Orion 120ST EQ, 9005 Note About Chromatic Abberation, How to Find Interesting Celestial Objects

lence causes objects viewed through the telescope to “boil.” If the stars are twinkling noticeably when you look up at the sky with just your eyes, the seeing is bad and you will be limited to viewing with low powers (bad seeing affects images at high powers more severely). Planetary observing may also be poor.

In conditions of good seeing, star twinkling is minimal and images appear steady in the eyepiece. Seeing is best over- head, worst at the horizon. Also, seeing generally gets better after midnight, when much of the heat absorbed by the Earth during the day has radiated off into space.

Avoid looking over buildings, pavement, or any other source of heat, as they will cause “heat wave” disturbances that will distort the image you see through the telescope.

Especially important for observing faint objects is good “transparency”—air free of moisture, smoke, and dust. All tend to scatter light, which reduces an object’s brightness. Transparency is judged by the magnitude of the faintest stars you can see with the unaided eye (6th magnitude or fainter is desirable).

Note About Chromatic Abberation

Chromatic abberation 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 prob- lem 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 different wave- lengths 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 slightly different point, which will provide unacceptable images.

Achromatic refractors, like the AstroView 120ST, are designed to minimize chromatic abberation to acceptable levels. The objective lens is actually comprised of two individual lenses, called elements, made of different materials, which bend light in slightly different ways. By precisely spacing and shaping the elements, the chromatic abberation incurred when light passes through air and the first glass element is reduced by the way the second element bends the light. The result is an image that is much better color corrected than a non-achro- matic (one element) objective lens.

Even with the achromatic lens design, however, the AstroView 120ST will suffer a bit from chromatic abberation due to its large aperture and short focal length. This will be noticeable, to some degree, on extremely bright objects, such as the brightest stars in the sky. What you will notice is that the object, when focused, has a slight “purple-halo” around it. This will not present a problem for most observers, as the eye readily adapts to the view and is still able to distinguish fine details. Chromatic abberation will never inhibit deep sky observing, as deep sky objects are too faint to cause any noticeable color distortion.

How to Find Interesting Celestial Objects

To locate celestial objects with your telescope, you first need to become reasonably familiar with the night sky. Unless you

know how to recognize the constellation Orion, for instance, you won’t have much luck locating the Orion Nebula, unless, or course, you look up its celestial coordinates and use the telescope’s setting circles. Even then, it would be good to know in advance whether that constellation will be above the horizon at the time you plan to observe. A simple planisphere, or star wheel, can be a valuable tool both for learning the constellations and for determining which ones are visible on a given night at a given time.

A good star chart or atlas will come in very handy for helping find objects among the dizzying multitude of stars overhead. Except for the Moon and the brighter planets, it’s pretty time- consuming and frustrating to hunt for objects randomly, without knowing where to look. You should have specific tar- gets in mind before you begin observing.

Start with a basic star atlas, one that shows stars no fainter than 5th or 6th magnitude. In addition to stars, the atlas will show the positions of a number of interesting deep-sky objects, with different symbols representing the different types of objects, such as galaxies, open star clusters, globular clus- ters, diffuse nebulas, and planetary nebulas. So, for example, your atlas might show a globular cluster sitting just above the lid of the “Teapot’ pattern of stars in Sagittarius.You then know to point your telescope in that direction to home in on the cluster, which happens to be 6.9-magnitude Messier 28 (M28).

You can see a great number and variety of astronomical objects with your AstroView 120ST, including:

A. The Moon

With its rocky, cratered surface, the Moon is one of the easi- est and most interesting targets to view with your telescope. The best time to observe our only natural satellite is during a partial phase, that is, when the Moon is NOT full. During par- tial phases, shadows on the surface reveal more detail, especially right along the border between the dark and light portions of the disk (called the “terminator”). A full Moon is too bright and devoid of surface shadows to yield a pleasing view. Try using a Moon Filter to dim the Moon when it is very bright. It simply threads onto the bottom of the eyepieces (you must first remove the eyepiece from the star diagonal to attach the Moon filter).

B. The Sun

You can change your nighttime telescope into a daytime Sun viewer by installing an optional full-aperture solar filter over the front opening of the AstroView 120ST EQ. The primary attraction is sunspots, which change shape, appearance, and location daily. Sunspots are directly related to magnetic activ- ity in the Sun. Many observers like to make drawings of sunspots to monitor how the Sun is changing from day to day.

Important Note: Do not look at the Sun with any optical instrument without a professionally made solar filter, or permanent eye damage could result.

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