From previous ObsParis / CDS meeting
Using Aladin prototype v10 (6/6/2017) and TOPCAT v4.4 on MacOS
- See also page: Aladin & planetary surfaces for older use cases / examples.
Standard planetary mapping in TOPCAT
Assuming longitude is E-handed (EPN-TAP convention, and IAU convention in planetocentric frames):
• Standard setup for planetary maps in TOPCAT SkyPlot:
(in Axes / Projection)
Projection = sin (i.e. sphere, for orthographic), but car (cylindrical) is also possible
Uncheck Reflect longitude axis,
View Sky System: Equatorial,
(in Axis / Grid)
Increase Grid Crowding cursor value to 30 or 60° tick
• Standard setup for planetary maps in TOPCAT PlanePlot (cylindrical):
(in Axes / Range)
Set Max X value to 360
Central meridian will be on left side
• To put the central meridian at the center:
(in Axes / Range)
Min / Max X = -180 / 180
(in Mark / Position)
X: LON > 180 ? Lon - 360 : Lon
(in Axes / Labels)
X Label = Longitude
Healpix generation from a table in TOPCAT
Healpix is a tesselation system on the sphere, depending on a resolution/scale parameter.
The point is to easily get a combined map from sparse data; the result is also scalable in resolution, at least degradation is performed correctly by combining healpix cells.
In TOPCAT, load a table containing a list of observations located in lon/lat
(working example is a large table of VIRTIS/Rosetta observations of 67P)
1) Standard way:
• Open Plane Plot panel, select table and Lon & Lat parameters in Position panel
• Go to Form panel, click on the big Forms button (green cross) and "Add SkyDensity" from the local menu. Uncheck the previous Mark Form
• Go back to Position panel, select Lon and Lat parameters (C1min and C2min for an EPN-TAP view)
• Back to the Form panel, select parameter to plot as Weight (will be weighted by density), select combine mode, and adjust scale. Healpix generation is performed on the fly, so all scales are OK.
• Possibly adapt color scale in the Aux Axis tab (left menu)
• In the Form panel, you can save the healpix map (cell #, plotting parameter) - saved at scale 12, apparently (FITS formatting is more efficient than VOTable, as it is compressed). You can also load an existing map from there.
• Add new synthetic column providing healpix cell number (right-click on table column header and enter new column description):
healpixNestIndex( 6, longitude, latitude )
(adapt scale to data in use, 6 provides a resolution of ~ 1° at the Equator)
• Open Skyplot panel, add New healpix layer, provide inputs
Correct Healpix Data Level in particular (TOPCAT will try to guess, but this is uncertain)
• In Skyplot / Style panel, use Degrade and Combine mode to reduce resolution on the fly (super-resolution can no longer be achieved)
Mars HiPS generation in Aladin
HiPS (Hierarchical progressive survey) are associations of healpix maps at different scales. They provide multiresolution scaling on the fly in Aladin.
• In Aladin, load file Mars_MGS_colorhillshade_mola_1024.jpg
(MOLA integrated relief map)
• Go to Image > astrometric calibration:
Coord = 0.,0.
pos x/y = 512, 256 (lon = 0°, lat = 0° at image center)
size = 21.1' (= 360. / 1024)
Symetry: No => this is mandatory to get the image in the correct orientation.
But this currently provides a W longitude frame (entering Yes results in a reverse image (seen from the inside), not in a reverse frame).
Edit > properties: ascending longitudes - does not change anything here
• Convert to HiPS :
Tool > Convert current image to a HiPS
• The hpx image should be saved on disk automatically
MOC use case in Aladin
MOC (Multi-Order Coverages) are footprints defined in terms of healpix cells list mixing various resolutions. They provide immediate location information e.g. about intersections.
- Load Mars MOLA hpx in Aladin
- Select two large craters in Mars Craters service from VESPA client
- send as s_region from VESPA client (to Aladin) — (s_region is currently provided in this service with W longitudes, so this will project correctly)
- select them in Aladin with pointer
- Generate common MOC for these 2 craters (Coverage > Generate MOC > from objects)
- You can save it from File > Export Planes (save preferably as fits for reuse)
This is OK and very quick.
• Then: test intersections with something else.
How do we do that?