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StarCharter

StarCharter is a command-line tool for producing vector-graphics charts of the night sky in SVG, PDF and PNG formats. It can also overlay the paths of solar system objects, such as planets and comets, across the sky if the tool ephemerisCompute is also installed.

StarCharter was written to produce all the star charts on the website https://in-the-sky.org, which is maintained by the author.

Supported operating systems

StarCharter is written in C and runs in Linux, MacOS, and other Unix-like operating systems. The installation scripts require python3. StarCharter uses libcairo to produce its graphical output.

The build process requires a minimum of 4GB RAM. Note that Docker Desktop for Mac imposes a default memory limit of 2GB, which needs to be increased to build StarCharter successfully.

License

This code is distributed under the Gnu General Public License. It is (C) Dominic Ford 2015 - 2022.

Set up

Before you start, StarCharter needs to download various data from the internet, including star catalogues, deep sky catalogues, and an image of the Milky Way to use to shade the background of star charts.

This can be done with the shell script setup.sh. The total download size will be around 500 MB.

Docker container

A Dockerfile is provided to build StarCharter. A docker compose script is provided to build a selection of example starcharts:

docker compose build
docker compose run star-charter

This builds some example starcharts, which are written to the directory examples/output. To make other star charts, open a shell within the Docker container as follows:

docker run -it star-charter:v1 /bin/bash

Generating a star chart

Once you have compiled the StarCharter code, you need to write a configuration file to generate each star chart you want, specifying which portion of the sky should be charted, and what labels you want on the chart. There are some examples in the examples directory, and so a good starting point is to generate one of these:

cd examples
../bin/starchart.bin orion.sch

This will generate three star charts in the output directory, in PNG, SVG and PDF formats.

The file orion.sch reads as follows:

DEFAULTS
ra_central=5.5
dec_central=4.0
angular_width=29.0
mag_min=7
width=15.0
aspect=1.41421356
ra_dec_lines=1
constellation_boundaries=1
constellation_sticks=1
coords=ra_dec
projection=gnomonic
star_names=1
star_flamsteed_labels=0
constellation_names=1
plot_galaxy_map=1
plot_equator=0
plot_ecliptic=0
plot_galactic_plane=1
font_size=1.2

CHART
output_filename=output/orion.png

CHART
output_filename=output/orion.svg

CHART
output_filename=output/orion.pdf

CHART
output_filename=output/orion.eps

Settings are arranged in blocks which are headed by the words DEFAULTS or CHART. Settings in a DEFAULTS block do not themselves produce a star chart, but change the default settings which are applied to all subsequent charts. Settings in a CHART relate to a specific chart that is to be rendered, and do not affect any subsequent charts which may be rendered later in the same configuration file.

The file above configures a large number of parameters to produce a star chart of the constellation Orion, and makes these the default settings. It then produces three identical copies of the star chart, in three different graphic formats.

The configuration settings which are recognised are listed below under 'Configuration settings'.

Paths of solar system objects

The draw_ephemeris option in a configuration file can be used to draw the path of a solar system object across the sky. This requires the tool ephemerisCompute to be installed.

The syntax is as follows:

draw_ephemeris = <body>,<jd_start>,<jd_end>

where body is the name of the solar system object to plot, and jd_start and jd_end are the Julian day numbers of the beginning and end of the time period for which the object's path should be plotted.

Recognised object names include any of the following:

  • p1, pmercury, mercury: Mercury
  • p2, pvenus, venus: Venus
  • p3, pearth, earth: Earth
  • p301, pmoon, moon: The Moon
  • p4, pmars, mars: Mars
  • p5, pjupiter, jupiter: Jupiter
  • p6, psaturn, saturn: Saturn
  • p7, puranus, uranus: Uranus
  • p8, pneptune, neptune: Neptune
  • p9, ppluto, pluto: Pluto
  • A<n>: Asteroid number n, e.g. A1 for Ceres, or A4 for Vesta
  • C/1995 O1. Comets may be referred to by their names in this format
  • 1P/Halley. Comets may be referred to by their names in this format
  • 0001P. Periodic comets may be referred to by their names in the format %4dP
  • CJ95O010. Comets may be referred to by their Minor Planet Center designations
  • C<n>: Comer number n. n is the line number within the file Soft00Cmt.txt, downloaded from the Minor Planet Center.

