[AstroPy] Problems with solar system ephemerides

[Michael Brewer]

Hi Stuart,

 > you could always raise an issue on the astropy github page requesting 
this.

   Will do.

 > you'd also like a geocentric apparent coordinate frame you can use as 
an alternative to CIRS?

    Yes, in the sense that "geocentric" also includes topocentric when a 
site location is present.

    Thank you for your reply,

       Michael Brewer

On 04/29/2020 12:00 PM, Stuart P Littlefair wrote:
> Hi Michael,
>
> Ignoring your third issue for the moment (for convenience).
>
> In summary you'd like an astrometric frame added to the built-in 
> frames, and you'd also like a geocentric apparent coordinate frame you 
> can use as an alternative to CIRS? Sounds eminently reasonable; you 
> could always raise an issue on the astropy github page requesting this.
>
> It's feature freeze week for V4.1, so things are a bit busy at the 
> moment, but I'm sure it'll happen if you ask.
>
> Stuart
>
> On Wed, 29 Apr 2020 at 16:48, Michael Brewer <brewer at astro.umass.edu 
> <mailto:brewer at astro.umass.edu>> wrote:
>
>     Dear List,
>
>        Every once in awhile, I have a colleague who wants to use the
>     AstroPy
>     solar system ephemerides. I am getting tired of having to dissuade
>     them,
>     so I'd like to discuss the issues that I have with these
>     ephemerides in
>     an attempt to get them resolved.
>
>     Issue #1: The positions of the bodies are returned in the GCRS and
>     there
>     appears to be no way to easily transform them to topocentric
>     astrometric
>     positions. By this I mean simply the difference in the ICRS
>     position of
>     the body compensated for light time and the ICRS position of the
>     observer. This is rather important if one wishes to place the body
>     on a
>     background map in the ICRS. It is also the only way to compare the
>     output of AstroPy's ephemerides with that of JPL Horizons or Brandon
>     Rhodes' Skyfield. Why isn't there a builtin frame for doing this?
>
>     Issue #2: Currently, there is also no builtin frame for
>     transforming the
>     returned positions to apparent place. By this I mean the topocentric
>     position with respect to the true equator and equinox of date.
>     This is
>     quite important to people such as myself who still like their
>     origin of
>     right ascension to be an actual location on the sky rather than a
>     convenient mathematical construct. It allows one to point an
>     equatorial
>     mounted telescope using the local sidereal time to calculate the hour
>     angle. And again, this is the only way to compare the output of
>     AstroPy's ephemerides with that of JPL Horizons or Skyfield. It is
>     also
>     quite simple to do. Just adjust the CIRS right ascension by
>     subtracting
>     the equation of the equinoxes. Note: I did find a function for doing
>     this in solar_system.py, _apparent_position_in_true_coordinates(),
>     but
>     it feels sort of kludgy to use this. There should be a builtin
>     frame for
>     this.
>
>     Issue #3: This is a fairly minor quibble, but the functions
>     atciqz() and
>     aticq() are calculating the gravitational light deflection from
>     the Sun
>     incorrectly. The third argument of erfa.ld() should be the time
>     delayed
>     heliocentric position vector of the target body. I do realize that
>     SOFA
>     has this problem also.
>
>         Sincerely,
>
>           Michael Brewer
>
>
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>
>
>
> -- 
> Stuart Littlefair
>
> -------------------------------------------------------
>
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> /
> /
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> -------------------------------------------------------
>
> Dept. of Physics & Astronomy,
> Univ. of Sheffield, Sheffield, S3 7RH.
>
> email: S.Littlefair at sheffield.ac.uk <mailto:S.Littlefair at sheffield.ac.uk>
> phone: +44 114 2224525
>
>
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