<html>
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
</head>
<body bgcolor="#FFFFFF" text="#000000">
Hi Stuart,<br>
<br>
> you could always raise an issue on the astropy github page
requesting this. <br>
<br>
Will do.<br>
<br>
> you'd also like a geocentric apparent coordinate frame you can
use as an alternative to CIRS?<br>
<br>
Yes, in the sense that "geocentric" also includes topocentric
when a site location is present.<br>
<br>
Thank you for your reply,<br>
<br>
Michael Brewer<br>
<br>
<div class="moz-cite-prefix">On 04/29/2020 12:00 PM, Stuart P
Littlefair wrote:<br>
</div>
<blockquote
cite="mid:CAOrDc-8T66WJ8dgynvt+jo2sX-AgyP3DcStCGWJLLs85isUGbg@mail.gmail.com"
type="cite">
<div dir="ltr">Hi Michael,
<div><br>
</div>
<div>Ignoring your third issue for the moment (for convenience).</div>
<div><br>
</div>
<div>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. </div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>Stuart</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Wed, 29 Apr 2020 at 16:48,
Michael Brewer <<a moz-do-not-send="true"
href="mailto:brewer@astro.umass.edu">brewer@astro.umass.edu</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">Dear
List,<br>
<br>
Every once in awhile, I have a colleague who wants to use
the AstroPy <br>
solar system ephemerides. I am getting tired of having to
dissuade them, <br>
so I'd like to discuss the issues that I have with these
ephemerides in <br>
an attempt to get them resolved.<br>
<br>
Issue #1: The positions of the bodies are returned in the GCRS
and there <br>
appears to be no way to easily transform them to topocentric
astrometric <br>
positions. By this I mean simply the difference in the ICRS
position of <br>
the body compensated for light time and the ICRS position of
the <br>
observer. This is rather important if one wishes to place the
body on a <br>
background map in the ICRS. It is also the only way to compare
the <br>
output of AstroPy's ephemerides with that of JPL Horizons or
Brandon <br>
Rhodes' Skyfield. Why isn't there a builtin frame for doing
this?<br>
<br>
Issue #2: Currently, there is also no builtin frame for
transforming the <br>
returned positions to apparent place. By this I mean the
topocentric <br>
position with respect to the true equator and equinox of date.
This is <br>
quite important to people such as myself who still like their
origin of <br>
right ascension to be an actual location on the sky rather
than a <br>
convenient mathematical construct. It allows one to point an
equatorial <br>
mounted telescope using the local sidereal time to calculate
the hour <br>
angle. And again, this is the only way to compare the output
of <br>
AstroPy's ephemerides with that of JPL Horizons or Skyfield.
It is also <br>
quite simple to do. Just adjust the CIRS right ascension by
subtracting <br>
the equation of the equinoxes. Note: I did find a function for
doing <br>
this in solar_system.py,
_apparent_position_in_true_coordinates(), but <br>
it feels sort of kludgy to use this. There should be a builtin
frame for <br>
this.<br>
<br>
Issue #3: This is a fairly minor quibble, but the functions
atciqz() and <br>
aticq() are calculating the gravitational light deflection
from the Sun <br>
incorrectly. The third argument of erfa.ld() should be the
time delayed <br>
heliocentric position vector of the target body. I do realize
that SOFA <br>
has this problem also.<br>
<br>
Sincerely,<br>
<br>
Michael Brewer<br>
<br>
<br>
_______________________________________________<br>
AstroPy mailing list<br>
<a moz-do-not-send="true" href="mailto:AstroPy@python.org"
target="_blank">AstroPy@python.org</a><br>
<a moz-do-not-send="true"
href="https://mail.python.org/mailman/listinfo/astropy"
rel="noreferrer" target="_blank">https://mail.python.org/mailman/listinfo/astropy</a><br>
</blockquote>
</div>
<br clear="all">
<div><br>
</div>
-- <br>
<div dir="ltr" class="gmail_signature">
<div dir="ltr">
<div>
<div>Stuart Littlefair</div>
<div><br>
</div>
<div>-------------------------------------------------------<br>
</div>
<div><br>
</div>
<div>
<div style="font-size:small"><font color="#38761d"><b><i>I
don't expect you to respond to my email outside
your working hours. </i></b></font></div>
<div style="font-size:small"><font color="#38761d"><i><br>
</i></font></div>
<div style="font-size:small"><font color="#38761d"><i>At
the University of Sheffield we value and encourage
flexible working patterns, so please be assured that
I respect your working pattern and I am looking
forward to your response when you are next working. </i></font></div>
</div>
<div style="font-size:small"><font color="#38761d"><i><br>
</i></font></div>
<div>-------------------------------------------------------</div>
<div><br>
</div>
<div>Dept. of Physics & Astronomy,</div>
<div>Univ. of Sheffield, Sheffield, S3 7RH.</div>
<div><br>
</div>
<div>email: <a moz-do-not-send="true"
href="mailto:S.Littlefair@sheffield.ac.uk"
target="_blank">S.Littlefair@sheffield.ac.uk</a> </div>
<div>phone: +44 114 2224525</div>
</div>
</div>
</div>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<br>
<pre wrap="">_______________________________________________
AstroPy mailing list
<a class="moz-txt-link-abbreviated" href="mailto:AstroPy@python.org">AstroPy@python.org</a>
<a class="moz-txt-link-freetext" href="https://mail.python.org/mailman/listinfo/astropy">https://mail.python.org/mailman/listinfo/astropy</a>
</pre>
</blockquote>
<br>
</body>
</html>