Use of 2" Eyepieces and Diagonals
A feature of the 80mm ED is its ability to also use 2" barrel- diameter eyepieces and diagonals. At low magnifications, 2" eyepieces can give a wider field of view than standard 1.25" eyepieces. This is especially desirable for observing deep-sky objects, as many of them appear quite large, but faint. As with 1.25" eyepieces, the 80mm ED will not reach focus with the 2" eyepieces unless a 2" diagonal or extension tube is used.
To use 2" eyepieces, simply loosen the two large thumb- screws on the 2" adapter (Figure). Once these thumbscrews are loosened, the entire back end of the focuser, including any 1.25" diagonal and eyepiece that may be attached, comes off, exposing the 2" diameter focuser drawtube. Now, insert your 2" diagonal into the drawtube and secure with the two thumb- screws loosened previously. Insert a 2" eyepiece into the diagonal, secure it in place with the thumbscrew on the diago- nal, and you’re ready to observe.
Note About the 2" Crayford Focuser
The 80mm ED comes equipped with a Crayford focuser. The Crayford design allows for smooth, precise focusing without any image shift that typical rack-and-pinion designs experi- ence. If you find that the focus wheels are too tight or too loose, you can make adjustments to the focuser tension by using the focuser tension thumbscrew located on the bottom of the optical tube, between the focus wheels. Make adjust- ments to this thumbscrew until the focuser motion feels comfortable. Please note that you must have at least some tension applied to the focuser drawtube or else it will not move when you turn the focus wheels.
Calculating Magnification (Power)
It is desirable to have a range of eyepieces of different focal lengths, to allow viewing over a range of magnifications. To calculate the magnification, or power, of a telescope, simply divide the focal length of the telescope by the focal length of the eyepiece:
Telescope F.L. ÷ Eyepiece F.L. = Magnification
For example, the 80mm ED, which has a focal length of 600mm, used in combination with a 25mm eyepiece, yields a power of
600 ÷ 25 = 24x.
Every telescope has a useful limit of power of about 45x-60x per inch of aperture. Claims of higher power by some tele- scope manufacturers are a misleading advertising gimmick and should be dismissed. Keep in mind that at higher powers, an image will always be dimmer and less sharp (this is a fun- damental law of optics). The steadiness of the air (the “seeing”) will limit how much magnification an image can tol- erate.
Always start viewing with your lowest-power (longest focal length) eyepiece in the telescope. After you have located and looked at the object with it, you can try switching to a higher- power eyepiece to ferret out more detail, if atmospheric conditions permit. If the image you see is not crisp and steady, reduce the magnification by switching to a longer-focal-length eyepiece. As a general rule, a small but well-resolved image will show more detail and provide a more enjoyable view than a dim and fuzzy, overmagnified image.
Note About Chromatic Aberration
Chromatic aberration literally means color distortion. Whenever light passes through one material to another, light of different wavelengths (color) is bent by different amounts. This is a problem that plagues refractor-type telescopes, since light passes through both air and glass to form an image. Most astronomical objects emit a spectrum comprised of many dif- ferent wavelengths of light, so each wavelength will be bent by a slightly different amount when passing through a lens. This results in each color of light reaching precise focus at a slight- ly different point, which reduces image sharpness.
The 80mm ED is designed to minimize chromatic aberration. The objective lens comprises two individual lens elements, one of which is made of “ED” (Extra-low Dispersion) glass, a special type of glass that has superior refractive properties compared to normal types of glass. The use of this ED glass minimizes the amount of chromatic aberration, resulting in a cleaner, sharper image compared to that in standard achromatic telescopes.
Photography with the 80mm ED Refractor
With an optional camera adapter, the 80mm f/7.5 ED becomes a 600mm f/7.5 telephoto lens for a single-lens reflex camera. For long-distance terrestrial or astronomical photog- raphy, you need only a T-ring for your particular camera model. The T-ring attaches to your camera and threads onto the 80mm ED’s focuser drawtube, coupling the camera body to the telescope.
Use the camera’s viewfinder to frame the picture. Use the tele- scope’s focuser to focus the image. Tighten the focuser tension thumbscrew to make sure the camera does not slip out of focus.
You may want to consider using a remote shutter release instead of the shutter release on the camera. Touching the camera can vibrate the system and blur the resulting photo- graphic image. Also, be sure to use a solid tripod.
Care & Maintenance
Give your telescope reasonable care and it will last a lifetime. When not in use, keep its dust cover on as well as the dust cap on the eyepiece opening. Store it indoors or in a dry garage. Do not leave the telescope outside except when using it. The optical tube is aluminum and has a smooth painted sur- face that should resist scratches and smudges. If a scratch does appear on the tube, it will not harm the telescope. Smudges on the tube can be wiped off with standard house- hold cleaners such as Windex or Formula 409.
Any quality optical lens tissue and cleaning fluid specifically designed for multi-coated optics can be used to clean the tele- scope’s objective lens as well as the lenses of eyepieces and finder scopes. Never use regular glass cleaner or cleaning fluid designed for eyeglasses. Before cleaning with fluid and tissue, however, blow any loose particles off the lens with a blower bulb or compressed air, or lightly brush the lens with a soft camel hair brush. Apply some cleaning fluid to a tissue, never directly on the optics. Wipe the lens gently in a circular motion, then remove any excess fluid with a fresh lens tissue. Oily fingerprints and smudges may be removed using this method. Use caution; rubbing too hard may scratch the lens! Clean only a small area at a time, using a fresh lens tissue on each area. Never reuse tissues.
