[AstroPy] RA/Dec to Alt/Az

Sat Apr 22 18:28:26 EDT 2023

No, the time that you set into obstime is the date of the observation
(epoch), not the equinox that the RA/Dec coordinates are referred to. If
you want to convert apparent rather than astrometrc RA/Dec coordinates to
AltAz, then you should set up a True Equinox True Equator of date (TETE)
frame for your RA/Dec coordinates.

On Sat, Apr 22, 2023 at 6:08 PM Richard Moffat <richard.moffat at gmail.com>
wrote:

> " 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 programs using a different equinox, or perhaps an outdated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:
>>>>>>
>>>>>>    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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