Contents - Index

Meridian Crossing



When tracking a target in the east with an equatorial mount, eventually the meridian is approached or even crossed.  While this is of no concern with a fork mount, a decision is required for equatorial mount users.  There are basically two choices.  Meridian Abort, whereby data acquisition for this target is halted and CCDAutoPilot moves on to the next target, or Meridian Flip, where CCDAutoPilot makes the changes necessary to pick up the target after flipping the mount on the west side of the meridian.
Automatic meridian flipping essentially uses software to turn an equatorial mount into the functional equivalent of a fork mount. 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 successful plate solving and Initialization.
  • Track Past Meridian: If the value entered is positive, this indicates the number of minutes your mount can track past the meridian and defers the flip for 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. It can 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. For some mounts, this may need to be used in conjunction with Safety Slew, below.
  • 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. 
  • Post Flip Delay: The session will pause for the entered time before proceeding. This is useful to allow domes that do not have direct synchronization with the mount to "catch up" to the mount. Some domes depend on a text file written periodically to determine where the mount is pointing for dome synchronization.
  • Allow Sync After Flip: 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 made 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 model. If checked, any Tpoint models should be disabled as repeated syncing into a Tpoint model is not recommended and will result in pointing inaccuracies. Use either Allow Re-sync or Tpoint but not both. 
  • Auto Guide Star Select After Flip: 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 After Flip: When checked, the focus method chosen on the Focus page will be executed immediately after the meridian flip and before the next exposure. 
  • ReverseY After Flip: In very rare circumstances, it may be necessary with some guide cameras and using a Guide Scope (only) for guiding to select this option.  This option is also required when using Maxim's Telescope or ASCOM Direct guide methods with some drivers, notably the AstroPhysics V2 driver. If your guide star moves out in the vertical direction after crossing the meridian, you should check this option. Most properly configured systems should have this option unchecked.


    Tips on efficient meridian usage
    With the various time settings, you can increase your meridian usage efficiency. Track Past Meridian 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. Let's assume you have determined your mount can safely track past the meridian for 15 minutes. Set the Track Past Meridian 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 size and orientation, declination, etc.

    Warning: It is up to the user to determine the proper Track Past Meridian. 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 this setting at various camera rotations.