[AstroPy] RA/Dec to Alt/Az

Sat Apr 22 18:08:26 EDT 2023

" AstroPy defaults to J2000"  <== that. I think you hit the nail on the
head there. I think my first attempts were assuming a local time (or GMT
with offset) gets carried through the classes.

Thank you.

*Noho ora mai,*
*All the best,*
Richard Moffat.

==============================================================

richard.moffat at gmail.com
==============================================================

On Sun, 23 Apr 2023 at 09:58, Michael Brewer <brewer at astro.umass.edu> wrote:

> 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 programs using a different equinox, or perhaps an outdated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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>>>>> Today's Topics:
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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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