[AstroPy] Problems with solar system ephemerides

[Stuart P Littlefair]

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> 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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email: S.Littlefair at sheffield.ac.uk
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