What is DestinationSideOfPier and why would I want to use it?
The DestinationSideOfPier property is provided for
applications to manage pier flipping during automated image sequences. Basically
you provide it with an RA and Dec, and it comes back telling you the pointing
state SideOfPier that would result from a slew-to at the
present time. Looking at the current SideOfPier and DestinationSideOfPier tells
you if the mount would flip on a slew to those coordinates. This info is based
on the given RA/Dec at the given time, so is not a static function.
The mount knows where all of its settings are, how they are applied, and what their effects are. All it needs to do is tell the app the outcome of a slew to a point. Obviously if trash RA/Dec are given the mount would raise an exception for invalid coordinates.
As your image sequence progresses, at the beginning of each image you add the exposure interval to the RA (RA is a time coordinate, right?) and if you’re really picky adjust by the 0.27% difference from sidereal to solar time, then call DestinationSideOfPier(RA + image, Dec). If it tells you the flip point will be reached before the end of the exposure, then you have some choices to make:
Will the mount track past the flip point far enough to allow the image to proceed “from here” and complete, so you could do the flip at the end while the image downloads?
If the mount is hard limited at the flip point then you would have to wait until the target drifts past the flip point, flip, then proceed. Not many mounts are hard limited against tracking past their flip points.
The tricky parts are
For #1 above, knowing whether, and how far, the mount can track past its flip point. Most German mounts can track at least one “typical” exposure interval past their flip points. In the old days this would be 1800 seconds for grungy CCDs with bad read noise and a narrowband filter, but nowadays, especially with CMOS, even narrowband exposures are significantly shorter. Even at the celestial equator, 1800 seconds is only 7.5 degrees, and less as declination increases (by cos(dec)). Tracking 7.5 degrees or less past a flip point seems within the capability of most GEMs. Also, if you can image past the flip point, you can download the image in parallel with flipping the mount, so the penalty for flipping is the flip time minus the image download time.
For #2 above, how long to wait before flipping? To handle this, stop tracking for safety, then periodically call
DestinationSideOfPier()for your target’s coordinates while the target itself drifts towards, then past, the flip point (which you don’t know but who cares?). Wait until it tells you that the mount will flip Turn on tracking, slew to your target, the mount will flip, and off you go toward the west with your image sequence.