Sync and Recalibrate (RCal) – We've taken the mystery out...

Sync vs ReCalibrate has been a single source of many user group posts over the years.  We can't make the concepts any easier, but we have been able to adjust some things so that you no longer need to worry much about it.  Our software now converts ALL calibration requests into ReCals with the exception of resumption from the pre-defined AP park positions.  If any of you care to read on, please feel free to do so.  You may find the information useful and historically interesting.  It is, however, no longer the major worry it was a few years back.  That being said, please don't try to defeat our safety measures by forcing full syncs, even if you know how to do so.

Sync and Recalibrate (RCal) – A Brief Explanation of the Differences

The Sync and Recalibrate commands are essential to the proper operation of your German Equatorial Mount. They allow the implementation of the meridian delay feature and movement of your telescope via the clutches. If used in the wrong way, the Sync and Recalibrate commands will cause the mount to slew incorrectly and possibly cause damage to equipment. They should be used with caution. The following information will provide a basis for understanding the purpose of each command. If you would like to have a more detailed explanation complete with diagrams, please consult the most recent version of the GTO Keypad Manual in the Technical Support section of the Astro-Physics website.

SYNC  

The Sync command, if used at all, is typically employed at the beginning of an observing or imaging session after your mount is polar-aligned. Sync tells the mount where it is pointing and defines which side of the mount is the telescope side, and which is the counterweight side.  Sync assumes that you have placed your telescope on the proper side of the meridian, i.e. a telescope placed on the west side of the mount pointing to a star in the east. This assures that the telescope will be positioned on the proper side of the mount for all subsequent objects.

Sync can also be used during your observing session if you have pushed the telescope by hand to a new position. Each time you use Sync, you redefine the orientation of the telescope and counterweight shaft.  If you use it incorrectly during your session, the mount may move in an unexpected direction.

Sync defines both where the scope is pointing and defines which side of the mount the telescope is on.

RECALIBRATE  

Recalibrate (RCal) is the “tweak” command.   It is quick and easy and is perfect for fine-tuning your pointing accuracy during your observing or imaging session.   Recalibrate is used when you have used the Go-To function and will be centering the object via the directional buttons on the virtual handbox. RCal updates the mount's internal memory position for that object without changing the correct orientation of scope on top of mount.  It does NOT send new RA and Dec coordinates to the mount.

Recalibrate is ONLY intended to confirm the coordinates of the last Go-To slew.   The idea is that you are telling the servo that it is actually still pointing exactly where the Go-To slew told the mount to point in the first place.

Since Recalibrate does not re-establish the mount’s position in memory, it doesn’t matter if the RCal object is past the meridian with the scope on the wrong side of the mount.   It is therefore a MUCH safer command to use during your session.    

Recalibrate reestablishes where the telescope is pointing, but does NOT redefine the position of the scope with respect to the mount.

SYNC VS. RECALIBRATE IN PLANETARIUM PROGRAMS

It is important to note that external software programs, including programs that use the ASCOM interface, may not make the same distinction between the 2 terms (most only have Sync as an option).   YOU MUST BE CAREFUL when using a Sync or Recalibrate (or whatever they call it) from a client software program!   Most often, only the Sync command is provided, which will encourage you to use it when your intent is to Recalibrate. This may result in unintended consequences.

In order to protect your mount from erroneous instructions, the Astro-Physics V2 ASCOM driver provides the checkbox to “Use RCAL for Syncs.”   If this box is checked, all sync commands that are issued from the planetarium program will be translated into Recalibrate before it goes to your mount.  We strongly advise that you keep this box checked unless you have just pushed the mount to a new position by hand via the clutches.   Then, once synced to the new RA and Dec coordinates, please remember to re-check the box so that you will be protected.

Unfortunately we cannot prevent all errors. If you select an object on the planetarium screen and then select Sync before you slew to the object, you will inadvertently calibrate on those coordinates.   You must be careful!