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the sky is visible, at a given time, with that telescope/eyepiece. In entering the aperture size, you can also enter 15' for a 15 arcminute circle, or 45.3" for a 45.3 arcsecond circle. Also: when any of these objects appears on-screen, you can adjust its display by right-clicking on it, then clicking "Display". The only time it is truly necessary to go through the "Ticks, Grids, Etc." menu option is when the object type you want isn't on screen (perhaps because it is turned off.) 7g: Background dialog In the Background dialog in the Display menu you may modify the usual "white stars on a black background" appearance of the screen. There are five different types of background: "normal colors", "chart mode", "red mode", "flashlight mode", and "realistic". "Normal Colors" is the default; the background is usually black. Printouts always have black stars on white (even with color printers). "Chart Mode" switches to a white background. This was first added to support color printing. If you have a color printer, you can switch to Chart Mode, set colors to match what you want to see printed, and then do a printout. It would be very clumsy to set up colors if you didn't have a way to turn the screen background to white. It has turned out to have other uses, however, and some people prefer it to "normal" mode. The "Red Mode" option converts everything in the chart, and many controls outside the chart, to shades of red and black. This can be significantly less damaging to dark-adapted vision, and is a common choice among people running Guide in the field. If you run Guide in the field without using this option, your eyes will never fully adapt to darkness, and you won't see as much in the sky as you ought to. By default, "red mode" is indeed red. But you'll see a button provided with that default shade of red, and can click on it to select a brighter or dimmer shade of red. Or you can choose a different color; there is a school of thought that holds that a dim shade of green is really the best color for preserving night vision. In "Flashlight mode", the background is red and markings are black. The result is a bright screen that won't damage night vision. It is sometimes useful in finding dropped eyepieces and such: you can switch to flashlight mode, find the missing object, and switch back to red mode. Of course, most people will use a real flashlight with a red filter for that purpose. "Realistic mode" shows a bright blue sky in the daytime and a dark black background at night. Between sunset and true night (when the sun is more than 18 degrees below the horizon), shades of blue denote the progress of twilight. This gives you some clue to the visibility of an
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