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Variation in RA with altitude??
Frustrating!!??
Looking at the symptoms; if there's no appreciable Dec drift then I would suggest it's either a balancing/ flexing issue with the scope, or maybe atmospheric displacement ( abberation, NB at the horizon it's almost 30 min arc).
Is the movement always in the same direction? ie RA appears to run faster/ slower?
Have you tried checked this RA movement for an object on the celestial equator at varying altitude both on the East side and the west side of the meridian??? Does the "error" definately vary with altitude?
Maybe with some more analysis we can crack it?
I had a similar but different problem; only showed at high hour angles, where my spectroscope would appear to loose the star???? Finally traced it down ( or the major effect) to movement in the adaptor tubes between the scope and the spectro ( infact one of the issues was the use of T2 threaded spacers... I thought they would be more robust and solid rather than 2" sleeves with grub screws, BUT they unscrewed themselves whenever the "load" was offset CCW!!)
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