en:documentation:chart_coordinates

# Chart, Coordinates Setting

From the menu: Setup → Chart, Coordinates
The Chart, Coordinates Setting has six tabs:

- Chart, Coordinates
- Field of Vision
- Projection
- Object Filter
- Grid Spacing
- Object List Setting

## Chart, Coordinates

### Chart Setting

Here, you can specify the coordinate system used for the chart. The four available systems are:

- Equatorial coordinates
- Azimuthal coordinates
- Galactic coordinates
- Ecliptic coordinates

### Type of coordinates

Precession, nutation and aberation recognition Your choice here determines which type of coordinates SkyCharts will display for a selected object on the status bar, on the bottom of the chart. If you don't check the Expert mode, you can choose in the groupbox Type of coordinates from four options:

- Apparent, true equator, equinox and epoch of the date
  This is the true apparent position for the equinox of date of the chart, including correction for proper motion, precession, nutation, aberration, light deflection. Normally you want this setting as this is the only to show the real position of the objects.
- Mean of the date, mean equator, equinox and epoch of the date
  The position for the equinox of date of the chart, including correction only for proper motion and precession. Use this setting only to compare the coordinates with other source that use this system, like printed almanac
- Mean J2000, mean equinox and epoch J2000
  The mean position for the date 2000.0, including correction for proper motion and precession at 2000.0. Use this setting only to compare the coordinates with other source that use this system, like printed atlas.
- Astrometric J2000 , mean equinox J2000 and epoch of the date
  Use the equinox 2000.0 and precession for 2000.0, but proper motion for the current chart date. This setting is only used to compare the coordinates with an astrometry software.

In Expert mode you can specify more details :

- Equinox year (from -20000 to 20000)
- Epoch year (from -20000 to 20000)
- Proper motion checkbox to correct positions for the given epoch
- Mean position or True position for Nutation and aberration.

Fast ways to make simple changes to the chart coordination system are from the menu by Chart → Coordinate Sytem → [Your Choice], or directly on the chart by the icons in coordinate system group.

## Field of Vision

Here you can specify eleven ranges as the Field of Vision (numbered from 0 to 10).

For each range you can specify the minimum FOV in degrees, which automatically will become the maximum for the preceding range.

The first range minimum is 0.0° and last range maximum is 360° (which funny enough appears as 0.0°), these values cannot be modified.

These ranges are listed at bottom of all the Catalog dialog box tabs and also are used with the Projection, Object Filter and Grid spacing tabs.
Changing the FOV itself can be done from the menu by Chart → Field of Vision or directly on the chart by the icons in the field of vison group. A very precise FOV can be manually set by the FOV part in the dialog box from the menu View → Field of Vision (FOV).

## Projection

For every FOV range you can choose from four projection types:

- ARC Zenithal equidistant. It corresponds to the projection of a Schmidt camera.
- TAN Gnomonic. This is the default projection up to 90°. Corresponds to the projection of a picture obtained with a telescope or a photographic lens. The tangent projection has a great distortion for fields larger than 90° and diverge for 180°
- SIN Slant orthographic. Used to display images in radio-astronomy. The sine projection overlaps at more than 90°.
- CAR Cartesian. It is of no great interest, apart from the fact that it can display very large field of views.
- MER Mercator. A good projection for large fields but diverge at the pole. This is the default projection between 90° and 360°.
- HAI Hammer-Aitoff. A compromise often use for large fields.

The three zenithal projections by E. Griessen, AIPS memo 27

By default the projection CAR, MER and HAI are oblique, i.e the projection equator is moved at the chart center. This minimize the distortion on the chart.
If you want the classic aspect of this projections you need to remove the corresponding check box.
See the projection comparison page for more information.

## Object Filter

By this tab you can configure magnitude limits to display stars and deep sky objects based upon the FOV of your chart.

The Stars Filter can be:

- disabled. (only practical with the smaller FOVs),
- automatic. By this you can specify a magnitude as Naked eye reference,
- manual. Now you can specify a magnitude limit for every FOV range.

A funny excercise: There's no technical problem to disable the stars filter in combination with a large active star catalog (i.e. HST GSC), a high value for field number max (i.e. 6) and a FOV of 20 degrees. Now you can see why a disabled Stars filter in a large FOV is not very convenient.

Deep Sky Filter can be:

- disabled (only practical with the smaller FOVs),
- manual. Here you can specify the magnitude limit for every FOV range.

You can also filter large deep sky objects specifying the maximum dimension in minutes.

## Grid Spacing

By this tab you can set the grid spacing for every FOV range.

- Degree is for the spacing altitude or declination direction
- Hour is for the azimuth or right ascension direction

You can enable or disable the grid display for every FOV range.

You can enable or disable the display of a compass, you also can adjust its size or disable it.
You can use a simple N/E pointer instead of the compass.

You can enable or disable the display of grids from the menu by Chart → Lines / Grid → [Show coordinate grid/add equatorial grid]

## Object List Setting

By this tab, you determine which object types are to be filtered from your chart to your Object List. Click on the icon from the main bar to retrieve your filtered list of objects displayed on the chart.