevant. You don't want to be way off, but you probably don't need to be
dead-on either. Here are some thoughts on time and location as it applies to the Astro-Physics servo system.
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The obsession with time and location accuracy probably comes from people who started with Alt/Az mounts. When a
mount is set up as an Alt/Az mount, it must convert its native Alt/Az system to RA/Dec for celestial viewing. To make
this conversion, the more accurate the time and location data, the more accurate the conversion. Time and location
are critical to the Alt/Az astronomical mount.
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Time and location come into play during initialization as discussed above. They are necessary for calculating the
current RA value at start-up. Precise time at power-off followed by precise time at the following initialization will lead
to a more accurate calculation of the RA However, think about it: How important is to-the-second accuracy of your first
GoTo of the evening? If your first target is off by 30 seconds of RA, or even a minute, won't it still be within the field of
view? After centering and doing a ReCal, the time relevance is over.
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Time and location are used to determine the meridian (aka z-value, LST, overhead hour, etc.). They are also used
to then determine the horizons. Again, the more accurate time and location are, the more accurate the meridian and
horizon values will be. As above, the question to ask is how important is it if the calculated meridian or horizon is off
by a little bit. You would probably never know it during a normal observing or imaging run.
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Time and location accuracy is VERY important for satellite tracking, and for astrometry. If you are using APCC's
Horizons program, or you are doing research on NEOs, by all means, use a time server and GPS!!
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Once you are calibrated on a star, apart from determining the flip-point, precise time and location play almost NO
ROLE WHATSOEVER in normal GoTo slews or pointing accuracy.
Many client software programs like planetarium programs and observatory control programs will demand that the mount
time matches the computer clock. This is necessary for coordination among the programs. However, if you are away from
an internet connection or GPS unit, and your computer clock is off from the atomic time in Boulder, CO by a little bit, you
really shouldn't worry about it.
Safety Slew Logic
Safety Slew Logic is designed into the GTOCP4 and GTOCP5 firmware and functions with all of our GoTo mounts. Its
purpose is to protect your scope from a possible crash into the pier when slewing out of some counterweight-up positions.
Our mounts have the unique ability to track past the meridian while imaging, such that the counterweight will move
higher than the scope (counterweight up). Normally when a GoTo slew is made, both the RA and the Dec axes begin
moving simultaneously. However, there are some counterweight-up areas from which a slew with the Dec axis moving
simultaneously with the RA axis would cause the scope to hit the pier before the RA axis can take it safely into a
counterweight-down position.
The Safety Slew Logic knows when the mount is in a counterweight-up position and will initiate an RA-only slew to bring
the mount back into the counterweight-down safe zone where it will pause momentarily. The mount will then automatically
make a second slew using both axes simultaneously to continue to your object.
Conversely, when using Meridian Delay to begin with a counterweight-up imaging session, the reverse happens. A dual-
axis slew will take place until the meridian is reached where the RA will stop and the Dec axis will continue to its position.
When the Dec has reached its position and stops, the RA will make its second slew to the final position.
It is even possible that a three-part slew may take place if you are slewing from a counterweight-up position and into
another counterweight-up position.
Having this firmware logic in the GTOCP5 control box means that it will work regardless of whether you are using computer
software or the keypad to control your mount.
