| article: barlow lens projection astro photography |
The Barlow Lens Projection increases the focal length of a telescope by a factor, called amplification factor.
It is also called negative projection.
It works close to a teleconverter known from photography, just the other way round.
If the Barlow Lens is a 2x one, the factor will be around 2x.
Barlow Lenses can be bought up to 5x.
When used for astrophotography the factor will slighty be bigger than noted on the Barlow lens because of the different distance of the lens to imager compared with an eyepiece.
You can expect a 2x to work closer to 2.5x effective amplification.
Of course it is possible to combine two different ones working like a single one with multiplicated amplification: 2x and 3x is finally like 6x.
a 3x Intes and a 5x TeleVue Powermate
The Barlow Lens itself is usually a group of lenses with a negative power and build into a tube that fits the focuser.
Prefereably the group of lenses is apochromatic (APO) not to introduce chromatic aberration.
The open side of the tube can hold an eyepiece or the imager like a CCD camera.
With the help of a Barlow lens it is possible to image small objects, e.g. the planets, in a size which is acceptable even for telescopes with small focal length of about 1 meter.
You can calculate the amplification factor M by this formula:
M = ( s2 - F2 ) / F2
The effective foacal length is then:
F = F1 * M
Because Barlow Lenses do not need much back-focus they can be used with any telescope including Newtonian Reflectors.
Another method of increasing the size of an object while imaging is eyepiece projection.
|
|
|
