Re: east/west Plate solving with an SCT


I tried plate-solving for a pointing model with my 300mm f/15 Cassegrain.  The model to the east worked fine.  The model to the west was a disaster and only about 10% of the attempted solutions would "solve." The partial results suggested I had a 7-degree orthogonality error.  For the life of me I couldn't see how that resulted given the setup. Everything is tight, mirrors are fixed, setup is balanced, the 1100GTO is barely working up a sweat given the load, etc. 

I then ran a careful assessment with ConeSharp which indicated I had a 10 arc-minute orthogonality issue -- not 7 degrees. Just last week I ran a manual TPoint model in TSX which suggested I had a 11 arc-minute orthogonality error.

I have never been able to explain the original APPM result and can only conclude it was the result of erroneous plate solutions caused by magnification beyond what the local seeing at the time could support, exacerbated by a moon on the western side of the meridian.

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