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

[Russell Valentine]

My attempt at a simple HaDec frame and converting ICRS to HA-DEC based 
on icrs_observed.py:

https://gist.github.com/bluthen/8a07259413647b69e045733e97ae9949

It looks like it gives pretty good results. Thank you for the help!


Russell Valentine


On 11/28/20 12:29 PM, Russell Valentine wrote:
> Yes, correct I do want HA-Dec frame.
>
> I had been trying to do that on my own, but it was pretty convoluted. 
> My current procedure was (psudocode):
>
> side_real_time = AltAz(90,0, location, time).to(ICRS).ra
> altaz = my_orig_coord.to(AltAz(location, pressure, time))
> # Make new altaz without pressure and go back to icrs for
> # refraction adjusted ra/dec
> adjusted_icrs = AltAz(altaz.alot, altaz.az, location, time).to(ICRS)
> adjusted_icrs = ICRS(side_real_time - adjusted_icrs.ra, 
> adjusted_icrs.dec)
>
> And then use adjusted_icrs.ra and adjusted_icrs.dec as telescope HA-DEC.
>
> It was not clear to me if the altaz frame took into account 
> precession. Maybe that happens when converting to topocentric CIRS.
>
> Thank you for the reply and suggestion to look at icrs_observed.py I 
> will do that, and try to learn more about these different coordinate 
> systems.
>
>
> Russell Valentine
>
> On 11/28/20 2:53 AM, Stuart P Littlefair wrote:
>> 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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