[AstroPy] Telescope pointing with astropy, atmospheric refraction and precession

[Stuart P Littlefair]

Hi Russel

You are correct that TETE does not include atmospheric refraction. What you want is an HA-Dec frame. 

It does not exist within astropy yet, but is on the roadmap for the near future. 

If you want something to work in the meantime, you could look at the code in coordinates/builtin_frames/icrs_observed.py in the current master branch on GitHub. 

This file contains the transforms from ICRS to AltAz and you will see that it actually calculates the HA as part of the function!

You could simply copy this function and return HA and Dec instead of Alt and Az. 

Stuart Littlefair
Dept. of Physics & Astronomy
Univ. of Sheffield, Sheffield, S3 7RH

email: s.littlefair at shef.ac.uk
Phone: +44 114 2224525
Sent from my iPhone

> On 28 Nov 2020, at 06:46, Russell Valentine <russell.valentine at gmail.com> wrote:
> 
> Thank you for your reply Eric. I do not think TETE can take a equinox parameter. I know FK5 has one. I do see what I think is precession corrections when I go from ICRS to TETE, however I do not think TETE does atmospheric refraction as well.
> 
> https://docs.astropy.org/en/stable/api/astropy.coordinates.builtin_frames.TETE.html#astropy.coordinates.builtin_frames.TETE
> 
> 
> >>> tete = icrs.transform_to(TETE(obstime=t, location=earth_location, equinox=t))
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
>  File "/home/russ/.local/share/virtualenvs/server-zkliv1pZ/lib/python3.7/site-packages/astropy/coordinates/baseframe.py", line 609, in __init__
>    list(kwargs)))
> TypeError: Coordinate frame got unexpected keywords: ['equinox']
> 
> 
>> On 11/27/20 8:24 PM, Eric Jensen wrote:
>> Hi Russell,
>> I think the thing you’re missing may be the ‘equinox’ argument to the ’transform_to’ call.  I believe that ‘obstime’ sets the time of your observation for calculating alt/az (and may also be used as the reference date for applying proper motion corrections if you do that), but I don’t think it sets the date for the equinox used for your coordinate system.  Try adding ‘equinox=t’ to your transformations between ICRS and TETE and see if that helps.    (I can’t test here because conda channels haven’t caught up with the 4.2 release yet, so I don’t have TETE in my installation.)
>> Best,
>> Eric
>>  
>>>> On Nov 27, 2020, at 4:20 PM, Russell Valentine <russell.valentine at gmail.com> wrote:
>>> 
>>> Hello,
>>> 
>>> I've been trying to figure out the best way to get correct coordinates for telescope pointing of a equatorial mount in astropy. How to, given a set of coordinates in ICRS, convert it to account for precession and atmosphereic refraction to correctly move the telescope. It is my understand that you can transform from ICRS to TETE to compensate for precession, but then how to also account for atmospheric refraction? I believe the AltAz coordinate is needed for that.
>>> 
>>> Things I was thinking and tried:
>>> 
>>> 1) ICRS->AltAz (w/pressure)->TETE
>>> I do not know if this is correct because you get just very close to just ICRS. I thought maybe it is right and the refraction just happens to be the counter the precession, but I tried it on both east and west horizons with the same result.
>>> 
>>> 2) ICRS->TETE->AltAz(w/pressure)
>>> 
>>> But as expected AltAz is the same as ICRS->AltAz
>>> 
>>> I feel like I am missing some key piece. Has anyone done this with astropy?
>>> 
>>> 
>>> 
>>> 
>>> from astropy.coordinates import EarthLocation, TETE, ICRS, AltAz, SkyCoord
>>> import astropy.units as u
>>> from astropy.time import Time as AstroTime
>>> from astropy.units import si as usi
>>> t = AstroTime('2020-11-27 19:12:16.894431')
>>> earth_location = EarthLocation(lat=38.9369*u.deg, lon= -95.242*u.deg, height=266.0*u.m)
>>> # East horizon object
>>> icrs = ICRS(ra=346.23611667*u.deg, dec=12.32287778*u.deg)
>>> pressure = 98170.13549857 * u.Pa
>>> altaz = icrs.transform_to(AltAz(obstime=t, location=earth_location, pressure=pressure))
>>> tete = icrs.transform_to(TETE(obstime=t, location=earth_location))
>>> tete_from_altaz = altaz.transform_to(TETE())
>>> 
>>> # West horizon object
>>> icrs2 = ICRS(ra=172.53106667*u.deg, dec=9.27663056*u.deg)
>>> altaz2 = icrs2.transform_to(AltAz(obstime=t, location=earth_location, pressure=pressure))
>>> tete2 = icrs2.transform_to(TETE(obstime=t, location=earth_location))
>>> tete2_from_altaz2 = altaz2.transform_to(TETE())
>>> 
>>> 
>>> altaz_from_tete = tete_from_altaz.transform_to(AltAz(obstime=t, location=earth_location, pressure=pressure))
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