Advanced Pointing Model

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Advanced Pointing Model

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The pointing model tab provides control and status of APCC's Pointing and Tracking Rate correction models. Using a pointing model can greatly improve the pointing accuracy of the mount. Using the tracking rate model can significantly increase the maximum duration that the scope can go unguided.

Pointing Model

Astro-Physics mounts are extremely accurate so one might ask why is a pointing model needed? The answer may not be obvious... because pointing errors can come from many sources other than the mount itself, including:

Polar misalignment

Telescope and focuser flexure

Sagging or tilted camera to telescope interface.

Sagging or tilting optics

Index errors (improperly synched to the current RA/Dec coordinates)


Dragging cables or moving optics.

To create a pointing model you must use APPM, the Astro-Physics Point Mapping software application. APPM will slew about the sky, taking pictures and plate solving them to extract pointing errors for many different points in the sky. Each plate-solved image is considered a measurement point for correcting pointing and tracking rates in that part of the sky. Having many measurement points allows APCC to "best-fit" a set of standard mechanical and environmental formulas to compensate for most errors.

It's very important to note that APCC's modeling can only correct repeatable errors. That is, pointing errors that reoccur with about the same magnitude every time the scope is slewed to any arbitrary position in the sky. Errors that usually do not repeat, such as dragging cables or moving optics cannot be modeled with APCC's modeling scheme because they can occur randomly and with different magnitudes. APCC cannot foretell the future! It's best to try to remove the random errors by locking optics if needed and securing cables so they cannot randomly slide around.

APCC's pointing model is different from most other pointing models because:

1) It actually consists of two separate models, one for the East side, and one for the West side of the pier. The dual model scheme is automatic. You do not need to do anything extra to get the dual models.

2) Modeling can be done for telescope positions where the counterweight is up. This is a critical feature not found in any other point modeling software that we are aware of at the time of this writing.

3) Modeling records for each measurement point the current temperature, humidity, and pressure, if they are available, at the time of the measurement. This can be very important if the temperature changes by 15 or more degrees when the scope is in use (because of changes in refraction).

It should be noted that from the AP V2 ASCOM Driver that the pointing corrections are transparent. That is the driver doesn't know that APCC is making corrections. If APCC is commanded by the driver to go to a specific RA/Dec coordinate, it will appear to the driver that the scope is at those coordinates, when in fact the mount may be at slightly different coordinates. Toggling "Enable Pointing Correction" will display the two different sets of coordinates (corrected and uncorrected).

Because APCC sits between the ASCOM driver and the mount all planetarium and other ASCOM compliant programs automatically get the benefit of the pointing model. Also, any application that connects to one of APCC's virtual ports will get the benefit of the pointing model.

Tracking Rate Model

Because of flexure, polar misalignment, refraction, and other effects, the apparent tracking rate of stars and deep sky objects in the sky is not exactly equal to the sidereal rate. The reason is that the magnitude of these pointing errors change slightly as the position of the telescope changes. This causes the target to drift in the telescope's view over time.

To help keep a target centered APCC Pro uses a Tracking Rate Model. Tracking rate correction is very important if you want to:

Reduce required autoguider movements which will result in sharper images.

Do long-exposure unguided images,.

Do long-duration autoguider cycles through a narrowband filter

Accurately track comets and/or asteroids.

The tracking rate model uses the pointing model to calculate the necessary adjustments to the tracking rate to keep a target centered. The adjustments are made to both the Right Ascension and Declination tracking rates once every second.

Near the Poles

Near the celestial poles, APCC will automatically turn off the pointing and tracking rate corrections to eliminate the possibility of fast RA/Dec corrections near the celestial pole. So, in this case APCC will display a message "Near Pole"  in the Pointing and Tracking Rate status boxes. Pointing and tracking rate correction may not be as well corrected in these areas.