Register images to az/el using the astrometry.net program
Azimuth/Elevation converter for Astrometry.net
Note: If you want to work with the intermediate steps (source extraction) or photometry, see my AstroPy-based examples.
python3 -m pip install -e .
Astrometry.net index files
If you use astrometry.net on your PC, you may need to install the index files and setup your config file to point at them:
Command line options
--args command line option allows passing through a variety of parameters to
solve-field, which underlies this program.
solve-field -h or
man solve-field for a brief description of the nearly 100 options available.
Be sure to enclose the options in quotes. For example, to specify that the image field is at least 20 degrees in extent:
PlateScale ~/data/myimg.jpg -a "-L 20"
Citizen science images often contain extraneous items in the image field of view.
These can very easily break
solve-field, which is designed for professional science-grade imagery from telescopes and narrow to medium field of view imagers (at least to 50 degree FOV).
To mitigate these issues, judicious use of arguments passed to
--args is probably a good start.
The parameters I find most useful for citizen science images include:
-L / --scale-low <scale>: lower bound of image scale estimate -H / --scale-high <scale>: upper bound of image scale estimate -d / --depth <number or range>: number of field objects to look at, or range of numbers; 1 is the brightest star, so "-d 10" or "-d 1-10" mean look at the top ten brightest stars only.
For extraneous regions of the image, try making a copy of the original image that has the offending regions cropped out. If the original image is in a lossy format such as JPEG, consider saving in a lossless format such as PNG after cropping.
FITS image input
FITS is a legacy file format commonly used in astronomy.
Astrometry.net installed on your PC
PlateScaleFITS myimg.fits -c 61.2 -149.9 -t 2013-04-02T12:03:23Z --nc --png
gives NetCDF .nc with az/el ra/dec and PNG plots of the data. Both files contain the same data, just for your convenience.
61.2 -149.9 is your WGS84 coordinates, 2013-04-02T12:03:23Z is UTC time of the picture.
wcs.fits from the Astrometry.net WEBSITE
first rename wcs.fits to myimg.wcs:
PlateScaleFITS myimg.wcs -c 61.2 -149.9 -t 2013-04-02T12:03:23Z --nc --png
JPG image input
JPG is commonly used by prosumer cameras. It's preferable to use lossless formats for scientific imaging such as JPEG2000 or newer file formats.
ways to use astrometry.net
astrometry.net source code releases
article on good robustness of Astrometry.net to shaky, streaked images.
We use Linux or Windows Subsystem for Linux as follows:
wget http://astrometry.net/downloads/astrometry.net-latest.tar.gz tar xf astrometry.net-latest.tar.gz cd astrometry* apt install gcc make libcairo2-dev libnetpbm10-dev netpbm libpng-dev libjpeg-dev python3-numpy python3-pyfits python3-dev zlib1g-dev libbz2-dev swig libcfitsio-dev make -j make py -j make extra -j make install -j INSTALL_DIR=$HOME/.local/astrometry
add to ~/.bashrc:
open a new terminal to use.
Download index files
You will get various weird errors without the necessary star index files.
python downloadIndex.py -source http://broiler.astrometry.net/~dstn/4100/ -i 8 20
Then, edit ~/.local/astrometry/etc/astrometry.cfg, making the following changes:
- be sure
add_pathpoints to /home/username/astrometry-data, where username is your Linux username. Don't use ~ or $HOME.
- optionally, set
minwidthsmaller than the smallest FOV (in degrees) you'd ever see. For example, if NOT using a telescope, perhaps minwidth 1 or something.
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