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mirror image. Lenses often spin the image 180 degrees: most refractors invert the image totally. So do Cassegrain telescopes, like the popular "SCT" (Schmidt- Cassegrain) telescopes. Coude telescopes will invert only on one axis. The next line states "Rotation 0"; you can use this to add any arbitrary rotation you want. People with unusual telescopes might want to add a final "spin" of 20 degrees to their charts, for example. The next item tells you that North is up (i.e., at the top of the screen). So what? Isn't North always up on a chart? Well, not quite. What Guide is telling you is that celestial North (the direction to the North Pole) is at the top of the chart. Most star atlases are printed that way. However, as the earth turns, the sky seems to turn. Thus, the Big Dipper, which seems to be "right side up" when it's close to the northern horizon, looks "upside down" when you see it high in the sky. You would usually turn a paper star chart until it looks correct. Also, if you have the "show ground" or "horizon objects" options set in the Background dialog, these may appear tilted. If you select "alt/az (zenith up)", the chart will rotate to show the zenith (point directly overhead) at the top of the chart. If you've entered your latitude and longitude correctly, and if the clock is set to the right time and time zone, the chart will now appear "right side up", and will match what you see in the sky. If you have the horizon turned on, and it happens to be in the field of view, it will now be drawn straight across the screen, instead of at an angle. Since Guide is still centered on the same point in the sky, the effect will be to take the chart and rotate it, possibly until it is upside down. Having the zenith at the top of the chart is particularly useful if you have an "altitude/azimuth" type of telescope mounting, such as a Dobsonian. It is also useful for making charts showing a large part of the sky (i.e., low level numbers) for naked eye observing. The last two radio buttons, "Ecliptic north up" and "Galactic north up", are not nearly as useful. But sometimes, people do want charts aligned with these systems. 9: OVERLAYS MENU The Overlay menu lets you create and edit your own overlays, showing lines, circles, and text, on top of the charts. It also provides a few example overlays, showing constellation borders, names, and outlines, plus the pages in the AAVSO atlas and the plates in the Palomar Sky Survey. For example, you can use your own overlays to show areas you want to view in an observing session, or to label some objects or points of interest, or to add comparison magnitudes to a chart, or for any other use. When you enter the Overlay menu, it will look like this:
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