[AstroPy] AstroPy Digest, Vol 114, Issue 12
Christoph Deil
Christoph.Deil at mpi-hd.mpg.de
Fri Apr 1 02:45:23 EDT 2016
There’s a function to compute the midpoint [1] available in this spherical_geometry package [2].
Looks like it uses xyz vectors, so if you have (lon, lat) you’d have to convert to xyz, compute the midpoint, and convert back to (lon, lat) using the helper functions in [3].
[1] http://spacetelescope.github.io/sphere/api/spherical_geometry.great_circle_arc.midpoint.html <http://spacetelescope.github.io/sphere/api/spherical_geometry.great_circle_arc.midpoint.html>
[2] http://spacetelescope.github.io/sphere/spherical_geometry/index.html <http://spacetelescope.github.io/sphere/spherical_geometry/index.html>
[3] http://spacetelescope.github.io/sphere/spherical_geometry/api.html#module-spherical_geometry.vector <http://spacetelescope.github.io/sphere/spherical_geometry/api.html#module-spherical_geometry.vector>
Christoph
> On 01 Apr 2016, at 00:54, Pistacho Datil <datilp at gmail.com> wrote:
>
> thanks Eric
>
> For the midpoint I went for
>
> def midpoint(coord1, coord2):
> ra1 = coord1.ra
> dec1 = coord1.dec
> ra2 = coord2.ra
> dec2 = coord2.dec
>
> delta_ra = ra2 - ra1
> bx = np.cos(dec2) * np.cos(delta_ra)
> by = np.cos(dec2) * np.sin(delta_ra)
> dec_term1 = np.sin(dec1) + np.sin(dec2)
> dec_term2 = np.sqrt(np.power((np.cos(dec1) + bx), 2) + np.power(by, 2))
> decMid = np.arctan2(dec_term1, dec_term2)
> raMid = ra1 + np.arctan2(by, np.cos(dec1) + bx)
>
> return SkyCoord(raMid, decMid, frame='fk5')
>
> not my code but it calculates the midpoint on sky.
>
> Thanks again,
>
> Frank
>
> On Thu, Mar 31, 2016 at 5:00 AM, <astropy-request at scipy.org <mailto:astropy-request at scipy.org>> wrote:
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> Today's Topics:
>
> 1. calculate a new coordinate on sky given RA and Dec offsets
> and midpoint coordinate between two targets on sky (Pistacho Datil)
> 2. Re: calculate a new coordinate on sky given RA and Dec
> offsets and midpoint coordinate between two targets on sky
> (Eric Jensen)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Wed, 30 Mar 2016 17:22:23 -0700
> From: Pistacho Datil <datilp at gmail.com <mailto:datilp at gmail.com>>
> To: astropy at scipy.org <mailto:astropy at scipy.org>
> Subject: [AstroPy] calculate a new coordinate on sky given RA and Dec
> offsets and midpoint coordinate between two targets on sky
> Message-ID:
> <CABH4OQKtS1s7js1s5823_EO4xJgOj8j19c1scTgTVeNK-jJFvA at mail.gmail.com <mailto:CABH4OQKtS1s7js1s5823_EO4xJgOj8j19c1scTgTVeNK-jJFvA at mail.gmail.com>>
> Content-Type: text/plain; charset="utf-8"
>
> Hi,
>
> First of all apologies if these two questions are too simplistic.
>
> 1) I wonder if there is a way in astropy, given a target and an offset in
> RA and Dec to calculate a new target on sky.
>
> 2) Given two targets t1 and t2, is there a way using astropy of calculating
> the midpoint coordinate also on sky?
>
> Many thanks in advance,
>
> Frank
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> ------------------------------
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> Message: 2
> Date: Wed, 30 Mar 2016 21:26:21 -0400
> From: Eric Jensen <ejensen1 at swarthmore.edu <mailto:ejensen1 at swarthmore.edu>>
> To: Astronomical Python mailing list <astropy at scipy.org <mailto:astropy at scipy.org>>
> Subject: Re: [AstroPy] calculate a new coordinate on sky given RA and
> Dec offsets and midpoint coordinate between two targets on sky
> Message-ID: <BE913B6C-F771-40D0-B316-AD15A6658362 at swarthmore.edu <mailto:BE913B6C-F771-40D0-B316-AD15A6658362 at swarthmore.edu>>
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>
> Hi Frank,
>
> For #1, here?s a way you can do it:
>
> from astropy.coordinates import Angle, SkyCoord
> from astropy import units as u
>
> # Define our initial coordinate position:
> old_pos = SkyCoord('00h42m30s', '+61d12m00s', frame='icrs')
>
> # Define the offsets:
> ra_offset = Angle(15, unit=u.arcsec)
> dec_offset = Angle(-4.5, unit=u.arcsec)
>
> # Add the offsets to create a new coordinate position:
> new_pos = SkyCoord(old_pos.ra + ra_offset, old_pos.dec + dec_offset, frame='icrs')
>
> print(old_pos.to_string('hmsdms'))
> print(new_pos.to_string('hmsdms'))
>
>
> 00h42m30s +61d12m00s
> 00h42m31s +61d11m55.5s
>
> One thing to watch out for if you do it this way: notice that the RA offset is 15 arcseconds *of RA*, not 15 arcseconds *on the sky*. At a dec of 61 degrees, 15 arcseconds of angle on the sky would be more like 30 arcseconds of RA (or in HMS, about 2 seconds of time rather than 1).
>
> If you want to specify the RA offset in real angle on the sky and convert it to the appropriate RA offset to add to the initial coords, you could change the above to do something like
>
> import numpy as np
> ra_offset = Angle(15, unit=u.arcsec) / np.cos(old_pos.dec.to <http://old_pos.dec.to/>('radian'))
>
> and then proceeding as before, you would get output:
>
> 00h42m30s +61d12m00s
> 00h42m32.0757s +61d11m55.5s
>
> For your question number 2, you should be able to just do the arithmetic with subtracting the two RA values to find the RA separation, then halve it and add it to t1?s RA, then similarly with Dec, following the example above, to get the midpoint coordinates.
>
> Both of the above answers probably break down at some level if you have large angular separations, but for small separations they should be reasonably good. I also don?t know how these coordinate object handle edge cases, like crossing over RA = 0 or Dec = 90, but you could test it out.
>
> Eric
>
>
>
> On Mar 30, 2016, at 8:22 PM, Pistacho Datil <datilp at gmail.com <mailto:datilp at gmail.com>> wrote:
>
> > Hi,
> >
> > First of all apologies if these two questions are too simplistic.
> >
> > 1) I wonder if there is a way in astropy, given a target and an offset in RA and Dec to calculate a new target on sky.
> >
> > 2) Given two targets t1 and t2, is there a way using astropy of calculating the midpoint coordinate also on sky?
> >
> > Many thanks in advance,
> >
> > Frank
> >
> > _______________________________________________
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