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Tracking Past the Meridian
Unlike other German Equatorial mounts, Astro-Physics GEMs do not require a flip at the meridian in order
to follow an object in its nightly journey across the sky. Tracking through the meridian is a unique function of
AP mounts. Most objects that are located south of the zenith (northern hemisphere) can be imaged without
interruption from horizon to horizon. The mount will happily track with the counterweights high in the air.
A visual observer will not be overly inconvenienced by the mechanics and delay of the flip; however, this becomes
much more complicated for an imager! Here is the normal sequence of events other mount owners must do
when their mount flips during an exposure: stop the imaging session (often with the loss of a 20 to 30 minute sub-
exposure that has reached 19 or 29 minutes), flip the mount and reposition cables, rotate the camera so that you
can use the same guide star, re-center the object to your desired composition, re-acquire the guide star and then
recalibrate it, begin the new imaging session. Aren't you glad you don't have to flip?
Remember, it is the GoTo slew that will flip the mount...not tracking!
Please note that with this convenient feature comes the responsibility of preventing your scope and camera
from striking the pier when imaging an overhead object or one more northerly. Fortunately, APCC software that
safeguards against this mishap! A custom contoured Meridian Limits outline can be created with APCC that
keeps everything safe and sound. You may wish to take a little extra time to set this up.
Meridian Delay or Meridian Advance
The Meridian Limits (Delay or Advance) are one of the most powerful and useful features of Astro-Physics
mounts. They open up the possibility of imaging on both sides of the meridian WITHOUT wasting time on a pier
flip at declinations where the scope will not strike the pier. The limits, when properly configured, allow you to
track safely past the meridian up to the limits you set based on your unique instrument package. In addition, they
allow you to begin an image ahead of the meridian by safely slewing you into a counterweight-up position to start
an imaging series that can then simply continue through the meridian. However, like any powerful feature, you
MUST understand how the meridian limits work. It is vital that you understand what they CANNOT do as well as
what they can do!
The AP V2 ASCOM Driver and the mount's keypad each have the ability to set a meridian delay which controls
the flip point of the mount. These are hour angles that follow the full north-to-south meridian line, as opposed
to the custom contoured meridian flip lines that are possible in APCC. The advantage is that they can be set up
quickly, though one must take care to be sure that your scope will clear the pier when changing the timing of the
flip.
APCC adds some powerful capabilities. Three of the features outlined in bullet points below apply to West
Meridian Limits. The West Limits come into play when a scope's counterweight is in the up position (scope under
the mount) and the scope is pointing west of the meridian. This normally occurs when the scope is allowed to
track past the meridian. The fourth feature below applies to East Meridian Limits and provides a unique and
powerful tool for safely starting images in the east from the east side (counterweight up) using the meridian
advance.
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Instead of just a single meridian limit for all declinations, you can configure an arbitrary number of different
limits at different declinations. This is useful because at each declination there can be a different hour angle
at which the telescope will touch the pier. In declination regions where the telescope can theoretically go
the full 6 hours past in the west, the meridian limits can establish horizon tracking limits for the practical
distance past the meridian where you can reasonably continue to work. Likewise, in the east, they can set
the practical horizon ahead of the meridian where you can reasonably start an image.
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When a meridian tracking limit is reached, tracking can be automatically stopped, the scope can be flipped,
or the scope can be parked.
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Meridian tracking limits can also be used to create a safe zone around your pier if it interferes with reaching
the zenith
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Remember that an APCC Meridian Limits safe zone that does not allow the scope to reach the
Meridian must be stopped or parked. It cannot be flipped.
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When East Limits are properly configured and employed, slews to targets within the east limits will
