[AstroPy] RA/Dec to Alt/Az

[Michael Brewer]

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)
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>>> ----------------------------------------------------------------------
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>>> 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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