Note also the setting ephemeris_autoscale, which overrides the specified celestial coordinates for the centre of the star chart, and the specified angular width, and scales the star chart to automatically show the requested ephemerides.

Configuration settings

The following settings can be included in a StarCharter configuration file:

  • angular_width - The angular width of the star chart on the sky; degrees
  • aspect - The aspect ratio of the star chart: i.e. the ratio height/width
  • axis_label - Boolean (0 or 1) indicating whether to write "Right ascension" and "Declination" on the vertical/horizontal axes
  • axis_ticks_value_only - If 1, axis labels will appear as simply "5h" or "30 deg". If 0, these labels will be preceded by alpha= or delta=
  • cardinals - Boolean (0 or 1) indicating whether to write the cardinal points around the edge of alt/az star charts
  • constellation_boundaries - Boolean (0 or 1) indicating whether we draw constellation boundaries
  • constellation_boundary_col - Colour to use when drawing constellation boundaries
  • constellation_highlight - Optionally, highlight the boundary of one particular constellation, identified by a three-letter abbreviation.
  • constellation_label_col - Colour to use when writing constellation names
  • constellation_names - Boolean (0 or 1) indicating whether we label the names of constellations
  • constellation_stick_col - Colour to use when drawing constellation stick figures
  • constellation_stick_design - Select which design of constellation stick figures we should draw. Set to either 'simplified' or 'rey'. See https://github.com/dcf21/constellation-stick-figures for more information.
  • constellation_sticks - Boolean (0 or 1) indicating whether we draw constellation stick figures
  • coords - Select whether to use RA/Dec or galactic coordinates. Set to either 'ra_dec' or 'galactic'.
  • copyright_gap_2 - Spacing of the copyright text beneath the plot
  • copyright_gap - Spacing of the copyright text beneath the plot
  • copyright - The copyright string to write under the star chart
  • dec_central - The declination at the centre of the plot; degrees
  • draw_ephemeris - Definitions of ephemerides to draw
  • dso_cluster_col - Colour to use when drawing star clusters
  • dso_galaxy_col - Colour to use when drawing galaxies
  • dso_label_col - Colour to use when writing the labels for deep sky objects
  • dso_label_mag_min - Do not label stars fainter than this magnitude limit (default: unlimited)
  • dso_mag_min - Only show NGC objects down to this faintest magnitude
  • dso_mags - Boolean (0 or 1) indicating whether we label the magnitudes of NGC objects
  • dso_names - Boolean (0 or 1) indicating whether we label the names of NGC objects
  • dso_nebula_col - Colour to use when drawing nebulae
  • dso_outline_col - Colour to use when drawing the outline of deep sky objects
  • dso_symbol_key - Boolean (0 or 1) indicating whether we include a key to the symbols used to represent deep sky objects
  • ecliptic_col - Colour to use when drawing a line along the ecliptic
  • ephemeris_autoscale - Boolean (0 or 1) indicating whether to auto-scale the star chart to contain the requested ephemerides. This overrides settings for ra_central, dec_central and angular_width.
  • ephemeris_col - Colour to use when drawing ephemerides for solar system objects
  • ephemeris_compute_path - The path to the tool , used to compute paths for solar system objects. See https://github.com/dcf21/ephemeris-compute-de430. If this tool is installed in the same directory as StarCharter, the default value should be <../ephemerisCompute/bin/ephem.bin>.
  • equator_col - Colour to use when drawing a line along the equator
  • font_size - A normalisation factor to apply to the font size of all text (default 1.0)
  • galactic_plane_col - Colour to use when drawing a line along the galactic plane
  • galaxy_col0 - The colour to use to shade the dark parts of the map of the Milky Way
  • galaxy_col - The colour to use to shade the bright parts of the map of the Milky Way
  • galaxy_map_filename - The binary file from which to read the shaded map of the Milky Way
  • galaxy_map_width_pixels - The number of horizontal pixels across the shaded map of the Milky Way
  • great_circle_key - Boolean (0 or 1) indicating whether to draw a key to the great circles under the star chart
