Hour angle constraint

[mcoughlin]

This PR adds possibility of an hour angle constraint.

[mcoughlin]

Apologies @bmorris3. I fixed the linting issue.

[bmorris3]

@mcoughlin Just a few more:

astroplan/constraints.py:947:69: W291 trailing whitespace
astroplan/constraints.py:948:26: W291 trailing whitespace
astroplan/tests/test_constraints.py:168:34: W291 trailing whitespace

[mcoughlin]

@bmorris3 I think I got those in the last commit.

[bmorris3]

Hi @mcoughlin,

Thanks for starting this constraint.

In astroplan we try to use full-precision astropy methods wherever possible, so that results from astroplan will agree with doing the calculation "the long way" via astropy. Astroplan already has a method for computing hour angle, so an hour angle constraint should use the result of this method:

astroplan/astroplan/observer.py

Lines 1917 to 1944 in 67c0c6b

	def target_hour_angle(self, time, target, grid_times_targets=False):
	"""
	Calculate the local hour angle of ``target`` at ``time``.
	Parameters
	----------
	time : `~astropy.time.Time` or other (see below)
	Time of observation. This will be passed in as the first argument
	to the `~astropy.time.Time` initializer, so it can be anything that
	`~astropy.time.Time` will accept (including a `~astropy.time.Time`
	object)
	target : `~astropy.coordinates.SkyCoord`, `~astroplan.FixedTarget`, or list
	Target celestial object(s)
	grid_times_targets: bool
	If True, the target object will have extra dimensions packed
	onto the end, so that calculations with M targets and N times
	will return an (M, N) shaped result. Otherwise, we rely on
	broadcasting the shapes together using standard numpy rules.
	Returns
	-------
	hour_angle : `~astropy.coordinates.Angle`
	The hour angle(s) of the target(s) at ``time``
	"""
	time, target = self._preprocess_inputs(time, target, grid_times_targets)
	return Longitude(self.local_sidereal_time(time) - target.ra)

[mcoughlin]

@bmorris3 We tried that and found the calculation quite slow for constraints. Maybe it makes sense to just have a warning in the doc string?

[bmorris3]

I wonder why it's so slow. Is it possible that a different combination of arguments in the local_sidereal_time method would be faster? https://docs.astropy.org/en/stable/time/index.html#sidereal-time-and-earth-rotation-angle

[mcoughlin]

@bmorris3 Not sure. I just fixed the PR to use Astropy by default though.

[bmorris3]

@mhvk We're using astropy.time.Time.local_sidereal_time to compute LST in astroplan:

https://github.com/astropy/astroplan/blob/67c0c6ba8467fc26ca79a7cef056eb38a066abd2/astroplan/observer.py#L1914C29-L1915

Do you know if there are more performant options to give the LST functions, so that we can compute many accurate LSTs quickly (see #560 (comment))?

[mhvk]

If you don't need full precision, you can avoid the correction for the TIO relative to longitude 0. Easiest is picking an old model, "iau1982", which doesn't have it -- this is a factor 4 faster and seems accurate to better than 4e-7 hours.

More accurate (1e-10 hours) and almost as fast is

time.sidereal_time(kind, longitude='tio', model=model) + self.location.lon

For planning purposes, either seems reasonable.

p.s. We probably should have something in our documentation about the speed differences...

[mcoughlin]

Thanks @mhvk ! @bmorris3 I gave that a whirl.

[mcoughlin]

@bmorris3 maybe you can retrigger the tests?