Quote:
Don,
I'm not clear on one point. If the entire primary mirror is utilised by any eyepiece, why does the brightness of the field reduce at higher magnifications? If an eyepiece uses a smaller portion of the telescope's focal plane, resulting in a smaller exit pupil, isn't this equivalent to reducing the aperture size?
Regards, Rob.
|
Rob,
The eyepiece magnifies the focal plane to a larger size. The higher the magnification of the eyepiece, the more the small amount of the focal plane being viewed is magnified. In the hypothetical example I listed, a 28.5' field is expanded to 68 degrees by the eyepiece. That is a 142X magnification of the field being viewed, in width. In area, it's 20,164X the area. That means the eyepiece spreads the background sky brightness out from 28.5' to 68 degrees. The same amount of light now covers an area 20,164X as large, meaning the surface brightness of the background went down 10.8 magnitudes.
It's not the equivalent of reducing the aperture. The brightness of each square millimeter on the focal plane is related to the aperture of the scope. The eyepiece has no effect on the brightness AT THE FOCAL PLANE, while reducing the aperture would.