  • grid_col - Colour to use when drawing grid of RA/Dec lines
  • label_ecliptic - Boolean (0 or 1) indicating whether to label the months along the ecliptic, showing the Sun's annual progress
  • label_font_size_scaling - Scaling factor to be applied to the font size of all star and DSO labels (default 1.0)
  • language - The language used for the constellation names. Either "english" or "french".
  • mag_alpha - The multiplicative scaling factor to apply to the radii of stars differing in magnitude by one <mag_step>
  • mag_max - Used to regulate the size of stars. A star of this magnitude is drawn with size mag_size_norm. Also, this is the brightest magnitude of star which is shown in the magnitude key below the chart.
  • mag_min - The faintest magnitude of star which we draw
  • magnitude_key - Boolean (0 or 1) indicating whether to draw a key to the magnitudes of stars under the star chart
  • mag_size_norm - The radius of a star of magnitude <mag_max> (default 1.0)
  • mag_step - The magnitude interval between the samples shown on the magnitude key under the chart
  • maximum_dso_count - The maximum number of deep sky objects to draw. If this is exceeded, only the brightest objects are shown.
  • maximum_dso_label_count - The maximum number of deep sky objects which may be labelled
  • maximum_star_count - The maximum number of stars to draw. If this is exceeded, only the brightest stars are shown.
  • maximum_star_label_count - The maximum number of stars which may be labelled
  • messier_only - Boolean (0 or 1) indicating whether we plot only Messier objects, and no other deep sky objects
  • must_show_all_ephemeris_labels - Boolean (0 or 1) indicating whether we show all ephemeris text labels, even if they collide with other text.
  • output_filename - The target filename for the star chart. The file type (svg, png, eps or pdf) is inferred from the file extension.
  • photo_filename - The filename of a PNG image to render behind the star chart. Leave blank to show no image.
  • plot_dso - Boolean (0 or 1) indicating whether we plot any deep-sky objects
  • plot_ecliptic - Boolean (0 or 1) indicating whether to draw a line along the ecliptic
  • plot_equator - Boolean (0 or 1) indicating whether to draw a line along the equator
  • plot_galactic_plane - Boolean (0 or 1) indicating whether to draw a line along the galactic plane
  • plot_galaxy_map - Boolean (0 or 1) indicating whether to draw a shaded map of the Milky Way behind the star chart
  • plot_stars - Boolean (0 or 1) indicating whether we plot any stars
  • position_angle - The position angle of the plot - i.e. the tilt of north, counter-clockwise from up, at the centre of the plot
  • projection - Select projection to use. Set to either flat, peters, gnomonic, sphere or alt_az
  • ra_central - The right ascension at the centre of the plot; hours, J2000.0
  • ra_dec_lines - Boolean (0 or 1) indicating whether we draw a grid of RA/Dec lines in background of star chart
  • star_allow_multiple_labels - Boolean (0 or 1) indicating whether we allow multiple labels next to a single star. If false, we only include the highest-priority label for each object.
  • star_bayer_labels - Boolean (0 or 1) indicating whether we label the Bayer numbers of stars
  • star_catalogue_numbers - Boolean (0 or 1) indicating whether we label the catalogue numbers of stars
  • star_catalogue - Select the star catalogue to use when showing the catalogue numbers of stars. Set to 'hipparcos', 'ybsc' or 'hd'.
  • star_col - Colour to use when drawing stars
  • star_flamsteed_labels - Boolean (0 or 1) indicating whether we label the Flamsteed designations of stars
  • star_label_mag_min - Do not label stars fainter than this magnitude limit (default: unlimited)
  • star_mag_labels - Boolean (0 or 1) indicating whether we label the magnitudes of stars
  • star_names - Boolean (0 or 1) indicating whether we label the English names of stars
  • star_variable_labels - Boolean (0 or 1) indicating whether we label the variable-star designations of stars, e.g. V337_Car
  • title - The heading to write at the top of the star chart
  • width - The width of the star chart, in cm
  • x_label_slant - A slant to apply to all labels on the horizontal axes
  • y_label_slant - A slant to apply to all labels on the vertical axes
  • zodiacal_only - Boolean (0 or 1) indicating whether we plot only the zodiacal constellations