Grid on an image - difference with the old sunpy version and the 5.0.0

SunPy
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1

Hello,

Plotting an heliographic grid on ground-based images, I have a different behaviour with the new version of SunPy and the old one I was using (2.0.3). It seems related to the value of CDELT in the meta-data.

The resulting grid fits well the sun disk with sunpy 2.0.3:
with it’s too big with sunpy 5.0.0:

sunpy_5_0_0
sunpy_5_0_01126×1083 70.2 KB

It gets the right size when I force CDELT1=CDELT2=1

The fits file is here: https://www.sidc.be/uset/data/cameras/output_img/FTS/L1c/USET_CalciumII-K/2023/09/UCC20230923092010.FTS

And the code is:

hdul = fits.open(filename)
header = hdul[0].header
uset_map = sunpy.map.Map((hdul[0].data, header))
fig = plt.figure()
ax = plt.subplot(projection=uset_map)
uset_map.plot()
uset_map.draw_grid()
plt.show()

Any idea what could cause this difference ?

Many thanks,
Sabrina

2

Not everyone defines the radius of the Sun the same way, so we look in the FITS header for the value that should be used. When a header does not explicitly define the physical radius of the Sun (via the keyword RSUN_REF), we infer it from the angular radius of the Sun (via the keyword RSUN_OBS or SOLAR_R or RADIUS, in decreasing order of priority).

The issue here is that your FITS file has the following:

SOLAR_R =    984.6052312720685 / [pixel] estimated radius of the sun

However, sunpy is hardcoded to assume that SOLAR_R is provided in arcseconds, not pixels.

I recommend you open a GitHub issue so we can discuss whether SOLAR_R being provided in pixels is something that should be supported. In the meantime, you can work around this issue by converting SOLAR_R in your header to arcseconds, e.g.:

uset_map.meta['solar_r'] *= uset_map.meta['cdelt1']
3

Thank you for your answer ! Following your suggestion, I have posted this on GitHub.

cheers,
Sabrina