Re: [APML] light loss

[Chris1011@aol.com]

In a message dated 9/20/2001 9:41:18 PM Pacific Daylight Time, Tonyhallas@aol.com writes:


> ,
>
>    It's true that the large obstruction blocks a significant amount of light, but remember that it is in the center... most of the area of a mirror system is at the edges...
>
>     


It's easy to figure the light loss, whether at the edge or center. A 25% obstruction loses 6.25%, a 35% loses 12.25%, a 40% loses 16%, and a 50% loses 25%. A secondary effect is that the peak of the central Airy disc also drops down significantly on top of the shading loss, so that faint stars will diminish faster than nebulae (extended objects are not affected by Airy Disc brightness). 

Further more, beyond about 40%, the first diffraction ring starts to become nearly equal in brightness with the central Airy disc, and begins to merge with it, so that even the slightest atmospheric disturbance causes a very dramatic loss of point resolution. In other words, between 40 to 50% obstruction, a scope starts to resolve stars as if it were only half or less in diameter, although the brightness of the image is not affected. If you go to my web site and examine the image of M13, for example, it would be difficult to resolve the globular to this level with a 50% obstructed system. But then, some people like the look of the cluster when the stars are all bloated because it makes the image look brighter. To each his own, I guess.

Roland Christen