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Effect Of Central Obstruction (secondary)

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Posted by Steve Bodenheimer on May 3, 2001 13:17:10 UTC

Hey Dude,

Light gathering capacity is calculated based on the surface area of the lens (aperture) of a telescope. For a refractor with no central obstruction (secondary mirror), the area is pi times (radius)squared. For a reflector, the area is the area of the primary minus the area of the secondary. For a reflector diameter of 4.5 inches, the secondary mirror would need to be about 70 mm in diameter to reduce the light gathering capacity to that of an unobstructed 90 mm aperture. However typical secondaries in Neutonians are on the order of 20% of diameter (23 mm in the case of 4.5 in aperture) and can range up to about 35% of diameter in Mak and Schmidt Cassegrain visual scopes (40 mm). There is also some loss of light each time it passes through a lens or is reflected by a mirror, but the loss is minor and doesn't significantly change the above. On the other hand there is a loss of contrast and resolution due to diffraction around the edges of the secondary, that quantitivly exceeds the loss in light gathering capacity. Unless the secondary is extremely large, even this loss wouldn't reduce the contrast of a 4.5 inch reflector to that of a 90 mm refractor. See:
http://perso.club-internet.fr/legault/obstruction.html and:
http://www.laughton.com/paul/rfo/obs/diffraction/diffraction.html
for more detail.

Hope this helps.

Steve Bodenheimer

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