[AstroPy] RA/Dec to Alt/Az

[Richard Moffat]

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:
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>> CA+1X2WFxX2RONmf_8-gaAqsNWKP5jcUT0RS2hTOyYDA-ub4RWw at mail.gmail.com>
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>>
>> 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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