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    Re: Landmark sights
    From: Peter Monta
    Date: 2019 Aug 9, 00:20 -0700
    Hi Paul,

    This interesting DECam paper [1] talks about horizontal refraction, but only over small fields of a few degrees (as would occur in an astronomical telescope), so "relative horizontal refraction".  I think it's the source of the 10 mas number I had in mind.  The case of "absolute horizontal refraction" over arbitrarily large angular spans is presumably worse than the ~10 mas mentioned in the paper.  Still, numbers like "3 to 6 seconds" sound really large.

    For example, how stable were the ground marks at transit telescopes?  As I understand it, they put a target, collimator, and light source some distance from the telescope to the south, to serve as an azimuth reference.  If it habitually moved around multiple seconds, I would think that would render it useless, and they would have gone with something internal instead, such as a rotary encoder on the telescope's vertical axis (microscope-reading scales).  Did they maintain statistics on reference-mark performance at places like Greenwich, Pulkovo, or USNO?  Of course they dealt with only a few marks directly to the south, not spread out over the entire horizon, but at least it's a data point.

    As for chimneys, I would call that "pathological", though I'm sure C&GS were in the right to devise methods to handle difficult cases.

    But for the non-chimney case, with normal urban or rural terrain, I'd be curious just what the statistics are for large-angle horizontal refraction.  It would be a fairly easy experiment to do, so it might have been done in the era when such things mattered:  fix a theodolite to a stable mount, then take a long time series of azimuths to stars.  For a catalog, FK5 would have been good enough.  Indeed, this data might be already available as a by-product of catalog construction.

    From a hobbyist CN point of view, which would be better, a collimator at 100 meters or a streetlamp at 5 km?


    [1] https://arxiv.org/abs/1703.01679

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