Re: Dec-Arc Tracking in APCC Pro v1.9 - an explainer please?

Ray Gralak

Hi Dean,


In a nutshell:


The all-sky models produce a tracking rate derived from the best-fit pointing terms across all its data points. However, it is possible that the equipment has some un-modeled pointing characteristics. If so, then the tracking rate will not account for those un-modeled characteristics.


The declination-arc model is designed specifically for calculating tracking rates inclusive of any un-modeled characteristics. It does this by measuring the changes in pointing error along the declination arcs like in your screenshot. The more values in each arc the better the tracking rate modeling will be. Tracking rates are interpolated between declination arcs so the entire region encompassed by the sky data points is covered.


Some notes about this:

1)    Dec-Arc Tracking is only used for tracking rate calculations. The all-sky model, which is also calculated by APCC, still handles pointing.

2)    Tracking rate correction only works within the area in which sky points are measured. When outside that area APCC uses the all-sky tracking rate model. The All-Sky model will also extrapolate outside of the sky data points that were collected.


BTW, when I mention “All-Sky” model above, I mean one of the four model combinations: East/West, Counterweight Up/Down.




From: [] On Behalf Of Dean Jacobsen
Sent: Wednesday, August 18, 2021 10:06 AM
Subject: [ap-gto] Dec-Arc Tracking in APCC Pro v1.9 - an explainer please?


Hi All,

I am interested in hearing how the new Dec-Arc feature in v1.9 works.

I set up my mount for each session.  I always know my object in advance of setup so I then run an abbreviated model which contains about 3 or 4 lines which bracket the declination of my object for the night.

This has been working great for me for about a year now at focal lengths of 380mm and 540mm.

The image below is an example of how I would set up a modeling run for an object at +60 degrees decination:

I have looked in the on line help file for APCC Pro and APPM and only see mention of the Dec-Arc feature in the "New Features" section.

So, my question is:  How do I implement this new feature other than setting the check box?  Does the Dec-Arc feature still require a "standard" APPM modeling run such as what I have been doing above?  Lastly, if I have already performed a APPM mapping run, then how will the Dec-Arc feature benefit me?

Thanks in advance for the help.

Dean Jacobsen
Astrobin Image Gallery -

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