The settings in this view let you select the coordinate system used to display the sky chart, and precisely center the chart on a particular set of coordinates. You can also set the field of view width and orientation, and display or hide the chart center coordinates and field of view while you are swiping or pinching the chart.
This sets the coordinate system used by SkySafari. There are two options that you can choose here:
Horizon - In the Horizon (or "Alt-Az") coordinate system, altitude is how high in the sky something is, and azimuth is the direction around the horizon. This system is used to show an object's position in the sky relative to your local horizon line.
Equatorial - In the Equatorial (or "RA-Dec") coordinate system, RA stands for Right Ascension, and Dec stands for Declination. These coordinates are akin to longitude and latitude on the Earth. The Equatorial system is aligned with the Earth's equator and rotates with the Earth, so the coordinates of objects in the sky do not change as the Earth turns. Hence, Equatorial coordinates are commonly used with printed star atlases.
When using Equatorial coordinates, the horizon is not visible in the sky chart.
Center Azm (or RA): Sets the azimuth at the center of the sky chart. North is 0°, East is 90°, South is 180° and West is 270°. You may enter a new azimuth to precisely set the chart's center.
Center Alt (or Dec): Sets the altitude at the center of the sky chart. At 0° the chart is centered on the horizon, at +90° it is centered directly overhead at the zenith, and at -90° it is centered directly under your feet.
When using Equatorial coordinates, the chart center RA and Dec are always assumed to be for the precession epoch specified in the Precession settings.
When using Horizon coordinates, the chart center altitude is assumed to be apparent (i.e. it includes the effects of atmospheric refraction) if the Refraction option is turned on in the Precession settings. If this option is turned off, the chart center altitude is assumed to be the true (un-refracted) altitude.
Field Width Angle: Sets the sky chart's field of view width angle using a slider control.
The largest field of view SkySafari can display is 180 degrees, letting you see the entire sky at once. As the field of view increases past 90 degrees, the horizon becomes curved, due to the distortion caused by projecting the entire celestial sphere onto the flat iPhone screen.
If you hold your iPhone at arm's length, about 2 feet from your eyes, its 2-by-3 inch screen has an apparent size of 4.8 by 7.2 degrees. So, if you set the field of view width to 4.8 degrees (portrait mode) or 7.2 degrees (landscape mode), and hold your iPhone out at arm's length, the view on your iPhone should appear at the same scale as the real sky.
The smallest field of view SkySafari can display is 0.1 arcminutes, or 6 arcseconds - about the average size of the planet Mars as seen from Earth. One arcsecond is the best resolution a typical 8" backyard telescope can achieve under good observing conditions.
For comparison, the Sun and Moon appear about 1/2 degree (or 30 arcminutes) across. The smallest angle the unaided human eye can resolve is about 1/30th of a degree, or 2 arcminutes - about 1/15th the width of the full Moon. At its closest approach to Earth, the planet Venus appears about 1 arcminute across; Jupiter typically appears appears 45 arcseconds across.
Flip Horizontally: "On" flips the sky chart display horizontally to match the view in a telescope whose optical design results in a mirror-image view.
Flip Vertically: "On" flips the sky chart display vertically to match the view in a telescope whose optical design results in an upside-down view.
SkySafari can use the compass and altimeter built into your device to center the sky chart on the direction you're holding your phone. If your device does not have a compass, this option is disabled.
Shake to Use: When turned on, you can shake your iPhone or iPod Touch to activate the compass and/or altimeter. One quick shake will do it! Once activated this way, touch the screen anywhere to turn the compass/altimeter off. Turn "Shake to Use" off if you find that you're accidentally activating the compass/altimeter too often, or if you prefer to activate them from the main toolbar.
Please Note: for best results with the compass, turn your phone sideways to landscape mode. Also, the compass and altimeter will be turned off if you connect to a telescope, or lock on the telescope's position in the sky chart. The sky chart cannot be centered on the telescope's position, and centered on the coordinates reported by the compass/altimeter, at the same time. See the Scope Control view Help page for more information.
When the compass/altimeter have been activated by shaking, the coordinate system will change to Horizon coordinates (see above).
This items in this section control how the chart center coordinates and field of view are displayed at the top of the sky chart.
Show Always: When turned on, the chart center coordinates and/or field of view are always displayed at the top of the sky chart. When turned off, these items are only displayed when you swipe or zoom the chart, and will fade after you remove your finger from the screen.
Field Width x Height: Sets whether the sky chart's field of view is shown in the upper right while changing the field of view.
Center Coordinates: Sets whether the coordinates at the center of the sky chart are shown in the upper left while changing the field of view.