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

Author

This code was developed by Dominic Ford https://dcford.org.uk. It is distributed under the Gnu General Public License V3.

dcf21/star-charter

dcf21

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