Orientation mask for Atmospheric Dispersion Corrector (ADC) and 200mm Newtonian.

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ADCs must be lined up with the horizon as viewed through the eyepiece, which is challenging with a Newtonian telescope. A special mask should be placed in front of the aperture and positioned horizontally using a bubble level as shown in the picture, available for £1.50 on eBay. This ensures it conceals the bottom half of the aperture. By focusing a star or planet, you'll see a half-disc image, indicating the horizon's position as seen through the telescope. Rotate the ADC accordingly, keeping in mind that the image is inverted! The smaller upper half-disc serves an important purpose: preventing the mask from accidentally falling inside the telescope. This device measures 220mm in diameter, fitting perfectly within a Sky-Watcher Explorer 200P telescope. It can be printed on a 200mm bed if you rotate it by 45 degrees.