[AstroPy] Vizier projection

[Gabriel Perren]

You are right Derek, it was just the conversion factor back to degrees.
Thank you!

El lun, 30 de ago. de 2021 a la(s) 15:02, Derek Homeier (
derek at astro.physik.uni-goettingen.de) escribió:

> On 30 Aug 2021, at 7:26 pm, Eric L. N. Jensen <ejensen1 at swarthmore.edu>
> wrote:
>
>
> The correction factor is to multiply by the cosine of the other
> coordinate.  So for example if you plot RA*cos(Dec) as your X coordinate
> and Dec as your Y coordinate, you should get a plot that is 0.2 x 0.2
> degrees, and which looks a lot like the _x, _y plot (but without the mean
> value subtracted).
>
> In a small field like this simply subtracting the mean will get you
> something that’s pretty close to the correct _x, _y values, except that it
> doesn’t take into account the curvature of the celestial sphere.
>
> Just from visually comparing your middle and right plots the results from
> your implementation of the rotation matrix are looking quite identical to
> the Vizier columns, Gabriel – except the scales are smaller by a factor
> ~60,
> or more likely, 180/pi. You are plotting the output from np.arccos(u) * …
> which will be in radians again, but the Vizier _x, _y are probably in
> degrees,
> just as the coordinates themselves.
>
> A more correct way to do it would be to convert each of your data points
> to a SkyCoord in astropy, and also to make a SkyCoord for your field center
> coords.   Then you could find coord1.separation(coord2) to get the angular
> distance between any two points.
>
> Yes, coord1.separation(coord2) * [np.cos(coord1.position_angle(coord2)),
> np.sin(coord1.position_angle(coord2))]
>
> should produce the same [x, y] results, but note that this operation is
> not commutative –
> if you exchange the centre-of-field coord and object coord, you will not
> get the same
> (rsp. exact negative) offset, since the cos(dec) term is used from the
> first position.
>
> HTH,
> Derek
>
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