DS9 can create and display coordinate grids as an overlay on an image. The Display Coordinate Grid Menu is used to display grids. A coordinate grid is composed of Grid Lines, Axes, Border, and Title. Axes include tick marks, title, and numbers. The appearance of the coordinate grid is specified by parameters. These parameters may be configured via the Coordinate Grid Parameters dialog box. In addition to the axes titles and the grid title, the following menus are available.
The user may specify custom numeric formats for either axes. The format specification can be empty (default) or a print function, based on the selected coordinate system:
The format specification string to be passed to the C "printf" function (e.g. "%%1.7G") in order to format a single coordinate value.
The Format string supplied should contain one or more of the following characters. These may occur in any order, but the following is recommended for clarity:"": Indicates that a plus sign should be prefixed to positive values. By default, no plus sign is used.
"z": Indicates that leading zeros should be prefixed to the value so that the first field is of constant width, as would be required in a fixed-width table (leading zeros are always prefixed to any fields that follow). By default, no leading zeros are added.
"i": Use the standard ISO field separator (a colon) between fields. This is the default behaviour.
"b": Use a blank to separate fields.
"l": Use a letter ("h"/"d", "m" or "s" as appropriate) to separate fields.
"g": Use a letter and symbols to separate fields ("h"/"d", "m" or "s", etc, as appropriate), but include escape sequences in the formatted value so that the Plot class will draw the separators as small super-scripts.
"d": Include a degrees field. Expressing the angle purely in degrees is also the default if none of "h", "m", "s" or "t" are given.
"h": Express the angle as a time and include an hours field (where 24 hours correspond to 360 degrees). Expressing the angle purely in hours is also the default if "t" is given without either "m" or "s".
"m": Include a minutes field. By default this is not included.
"s": Include a seconds field. By default this is not included. This request is ignored if "d" or "h" is given, unless a minutes field is also included.
"t": Express the angle as a time (where 24 hours correspond to 360 degrees). This option is ignored if either "d" or "h" is given and is intended for use where the value is to be expressed purely in minutes and/or seconds of time (with no hours field). If "t" is given without "d", "h", "m" or "s" being present, then it is equivalent to "h".
".": Indicates that decimal places are to be given for the final field in the formatted string (whichever field this is). The "." should be followed immediately by an unsigned integer which gives the number of decimal places required, or by an asterisk. If an asterisk is supplied, a default number of decimal places is used which is based on the value of the Digits attribute.
All of the above format specifiers are case-insensitive. If several characters make conflicting requests (e.g. if both "i" and "b" appear), then the character occurring last takes precedence, except that "d" and "h" always override "t".
The default formats are d.3 for degrees and hms.1 / dms.1 / ldms.1 for sexagesimal.