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Re: Landmark sights
From: Peter Monta
Date: 2019 Aug 9, 00:20 -0700
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?
Cheers,
Peter
[1] https://arxiv.org/abs/1703.01679
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?
Cheers,
Peter
[1] https://arxiv.org/abs/1703.01679
