[AstroPy] RA/Dec to Alt/Az

[Michael Brewer]

I guess I should explain that. The equinox of the coordinates determines
the precession and nutation corrections that are applied to transform from
the equinox of the coordinates to the time of the observation. AstroPy
defaults to J2000, so the difference you see is either due to the other
programs using a different equinox, or perhaps an outdated
precession/nutation model. Your location on the surface of the Earth isn't
going to matter since you didn't provide a distance and even if you did
Alpha Centauri is far enough away that diurnal parallax is insignificant.

On Sat, Apr 22, 2023 at 5:40 PM Michael Brewer <brewer at astro.umass.edu>
wrote:

> You just need to use the equinox of these coordinates is all:
>
> # Alpha Centuri
> ra = '14h41m13.3s'
> dec = '-60d55m52.3s'
>
> Normally it's J2000.
>
>
>
>
> On Sat, Apr 22, 2023 at 4:54 PM Richard Moffat <richard.moffat at gmail.com>
> wrote:
>
>> Thank you, Ben. This is most useful.
>>
>> I didn't do astronomy in my physics degree (quite some time ago). This
>> has given me a great pointer to read up on.
>>
>> We can consider this question closed unless anyone else has anything else
>> to add.
>>
>> *Noho ora mai,*
>> *All the best,*
>> Richard Moffat.
>>
>> ==============================================================
>>
>> richard.moffat at gmail.com
>> ==============================================================
>>
>>
>>
>>
>>
>>
>> On Sat, 22 Apr 2023 at 23:30, Benjamin Weiner <bjw at as.arizona.edu> wrote:
>>
>>> Errors in RA/Dec calculations of order several arcminutes are frequently
>>> due to inconsistency in the equinox used. Epoch and equinox are not the
>>> same: epoch is time of observation (important for moving objects), while
>>> equinox is coordinate reference frame, ie where the Earth’s axis is
>>> pointing. The precession from 2000 to 2023 can cause an offset of several
>>> arcminutes. FK5 and ICRS are referred to 2000, but to point an earthbound
>>> telescope in 2023 one also needs equinox of the current time.  Any
>>> reference on celestial coordinate systems will give more detail.
>>>
>>> Ben
>>>
>>> On Sat, Apr 22, 2023 at 12:46 PM <astropy-request at python.org> wrote:
>>>
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>>>>    1. Re: RA/Dec to Alt/Az (Richard Moffat)
>>>>
>>>>
>>>> ----------------------------------------------------------------------
>>>>
>>>> Message: 1
>>>> Date: Sat, 22 Apr 2023 22:45:22 +1200
>>>> From: Richard Moffat <richard.moffat at gmail.com>
>>>> To: Astronomical Python mailing list <astropy at python.org>
>>>> Subject: Re: [AstroPy] RA/Dec to Alt/Az
>>>> Message-ID:
>>>>         <
>>>> CA+1X2WFxX2RONmf_8-gaAqsNWKP5jcUT0RS2hTOyYDA-ub4RWw at mail.gmail.com>
>>>> Content-Type: text/plain; charset="utf-8"
>>>>
>>>> Hmm... Replying to my own post :-)
>>>>
>>>> So, looking at the SkyCoord class, it looks like I need to specify a
>>>> frame
>>>> and possibly the equinox. I'm guessing this has something to do with the
>>>> sidereal time calculation and Julien dates and things.
>>>>
>>>> coord = SkyCoord(ra, dec, unit=(u.hourangle, u.deg), frame=FK5,
>>>> equinox=Time(gmt_time_str))
>>>>
>>>> Now I'm only 2-5 seconds out.
>>>>
>>>> Time to read up more on the parameters.
>>>>
>>>> *Noho ora mai,*
>>>> *All the best,*
>>>> Richard Moffat.
>>>>
>>>> ==============================================================
>>>>
>>>> richard.moffat at gmail.com
>>>> ==============================================================___
>>>>
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