Offset when plotting on a blank map

[Jasmine Saif]

I'm trying to run this script, and it runs beautifully except that there is an obvious shift when plotting the y axis as evident in the image produced by the script itself. If you observed the (0,0) coordinates, you'll find it's not exactly at the center.

is there a way to workaround this offset?

thanks.

[Albert Y. Shih]

Hi, Jasmine,

     I believe you are asking why the blue dot corresponding to (0, 0) in helioprojective coordinates is not at the intersection of the heliographic equator and the heliographic prime meridian.  (0, 0) in helioprojective coordinates *is* at the center of the solar disk, and at the center of the overall image.  The reason that the intersection of the two heliographic grid lines is not also at disk center is because the observer (specified as Earth in the example) is at non-zero heliographic latitude (because the Sun's equatorial plane is tilted relative to the ecliptic plane that contains Earth's orbit).  Disk center as seen by an observer has the same heliographic latitude as the heliographic latitude of the observer, and this value is known as the B0 angle, and varies over the year.

     If you want to draw a blank map where there is no offset (i.e., the heliographic equator is a horizontal line), you can specify an observer on the Sun's equatorial plane instead of using Earth as the observer, e.g.:

```

observer = SkyCoord(0*u.deg, 0*u.deg, 1*u.AU, obstime='2013-10-28',
                    frame=frames.HeliographicStonyhurst)

skycoord = SkyCoord(0*u.arcsec, 0*u.arcsec, obstime='2013-10-28',
                    observer=observer, frame=frames.Helioprojective)

```

     Let me know if you have further questions!

Albert

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[Jasmine Saif]

That did work Albert!

thanks a ton.

best,

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