ACP's built-in telescope pointing corrector is the only one that learns as it goes. There is no need for "mapping runs". And it continually adjusts itself to changes in your mount's characteristics. Pointing models are saved on disk and may be imported into MaxPoint or TPOINT for analysis. It works equally well with fork or German equatorial mounts.

ACP pioneered accurate pointing via its Every Image Centered™ feature. Each time ACP has to adjust the telescope to center an image, it sends the correction to the pointing corrector as a mapping point. Thus, as you image more and more, the corrector gets smarter and smarter. Eventually, the need for pointing adjustments reaches a low level, giving you maximum imaging time.

Theory of Operation

The pointing corrector takes your desired coordinates and adjusts them to get the imperfect mount to point where you want it. It also takes the imperfect telescope coordinates and reverses out the correction before displaying them. Thus, you only see "ideal" coordinates in ACP's displays, and you only use ideal coordinates when specifying a slew or sync.

Based on the mapping points supplied (via ACP sync operations), the corrector calculates the following mechanical errors in the mount:

• Hour angle and declination bias
• Polar axis azimuth and elevation errors
• Non-orthogonality of axes
• Bore sight (collimation) error
• Declination axis flexure (GEM only)
• Fork flexure (fork mount only)
• Optical Tube Flexure

Mapping points are processed by the corrector engine, which calculates a "best estimate" of the eight active model parameters. It does this each time a new mapping point is added (via a sync in ACP or one of its hub clients) and uses all of the mapping points added so far.

Analysis of Pointing Errors

ACP stores its pointing model as a set of mapping points in a disk file. The format of this file is compatible with Diffraction Limited's MaxPoint correction and analysis tool. If you have MaxPoint, you can load ACP's model and analyze the pointing errors. If you have TPOINT, you can convert ACP's model file into a format that can be loaded into the TPOINT Telescope Analysis tool and analyzed as well.

How ACP Learns

The corrector engine learns about pointing errors whenever ACP is called upon to synchronize the telescope coordinates. When correction is enabled, sync operations don't actually reset the telescope coordinates. Instead, the desired and actual coordinates at the time of sync are sent to the corrector as a mapping point where they immediately update the model used to predict the telescope's pointing behavior. Each time a sync is performed, whether manually through ACP's controls, from an Every Image is Centered adjustment, or from a client (such as MaxIm DL or a planetarium program) that is using ACP's ASCOM telescope hub, the pointing engine is immediately updated with a mapping point.

When ACP is first started with a new telescope and with correction enabled, it needs a minimum of six accurate syncs at fairly widely spaced points in the sky before the correction starts to take effect. For most mounts, after seven or eight syncs, the pointing accuracy will be below two arc minutes, often below one arc minute, regardless of mechanical flexure or polar alignment errors (within reason!). If the mount has poor slewing position repeatability, more pointing observations will be required. In addition a mount with poor repeatability will never achieve excellent corrected pointing.

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