What is meant by "stopping down a telescope"? (Advanced)

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.

This page was last updated June 28, 2015.

About the Author

Jagadheep D. Pandian

Jagadheep D. Pandian

Jagadheep built a new receiver for the Arecibo radio telescope that works between 6 and 8 GHz. He studies 6.7 GHz methanol masers in our Galaxy. These masers occur at sites where massive stars are being born. He got his Ph.D from Cornell in January 2007 and was a postdoctoral fellow at the Max Planck Insitute for Radio Astronomy in Germany. After that, he worked at the Institute for Astronomy at the University of Hawaii as the Submillimeter Postdoctoral Fellow. Jagadheep is currently at the Indian Institute of Space Scence and Technology.

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