[AstroPy] help with SkyCoord

[Emanuella Puddu]

Thank you Ivan,
your last suggestion should work, but I don't have the
"directional_offset_by()" method of SkyCoord in my astropy version (2.0.2).
Therefore, I build a grid of SkyCoord objects, I compute the "separation"
between the centre and each point of the grid, and then I only select and
plot the points with separation equal to the radius (within a tolerance
range).

E.













Il giorno gio 21 gen 2021 alle ore 09:03 Ivan Valtchanov <ivvv68 at gmail.com>
ha scritto:

> I may have misunderstood what was the goal.
>
> So, in this case you may try with
> centre.directional_offset_by(phi,theta) method in SkyCoord. You fix
> the centre and the offset and calculate the SkyCoord for points in a
> range of azimuth angles phi. At each phi yu will have a new SkyCoord.
>
> Cheers,
> Ivan
>
> On Thu, 21 Jan 2021 at 08:22, Mohammad Shameoni Niaei
> <m.shemuni at gmail.com> wrote:
> >
> > Hello Emmanuella.
> >
> > Thank you for sending this question. I found it challenging.
> >
> > As Ivan suggested, I tried to understand regions, CircleSkyRegion and
> find the boundaries to no avail.
> > I even tried to solve the angular distance equation for known one
> coordinates and distance. That failed too.
> >
> > But I think I have a solution. We can use Vectors for this problem.
> Let's say we have a vector (v1) pointed to x, y, z (Spherical form would be
> r, theta, phi. Where theta and phi is our reference point and r can be
> anything but better to be 1.) We can create another vector (v2) with
> angular distance of your R (Where R is your radius) and rotate the v2 about
> v1. see: stackexchange
> >
> > I once created a simple Vector library, which is installable with pip,
> named v3d .
> >
> > You can easily do the same calculations with numpy if v3d is bugged or
> is not trustable enough for you.
> >
> > An example is in attachment
> >
> > Links:
> > angular distance equation:
> https://en.wikipedia.org/wiki/Angular_distance#Equation
> > stackexchange:
> https://math.stackexchange.com/questions/511370/how-to-rotate-one-vector-about-another
> > v3d: https://pypi.org/project/v3d/
> >
> > On Thu, Jan 21, 2021 at 1:28 AM Ivan Valtchanov <ivvv68 at gmail.com>
> wrote:
> >>
> >> Hi Emmanuella,
> >>
> >> Have a look at astropy regions package:
> >> https://astropy-regions.readthedocs.io/en/latest/
> >>
> >> hope this helps,
> >> Ivan
> >>
> >> On Wed, 20 Jan 2021 at 19:11, Emanuella Puddu <manupu08 at gmail.com>
> wrote:
> >> >
> >> > Hi everyone!
> >> > I have to draw a circle around an object's center, a circle made of
> points which have the same distance from the object. This all in spherical
> projection on the sky (package SkyCoord). Does anyone have an idea about
> how this can be done?
> >> > Many thanks in advance for the help ...
> >> > _______________________________________________
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> >> AstroPy mailing list
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> >
> >
> >
> > --
> > Mohammad Shameoni Niaei
> > Astronomer
> > Atatürk University, Astrophysics Research and Application Center.
> > ERZURUM-TURKEY
> > _______________________________________________
> > AstroPy mailing list
> > AstroPy at python.org
> > https://mail.python.org/mailman/listinfo/astropy
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