Dale Ghent

Of your 3 questions, the 2nd one is the most pertinent and the answer to it informs the answers for the 1st and 3rd.

We all live under one giant lens called the atmosphere. We all know it's dynamic with many layers and variations in density between not only those but also different regions of the visible sky. All of this affects how light from space reaches any given point on earth. Mapping the refractive variations of the atmosphere allows a mount to compensate for the effects of atmospheric refraction in any location in the sky that is modeled. The model itself can be adjusted based on measured changes to temperature, to an extent.

Think of this mapping as making a PEC, but for the sky. We have PECs and encoders for compensating for the mechanical variances of the mount, and point mapping brings (broadly) the same concept to the sky. Point mapping can also compensate for non-mount mechanical variations such as any minute flexure of your OTA as it gets pulled by gravity differently when in different orientations (aka flexure).

Functionally, APPM creates a model that APCC then uses to influence the mount with. Yes, you get pointing accuracy, but APCC will also interpolate any needed changes to tracking speed when traversing between mapped areas of the sky. Needless to say, a more dense point model will give more accurate results than a sparse one.

So, with that, it's not a stretch to realize the benefits it has for tracking. A good model is essential for unguided imaging. How long you can go without noticing an excursion depends on your setup and optics, though.

On Aug 10, 2021, at 09:56, Bruce McMath <bruce.mcmath@...> wrote:

three quick questions:

• In a very small nutshell, how can a pointing model improve tracking?
• Why would you need such a model on a scope with encoders?
• How much improvement can you expect in unguided imaging with a mount without encoders?

I have a 900 mount and take lots of unguided images for photometry - over a hundred thousand todate. A big factor in the quality of my data is tracking-related, costing me a lot of time filtering out the worst. I am in line for a Mach2 but who knows when that will become reality. I have been hesitant to buy the APPC given that it comes with the Mach2. Hate to pay for the same thing twice. :-) But, if it really works.....
James (Bruce) McMath

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