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#### A.4.10.2 Sky Axes Control

The Axes control () for the sky plot window has the following tabs:

Projection Tab

Projection tab of the sky Axes control

The Projection tab controls how the sky position coordinates are projected onto the screen.

Projection
  Selects the sky projection to use. The options are Sin; Aitoff or Aitoff0 for Hammer-Aitoff; and Car or Car0 for Plate Carrée. Sin is rotatable, the other two are essentially flat all-sky projections. Car and Aitoff have longitude=0 at the center of the plot, while Car0 and Aitoff0 have it at the left/right edge. Note that for historical reasons the projections named Aitoff denote the equal-area Hammer-Aitoff projection and not the Aitoff projection itself. The Hammer-Aitoff projection is defined as:

             x = [2sqrt(2)/sqrt(1+cos(lat)cos(lon/2))]cos(lat)sin(lon/2)
             y = [sqrt(2)/sqrt(1+cos(lat)cos(lon/2))]sin(lat)

Reflect longitude axis
  Determines whether longitude increases left to right or right to left.
View Sky System
  Determines the sky coordinate system in which the data positions will be viewed. This interacts with the Data Sky System selected in the Positions tab of the table layer control for data coordinates; supplied data points are projected from the data system to the view system before being plotted. If you have (for instance) data in equatorial coordinates that you want to view in galactic coordinates, then select the Data Sky System as Equatorial and the View Sky System as Galactic. If the data and view systems are the same, it's OK to leave both as their defaults, even if they're not equatorial - the only effect that the chosen Data and View sky systems has is from the transformation between them.

Navigation Tab

Navigation tab of sky Axes control

The Navigation tab controls details of how the navigation works. It has the following option:

Zoom Factor
  Controls the factor by which each zoom action zooms the plot. Moving this slider to the left/right makes the mouse more/less sensitive (one wheel click or dragging a fixed distance has more/less zoom effect).

Field of View Tab

Field Of View tab of the sky Axes control

The FOV (Field Of View) tab allows you to enter a sky position or object name and a radius and positions the view at that region. Filling in the position and radius fields and hitting return will reposition the sky view, but not vice versa; normal pan/zoom operations will not affect the content of this panel.

Object Name
  If you fill this in with the name of a celestial object and hit the Resolve button, a Simbad query will execute to fill in the RA and Dec fields with its position.
RA/Dec
  J2000 positions of the required field centre. These values can either be filled in by the object name resolution as described above, or by hand.
Radius
  Gives the radius of the desired field of view.
Clear
  Clears the fields in this tab.

Grid Tab

Grid tab of the sky Axes control

The Grid tab controls how the sky coordinate axes appear.

Draw Grid
  If selected, grid lines will be drawn on the plot.
Draw Scale Bar
  If selected, a small annotated scale bar will usually be drawn at the bottom left of the plot indicating the scale of the image in degrees, minutes or seconds.
Sexagesimal
  If selected sky coordinate annotations of the grid will be in sexagesimal format, otherwise in decimal degrees.
Grid Colour
  Selects the colour with which grid lines will be drawn.
Grid Transparency
  Controls the transparency of the grid lines, which are drawn over the plot content.
Label Colour
  Selects the colour in which axis label text will be written.
Grid Crowding
  Use the slider to control how closely packed grid lines are on the axes. If you want to control the crowding separately on the two axes, you can use the SkyGrid layer control instead.
Label Positioning
  Controls whether and where the numeric annotations of the lon/lat axes are put. The default option Auto usually does the sensible thing, but other options exist to force labelling internally or externally to the plot region, to further annotate the axes with View Sky System coordinate names, or to remove numeric labels altogether.
Antialiasing
  Controls whether grid lines will be drawn antialiased (smoothed) or not. This option does not affect exported plots.

Font Tab

Font tab

The Font tab configures the font used for axis annotation. It also affects some other things like the legend.

Text Syntax
  How to turn the text into characters on the screen. Plain and Antialias both take the text at face value, but Antialias smooths the characters. Antialiased text usually looks nicer, but can be perceptibly slower to plot. At time of writing, on MacOS antialiased text seems to be required to stop the writing coming out upside-down for non-horizontal text. LaTeX interprets the text as LaTeX source code and typesets it accordingly.
Font Size
  Size of the font in points.
Font Style
  Style of the font.
Font Weight
  Whether the font is plain, bold or italic.

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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