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From Follow-up of previous ObsParis / CDS meeting



Set up for 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)
Uncheck Sexagesimal
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


(in Axes / Labels)
X Label = Longitude

Healpix generation from a table in TOPCAT

Healpix is a tesselation system on the sphere, which depends on a resolution/scale parameter. It provides regular sampling in surface, with no singularity (poles).
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.
Cell size (in FoV size, ie in longitude at Equator): scale 6 ~ 1°, 12 ~ 1', 18 ~ 1"
Map projections available with this mode are (in Axes panel) "sin" (i.e.: orthographic), Aitoff (all surface visible, minimize deformations near the poles), and "car" (cylindrical).


• 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 efficient multiresolution scaling on the fly in Aladin.


• 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)
Equatorial, Cartesian
RA symmetry = False   => this is mandatory to get the image in the correct orientation.

• Convert to HiPS :
Tool > Generate HiPS based on… > Convert current image to a HiPS

• The hpx image is saved (somewhere) on disk automatically

Open issue: this provides a correct E longitude frame. However, s_region footprints sent from an EPN-TAP service will now plot at the wrong longitude (see figure below with Huygens and Schiaparelli)
E.g. for Huygens crater, s_region = Polygon UNKNOWNFrame 55.582 -17.8305918846 54.7737623408 -17.7532983656…
(in short, s_region Unknown frame is incorrectly assumed W-handed)


TBC, did not work on my machine early 2017.

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.


- send as s_region from VESPA client (to Aladin) — (s_region is currently was then provided in this service with W longitudes, so this will project projected correctly)

- select them in Aladin with pointer