Setup Astrometry.net and usage tips

Astrometry.net is easy to use on Linux and Windows. Windows uses Windows Subsytem for Linux for Astrometry.net.

Install

After install, if you need the star index files, use: downloadIndex.py

Linux / Windows Subsystem for Linux

  1. Download/install the pre-compiled binary code

    apt install astrometry.net

  2. Install the star map data necessary to solve images.

    apt install astrometry-data-2mass-08-19 astrometry-data-tycho2-10-19

MacOS

Using Homebrew:

brew install astrometry-net

Usage

The major steps in achieving a useful WCS starfield polynomial fit are:

  1. source extraction (identifying star position in image frame)

  2. quad asterism hashing, including tolerance for star position wiggling due to noise.

  3. Match hash to star catalog, at least 3 choices are available:

  4. Bayesian decision process, find extremely high probability solution or reject.

We have written a separate article on Astrometry.new tips and techniques

Notes

[Optional] compile

This is normally not necessary, unless you want to customize/optimize.

Prereqs:

apt install libcairo2-dev libnetpbm10-dev netpbm libpng12-dev libjpeg-dev zlib1g-dev swig libcfitsio-dev

Install: If you have Anaconda/Miniconda Python as your default, Astrometry.net will use them (or whatever your default Python is). Back to at least Astrometry.net 0.70, Astrometry.net is Python 3 compatible.

curl -O https://astrometry.net/downloads/astrometry.net-latest.tar.gz

tar xf astrometry.net-*.gz

cd astrometry.net-*

make
make py
make extra

make install INSTALL_DIR=~/astrometry.net

Add to ~/.bashrc

export PATH="$PATH:$HOME/astrometry.net/bin"

do not use ~ or you’ll get error

cannot find executable astrometry-engine

Uncomment inparallel in ~/astrometry.net/etc/astrometry.cfg (or /etc/astrometry.cfg)

Copy the star index files with downloadIndex.py

python downloadIndex.py

If it can’t find the index file, be sure ~/astrometry.net/etc/astrometry.cfg contains:

add_path /home/username/astrometry/data

~ or $HOME will NOT work!

Reference paper

Program giving azimuth/elevation for each pixel of your sky image

Alternative: online astrometry.net image scaling