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A.4.4.3 Healpix Layer Control

The Healpix layer control () is available from the Sky Plot Window for plotting tables that represent HEALPix maps on the celestial sphere. Each row in the table must represent a single healpix tile, and a value from that row is used to colour the corresponding region of the sky plot. The resolution (healpix order) of the input table is supplied or may be guessed in order to do the plot, but the plot may be drawn at a degraded order (bigger pixels) if required.

Note this is different from the SkyDensity form, which takes a table containing sky positions, and represents that as a density map on the healpix grid by gathering the rows up into bins. The control described here works on a table for which that binning has already been done, for instance as a prepared sky map data product, or by executing a suitable (GROUP BY healpix_index) database query.

Sky Plot Window with a Healpix layer

Sky Plot Window with a Healpix layer

This control has two tabs, Data and Style.

Healpix control Data tab

Healpix control Data tab

The Data tab lets you specify the table and columns containing the healpix tile data. It has the following fields:

Table
The table supplying the data.
Data Sky System
The sky coordinate system of the grid on which the pixels in the input table are laid out.
HEALPix Data Level
HEALPix level of the (implicit or explicit) tile indices. Only values up to 13 are currently supported. This must be the value assumed by the data in the input table. If -1 is selected, an attempt is made to determine the correct level from the data; this may or may not be successful. Using the wrong value here will result in a nonsensical plot, or no plot at all.
HEALPix index
Column in the input table giving HEALPix index value. If this is left blank, then the row index is used, i.e. pixel #0 is assumed to be in the first row etc. This only works if the input table has full sky coverage and the rows are in the correct sequence.
Value
The column, or other expression, giving the colour to plot for the pixel at each row.
Row Subset
May be used to restrict the plot to one of the defined row subsets.

To control the colour map used to colour the sky tiles, use the Aux fixed control.

Healpix control Style tab

Healpix control Style tab

The Style tab lets you configure additional details of the plot's appearance. It has the following fields:

Degrade
Controls the resolution at which the pixel grid is actually plotted. If the value is zero, then the grid plotted is the same as that of the input data, but if a positive value is supplied then the healpix order will be reduced by that many. Each increment means that 4 pixels from the previous order will be combined into one. The way that combination is done is controlled by the Combine option.
Combine
Determines how pixels are combined together when the resolution is Degraded. Options are currently sum, mean, min, max.
Transparency
Adjusts the transparency of the filled area.


Next Previous Up Contents
Next: Histogram Layer Control
Up: Layer Controls
Previous: Pair Position Layer Control

TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk