Uranus – Stop Snickering

Stack of 10 images 800 iso 45 seconds 200mm f/4 cropped to about 1/3

The image is not inspiring and, at magnitude 5.7 Uranus is not a tough target but i’m pleased for a number of reasons:

  • It’s Uranus for pete’s sake, 3 billion kilometers away
  • I found it in the image by using the WCS file that astrometry.net produces
  • I learned about relating my images to stellarium’s displays
  • There are a bunch of faint stars that i can get my eye in on

I routinely do image recognition by uploading to astrometry.net. Besides the annotated image as below, it provides a WCS.fits file that provides the info you need to map from astronomical coordinates (RA and Dec) to pixel locations in the image. The python script below reads the WCS file and uses astropy to determine the pixel location for uranus.

from astropy import wcs
from astropy.io.fits import getheader
import numpy as np
from astropy.io import fits
from astropy import units as u
from astropy.coordinates import SkyCoord as SC

hdulist = fits.open(r"c:\users\bill\downloads\wcsuranus.fits")
w = wcs.WCS(hdulist[0].header)

pixcrd = np.array([[0, 0]], dtype=np.float64)
world = w.wcs_pix2world(pixcrd, 0)
print("RA/DEC for [0,0]: ",world)

wcrd=np.array([[36.542, 14.101]])
print("Pixel Coords for center as given by astrometry: ",pixcrd2)

uc=SC('2h27m45.84s', '14d6m14.0s')
print("Uranus's Coordinates(J2000): ",uc)
print("Uranus's Pixel Coordinates: ",uc.to_pixel(w))

I finally realize that i can line up my images with the stellarium displays if i put stellarium into equatorial mount mode and rotate it’s image right 90 degrees. I still find this a bit confusing but the equatorial mode is key.

Oh, also, I think the faintest stars I can reliably pick out are around magnitude 11-12 so i’m a long way from pluto’s 15.


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