Products

Accessories: Binoviewer: Focusing

Under Construction! Please check back.

Reaching Focus

Currently, the Burgess Binoviewer does not come with a corrector (commonly known as an OCA - an Optical Corrector Adaptor). In order to reach focus in a telescope, the focus tube needs to be racked in an amount equal to the optical length added by the binoviewer, which is about 4 inches (100mm). That is, from the point where focus is normally reached for an eyepiece, the focuser must be moved in by about 4 inches to accommodate the optical length added by the binoviewer. A corrector works like a Barlow and allows the binoviewer with eyepieces to be almost parfocal with just the eyepiece without the binoviewer.

Reflectors and Newtonians: The binoviewer must be used with a corrector in order to reach focus in a reflecting type telescope. Alternatively, the distance between the primary mirror and the focuser can be shortened to accommodate the extra optical path used by the binoviewer and the corrector need not be used. Some users have moved their primary mirror up the tube to use the binoviewer without a corrector. Unless the secondary mirror is increased in size, the fully illuminated spot at the focal plane may be reduced by moving the primary mirror.

Schmidt-Cassegrain (SCT) and Maksutov-Cassegrain (mak): The binoviewer can be used without a corrector with most SCTs and maks. These types of telescopes typically move the primary mirror to focus the telescope. The range of focus adjustment is typically sufficient that the binoviewer can be used without a corrector. Please note that when the primary mirror of a Cassegrain type telescope is moved, the focal length of the telescope changes slightly.

Refractors: The binoviewer must be used with a corrector in order to reach focus in most refractors. However, some refractors will reach focus with only the binoviewer. Some manufacturers, such as TMB, have a removable section in or near the focuser that effectively shortens the telescope tube, thereby accommodating the additional optical path of the binoviewer. Other refractors, such as the Burgess Optical Backpacker scopes, have a slightly shortened tube that allows the Binoviewer to be used with a 1-1/4" diagonal without a corrector.

Barlow as Corrector

It is possible to use a Barlow in place of a corrector. A Barlow is a negative lens, and the further the eyepiece focal plane is moved away from the negative lens, the greater the Barlow magnification.

If the binoviewer is inserted in the Barlow just as an eyepiece would be inserted, the Barlow will result in almost double the Barlow's rated magnification. That is, a 2x Barlow used this way will result in almost a 4x magnification factor.

If the Barlow has a removable element and that element is screwed into the binoviewer nosepiece much as a filter would be, then the 2x Barlow will have approximately 3x magnification factor.

The Televue Powermate is not a Barlow. A Powermate will have its magnification slightly decreased with a binoviewer inserted. For example, a 2.5x Powermate will magnify 2.25x with the binoviewer.