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

ADCs must be aligned with the horizon as seen through the eyepiece, which is difficult with a Newtonian.
This mask should be placed in front of the aperture and orientated horizontally using the bubble level as shown in the picture (£1.50 on ebay) so it hides the bottom half of the aperture. A defocused image of a star or a planet will then look like a half-disc, showing the position of the horizon as seen through the telescope. You can then rotate the ADC accordingly. Don't forget the image is