Features

  • Smooth, accurate Crayford-style focuser accepts 2" and 1.25" eyepieces
  • Roller-bearing design for zero image shift through focusing range
  • Includes deluxe machined focus knobs with rubber grips and tension control
  • Lets you easily achieve the sharpest possible focus even at high magnifications

The Crayford-style focuser provides greater focusing smoothness and accuracy, and the cast-aluminum focuser features a smooth roller bearing motion designed to eliminate image shift through the entire focusing range. With my scope I have not experienced any image shift when adjusting the focuser.

One of the modifications suggested by XT owners is to paint the interior of the focuser tube matt black, but when I inspected my telescope with this intention I found that the focuser tube is already internally blackened to improve contrast and reduce glare. I surmise from this that the older XT scopes may not have had this?

Crayford FocuserThe machined metal knobs with rubber grips have a quality feel, and a tension control knob provides precise focusing adjustment.

Th focuser has a racked-in height of 2-11/16" and 1.4" (35mm) of drawtube travel. The base is contoured to fit Orion XT scopes from 8" to 12" aperture on Dobsonian and other reflector tubes.

Warning: For readers who already have an older style Orion scope and are thinking of upgrading to use this new Crayford, be sure that you read the information on the Orion website regarding compatibility with the Skyquest XT8, or Classic Dobs prior to 2002, which may require modification in order to use this focuser.

The Focuser Base Plate incorporates user adjustable set screws to facilitate collimation of the telescope, although it is collimated at the factory and should never need to be adjusted. Focuser collimation is only required under very rare circumstances, but is available should the need arise.

 

Now a short lesson to help you understand the next couple of paragraphs;

In optics, the Exit Pupil is a "virtual aperture" in an optical system. Only light rays which pass through this virtual aperture can exit the system (shine out of the eyepiece). The Exit Pupil is the image of the aperture stop in the optics that follow it (i.e. the interior diameter of the apparent 'hole' which restricts how much you can see of the night sky when peering into an eyepiece). The same term is used to refer to both the virtual aperture and its diameter.

In a system designed for direct visual observation, the Exit Pupil should be about the size of the pupil of the observer's eye, and it should be placed a comfortable distance past the eyepiece, so the observer can place his or her pupil in the plane of the Exit Pupil. This allows the maximum amount of light to reach the retina, and the image is not vignetted.

For example, a set of 7×50 binoculars has an exit pupil just over 7mm, which is the diameter of the average human eye pupil size at night. (During daytime the human eye narrows its pupil down to about 4mm to let less light in).

The Exit Pupil and distance it is projected "out" of the eyepiece are important factors for people who wear glasses when choosing eyepieces, because their glasses prevent them getting very close to the eyepiece.

Many observers find that eyepieces with focal lengths smaller than 7.5mm can be less comfortable to view through (you generally need to hold your eye closer to the lens as the eyepiece size decreases). This makes it less comfortable to view through a "fast" telescope at high power, especially if you wear eyeglasses. [The focuser can compensate for typical near or far-sighted conditions, and so sometimes you can focus through the eyepiece without actually wearing your glasses].

This effect can be relieved somewhat by using a Barlow lens with an eyepiece. A 2x Barlow lens will make a 15mm eyepiece act at 100x in an f/5 telescope while maintaining the same comfortable eye relief. In addition, there are several lines of eyepieces currently available which combine high power with very comfortable eye relief, but they can be expensive.

With suitable eyepieces in the focuser it is possible to reach 23x observing areas with a Field of View as large as 2.1 degrees across (i.e. you will be able to see a nice wide 2.1 degrees of night sky at 23x magnification). Bear in mind, however, that at such a low magnification the Exit Pupil (the diameter of the cone of light coming out of the eyepiece) is quite large at 11.13mm, and this may not be such a good choice as using an eyepiece that provides a higher magnification of 36x showing less field of view at 1.77 degrees, but now it has a 7.08mm Exit Pupil, which is a much closer match to the size of your pupil at night.

If the XT scopes were equipped only with a 1.25" focuser, then the maximum Field of View would be only 1.23 degrees at 31x - with 8.1mm Exit Pupil; while a more practical 32mm eyepiece would provide 1.23 degrees at 39x with a more comfortable 6.48mm Exit Pupil.

Rather a lot to think about there, but basically we are saying that a 2" eyepiece in the focuser provides a wider field of view than the 1.25" holder (1.77 degrees compared to 1.23 degrees) so gathers more light, and using a slightly higher magnification eyepiece the Exit Pupil is narrowed down for a better match to our eye, therefore it makes better use of the light coming through the telescope, i.e. you will be able to see dimmer objects because it is concentrating the light tighter into your eye, rather than spreading out wider than your eye's pupil can take in.

You are not wasting any of that all important light!

The factors of aperture and the 2" focuser are very important to astronomers who seek better views of the "extended" (it covers a wide field of view), faint deep sky objects (nebulae, star clusters, galaxies, comets, and so forth).

With the 10" aperture of the XT10, the stellar magnitude reach is extended from about 13.8 of an 8" telescope, to about 14.3 so much dimmer objects are visible.

  • The brightest stars are said to be of first magnitude (m = 1), while the faintest are of sixth magnitude (m = 6), the limit of human visual perception without the aid of a telescope. The faintest stars visible in an urban neighborhood with naked eye are third magnitude (m = 3).

And still the XT10 affords the contrast and clarity required to observe the Moon, and planets. With a comparatively small central obstruction, and fewer optical surfaces, the contrast and brightness in some cases can surpass more expensive designs like some 8" and 10" Schmidt-Cassegrain telescopes.

Next section: Eyepieces included with the Orion XT Telescopes

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