Aladin Desktop is CDS’s astronomical atlas for exploring sky images, catalogues, and survey coverage on Windows, macOS, and Linux. It combines remote astronomical services with local data for visual inspection, measurements, and catalogue analysis. The stable application is free and open source.
Finding a field
Users enter an object name or sky coordinates to locate a target. Aladin resolves names through Sesame and centres the view on the resulting position. The Data Discovery Tree lists available surveys, databases, and catalogues, including Simbad and VizieR resources.
Keyword, position, time, and energy constraints narrow the collection list. A collection’s access selector offers its available query methods, such as searching around a position or within a region. Collection coverage indicators show whether the current field contains data when the provider supplies that information.
A stack of sky layers
Each image, catalogue, or graphical overlay occupies a plane in the stack. Users toggle planes, reorder them, organize folders, and adjust opacity. Catalogue positions and drawings follow the reference image’s sky projection.
Several views can compare different observations. The matching control aligns compatible views to a common scale and orientation. HiPS surveys use hierarchical tiles, bringing finer detail into the display as users zoom.
Local inputs include FITS images, JPEG and PNG pictures, and FITS, VOTable, or CSV tables. Ordinary pictures often lack sky calibration. Astrometric controls add it through parameters, star matching, or direct header editing before catalogue overlays can align.
Image calculations
Pixel controls adjust contrast, colour maps, and display transfer functions. Image arithmetic adds, subtracts, multiplies, or divides images; Aladin resamples the second image when necessary to match the first. Basic mosaic resampling uses display values, so its output cannot support photometric calculations. A survey-generation tool turns a local FITS image collection into a tiled HiPS for progressive browsing.
Users draw a circle or polygon to measure a region’s pixels. The photometric tools calculate sums, averages, medians, dispersion, and area. Saving these measurements creates a table. Pixel extraction instead produces a table of individual sky positions and values. These measurements require suitable FITS data or a HiPS survey in full-pixel mode.
Spectra from cubes
FITS cubes hold an additional sequence of image planes. Users play those frames as a movie, change their orientation, or extract an individual image. The spectrum tool samples a movable position and plots values along the cube’s third dimension.
CASSIS integration supports further analysis of spectra from points or drawn regions. Aladin does not save the cube itself.
Catalogue correlation
Users apply predefined catalogue filters or edit column conditions to control which sources appear and how their symbols look. For example, a measurement can determine a symbol’s size or colour. Scatter plots compare two columns, and selecting plotted sources identifies their locations in the sky view.
Local cross-matching compares two catalogues by position, shared field values, or positional error ellipses. For positional matching, users choose separation limits and keep nearest matches, all pairings, or sources without counterparts. Aladin creates a new result catalogue and lets users choose its columns.
Remote cross-matching sends a local table to the CDS service and joins it with a hosted catalogue. The result returns as another catalogue plane.
Queries against remote tables
The TAP interface queries remote astronomical databases. Users choose output fields and constraints through the form, or enter ADQL query text directly. An asynchronous request can continue separately, with results available for retrieval in another session. A Join control also incorporates a local catalogue into the remote query.
Coverage maps
Multi-Order Coverage maps describe where observations exist across the sky, through time, or across both dimensions. Aladin generates these maps from images, tables, or shapes that users draw. Logical operations combine them through union, intersection, subtraction, or complement.
Users can submit a selected region or coverage map to a compatible data service. A time interval also restricts the display of data that carry timestamps. Logical operations continue to use the original coverage resolution when the display reduces detail.
Sharing planes and sessions
SAMP exchanges images, tables, selected rows, and sky positions with compatible applications such as TOPCAT and DS9. Aladin provides an internal communication hub or joins an existing one. Scripts repeat loading, calculations, and exports; Java plugins extend image or catalogue processing.
Export separates reusable data from the rendered view:
- Image planes export as FITS, JPEG, or PNG.
- Catalogues export as TSV or VOTable.
- Coverage maps export as FITS or JSON.
An AJ backup preserves the stack and views for another session. View saving supports BMP, JPEG, PNG, and EPS; printing uses the displayed views.
