What is meant by "stopping down a telescope"?
The reason why astronomers prefer big telescopes is that big telescopes collect more light and this enables the observer to detect much fainter objects. But sometimes, one might want a low power wide angle view of a solar system object. A classic example is an attempt to see the full disk image of the Moon. In this case, a large reflector will overwhelm the retina of the eye with excessive brilliance in the image. In fact, you can easily burn your eyes if you attempt to look at the full moon through the telescope without using any filters. So in these cases, where we observe bright objects at low magnifications, we need a method to reduce the amount of light transmitted through our telescope.
Stopping down an aperture (not stopping down a telescope) means putting a diaphragm across the telescope aperture. The diaphragm has a small aperture which is offset between the edge of the primary mirror and the edge of a secondary mirror. This eliminates the extra diffraction caused by the secondary mirror and causes an increase in image contrast and sharpness.
For more details and images, see the website http://www.atmsite.org/contrib/Harbour/StoppingDown.html.
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