Re: PHD 2 Guiding "Good Enough"


Ray Gralak
 

Hi Bryan,

That said, PEC in the mount is never perfect, and you will often see residual repetitive errors even when PEC is
active. These often arise when the tracking errors occur with a frequency that is not a harmonic (integer fraction)
of the mount’s worm period – most PEC implementations can’t deal with those. You can also get residual
periodic errors if they are dependent on the mechanical loading of the mount or if the mount’s behavior has
changed since the PEC was programmed..
I would not rely on the PHD2 algorithm for most AP mounts. It will not be able to accurately predict any residual periodic error that could already be taken out by AP's PEC mechanism. It may be useful if there is periodic error not correctable by PEC because it doesn't repeat in phase every worm cycle, but there are very few cases where that might happen.

Even then, It may not be very effective unless the residual error is truly periodic and you can wait for multiple periods for the phase and amplitude to be determined to reasonable accuracy. For example, the result may inaccurate because portions of continuous timeline tracking data may be missing while image downloading or dithering operations. Just check your PE every few months and update it if necessary. This will provide better overall results. (And a more advanced solution is coming. :-)

-Ray Gralak
Author of APCC (Astro-Physics Command Center): https://www.astro-physics.com/apcc-pro
Author of PEMPro V3: https://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: https://www.siriusimaging.com/apdriver


-----Original Message-----
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Worsel via groups.io
Sent: Sunday, May 3, 2020 11:59 AM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] PHD 2 Guiding "Good Enough"

Charles

See Slide 14 in Best Practices for PHD2 <https://openphdguiding.org/PHD2_BestPractices_2019-12.pdf>

Also. from the Help file discussing the PPEC algorithm in PHD2 (note that PPEC guiding algorithm is NOT the
same as PEC in the mount)

·Short-term: for high-frequency errors such as those caused by gear roughness or seeing

·Medium-term: for residual periodic errors, typically occurring at intervals less than or equal to the worm period

·Longer-term: for steady drift and for lower frequency (longer time interval) harmonics that can be caused by the
interaction of multiple gears in the drive train

The short-term behavior is used to identify the unpredictable noise in the system, which is essentially filtered out
in order to identify components that are predictable. For most mounts, the medium-term component is likely to
be the most important. If you’re following best practices, you will have programmed periodic error correction in
your mount (assuming that feature is available to you). Doing this reduces the amount of work that needs to be
done by PHD2, and the PEC correction in the mount is normally saved permanently. This approach is preferable
to having to measure and infer the periodic error behavior every time you set up your equipment.

That said, PEC in the mount is never perfect, and you will often see residual repetitive errors even when PEC is
active. These often arise when the tracking errors occur with a frequency that is not a harmonic (integer fraction)
of the mount’s worm period – most PEC implementations can’t deal with those. You can also get residual
periodic errors if they are dependent on the mechanical loading of the mount or if the mount’s behavior has
changed since the PEC was programmed..

Bryan

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