|
Barlow Lenses, Focal Reducers
|
Barlows Lenses Barlow lenses are negative elements which increase the effective focal length of the objective. When used with an eyepiece, the magnifying power is multiplied with virtually no optical penalty.
In fact the benefits are substantial, the spherical abberation of the exit pupil is reduced, and you get the luxury of the extra eye relief afforded by a longer focal length eyepiece. If you have a Newtonian telescope, or a short-focus refractor, this is absolutely the first accessory you should buy.
|
Focal Reducers A focal reducer is a lens system that mounts on the rear cell of a telescope in
front of a flip-mirror, camera, or diagonal and eyepiece. The focal reducer acts as a wide-angle converter and changes the focal
ratio; the effective focal length of a telescope. Focal reducers are particularly useful when using affordable CCD cameras
because they use small CCD chips and thus do not view a large area of sky. Most deep-sky objects, nebulae and galaxies, are
larger than this view. A focal reducer helps to fit the image on the CCD chip. If you plan to do deep-sky photography with a CCD
camera, you probably will need to get a focal reducer. Focal reducers increase the effective viewing area of a scope and increasing
the telescope's optical, and thus photographic, speed which is beneficial for dim deep-sky objects, especially if you are using
35mm film. They also flatten the field of view for better resolution at the edge of the field.
|
|
