ITRS Hour angle and declination

To complement altaz, it would be good if Skyfield could also provide hour angle and declination (hadec) for an equatorially mounted telescope. This is the ITRS hour angle and declination taking into account both polar motion and atmospheric refraction.

[brandon-rhodes]

I’ve been hesitant to try installing from the development branch because I’m unsure what that will do. Will it write over my current installation of Skyfield without changing the version number?

Yes, that’s what it would do. It removes and replaces the current install but the version number will be the current one from PyPI, as I don’t increment it til it’s time to release (lest I accidentally release early). If you’re wary I can close the issue on the strength of the passing test, then you can re-open if you see problems after the next version comes out.

First of all, rotating by GAST will rotate from the equinox to the TIO, so you need to add the TIO locator to the geographic longitude there. Another way to think about this is that LST contains the TIO locator, so HA must also so that the ITRS “observed” RA = LST - HA is independent of the TIO locator.

The raw position vector is topocentric GCRS. The geographic longitude is an ITRS angle, and thus is over on the other side of the transform that includes the TIO locator. So when I apply the ITRS transform to the topocentric GCRS, which includes the TIO locator, aren’t I generating an angle that’s directly comparable to the geographic longitude? Your suggestion sounds like it would apply the TIO locator twice, but maybe I don’t see where exactly it would go in the code.

Second, this implementation is only valid in an airless model as it doesn’t include refraction.

Understood: at the moment my target is the Hour Angle described in the Supplement, which accounts for polar motion but not for atmosphere. The code will indeed have to become more complicated, as shown in your script, if in the future we go beyond the almanac concept of HA.

[brandon-rhodes]

I tried installing the development branch using your instructions, but it didn’t work. After downloading 93.6 MB, I got…

I’ll adjust the install-from-a-GitHub-zip template I use to add -U so that it doesn’t skip an install of what looks like the same version.

And, yes, I guess the zip includes all the files in the repository, even the ones that aren’t needed for the install. I’ll think about whether there’s a way around that.

You should probably document the fact that your ITRS hour angle and declination don’t include refraction.

Done.