Contents - Index

Meridian Flip

Automatic meridian flipping essentially uses software to turn an equatorial mount into a fork mount functional equivalent.  This is achieved by allowing the telescope to track a target up to the meridian, stop the exposure, move the telescope to the other side of the meridian and then continue tracking.  If a rotator is employed, the camera is rotated by 180°, the guide star is reacquired and guide calibration is appropriately adjusted.  All this is accomplished automatically.  Automatic meridian flipping depends on plate solving and proper settings on the Settings page.

  • Enable Meridian Flip: Checking this enables the telescope to track through the meridian when going from east of the meridian.  If checked and the target is west of the meridian, it will of course have no effect.  For most users, this option can be left checked.  Uncheck it if you are using a fork mount or an equatorial mount that is capable of tracking through the meridian without damage.
  • Rotate on Flip: If checked and a rotator is connected to CCDAutoPilot, the meridian crossing event will also trigger a rotator movement by 180°.  Thus the images on the west of the meridian will be the same orientation as on the east side of the meridian.
  • Safety Slew: Some mounts, notably the Gemini, will not flip even though the target has crossed the meridian.  Checking Safety Slew will slew the mount one hour past the meridian, which will force the flip, and then back to the target.  This should eliminate the need for the AM stop offset setting and it should be set to 0.
  • Allow PM Re-sync: If this option is checked, an image is taken and plate solved after a meridian flip. The mount is then sync'd to the center of the solved image. The correcting slew is then from this reference. Recommended for portable setups or for setups with poor pointing accuracy. If this option is cleared, there is no sync and the correcting slew is from the plate solved position. This option should be cleared for permanent setups with good pointing accuracy as may be obtained with a suitable Tpoint mode. If checked, any Tpoint models should be disabled for best results.
  • Auto guide star select: If checked, an automatic detect of the brightest star in the field will be used to guide. If unchecked, the run will pause until you select a guide star and tell it to continue. 
  • Focus of flip: When checked, the focus method chosen on the Focus page will be executed immediately after the meridian flip and before the next exposure. 
  • Flip Delay: This is the number of minutes to delay the meridian flip. Some mounts may require a longer delay than the default 1 minute to determine the target has passed the meridian. You should experimentally determine you particular mount's characteristic. 
  • Meridian Tracking: If the value entered is positive, this indicates the number of minutes your mount can track past the meridian and defers the flip for the your specified number of minutes.  Depending on the above two settings, you may be able to get another exposure in before meridian crossing and eliminate the wait time for the meridian flip.  Observatory users using DDW or ASCOM for observatory control should insure their shutters permit crossing the meridian by the PM Tracking time since dome tracking is disabled 2 minutes before a meridian flip.  AutomaDome has no such limitation since it integrates tightly with TheSky.  If the value entered is negative, the mount backs up 1 hour in RA in the east and the session waits for the specified number of minutes.  This is helpful if you cannot allow your OTA to point to the zenith (Alt. = 90°) due to interference with a camera.  It can also be used if your mount has some ambiguity in slewing to a location whose RA has passed the meridian.  By setting Meridian Tracking to -10 or so, the target RA will be 10 minutes past the meridian before meridian flip is enabled.  Some experimentation may be required for the smallest negative value that will work.

    Tips on efficient meridian usage
    With the various time settings, you can increase your meridian usage efficiency.  AM stop offset is generally not needed and can be left at 0.  Flip delay is the amount of time your mount has to track past the meridian before it (the mount) knows it is time to flip and will do so.  For most mounts, this can be set to 1.  PM Tracking requires a bit more discussion.  Let's assume you have determined your mount can safely track past the meridian for 15 minutes.  Set the PM Tracking to 12 minutes for a safety margin.  Now, assume you are taking 10 minute sub-exposures at the time the mount approaches the meridian.  Assume your meridian crossing is at 22:05.  Here is a schedule of events you might see:

    21:50 Take 10 minute exposure
    22:00 Take 10 minute exposure
    22:05 (Time to flip - deferred)
    22:15 Flip meridian
    22:18 Take 10 minute exposure

    Now, if PM Tracking were set to 0, this would be the result

    21:50 Take 10 minute exposure
    22:00 Wait for meridian flip
    22:05 Flip meridian
    22:09 Take 10 minute exposure

    Here are some excerpts from an actual log with PM Tracking set at 10 minutes:

    >>> Here is the target information
    20:02:28 Target: M1
    20:02:28   Rise: 13:51 6 Feb
    20:02:28   45° E elevation: 17:32 6 Feb
    20:02:28   Transit: 20:50 6 Feb
    20:02:28   45° W elevation: 00:09 7 Feb
    20:02:28   Set: 03:50 7 Feb

    20:05:49 Solved RA: 05 34 30.2, Dec: +21 59 17 
    20:05:49 Target RA: 05 34 30.2, Dec: +21 59 17 , PA: 358.4
    20:05:49 Meridian flip after 20:51 Tue 6 Feb
    >>> The mount would have flipped at 20:51 without the buffer time

    >>> The exposure completed after the meridian flip time but before the 10 minute buffer elapsed.
    20:43:31 Exposing...
    20:54:15 D:\Astronomy\070206_M1\Clear356E_M1_00006.fit

    >>> And then the mount flipped immediately.
    20:54:18 Waiting for meridian flip time...
    20:54:18 Meridian flip starting...

    If your mount is capable of longer tracking past the meridian, the flip can be deferred longer, for example in the case of an Astro-Physics mount which can track for a number of hours past the meridian, depending on telescope, camera, declination, etc.

    Warning: It is up to the user to determine the proper setting for PM Tracking.  In addition to consulting your mount's documentation, you should also experiment   at various declinations, while watching the mount.  If you rotate your camera, you should also verify the PM tracking at various camera rotations.