Orion XT10 CLASSIC Eyepiece Selection, Tracking Celestial Objects, Using 2 Eyepieces XT8 and XT10

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Eyepiece Selection

By using eyepieces of varying focal lengths, it is possible to attain many magnifications with the SkyQuest XT. The tele- scope comes with one high-quality Sirius Plössl eyepiece: a 25mm, which gives a magnification of 48x. Other eyepieces can be used to achieve higher or lower powers. It is quite com- mon for an observer to own five or more eyepieces to access a wide range of magnifications. This allows the observer to choose the best eyepiece to use depending on the object being viewed.

Whatever you choose to view, always start by inserting your lowest-power (longest focal length) eyepiece to locate and center the object. Low magnification yields a wide field of view,

Figure 19. Megrez connects the Big Dipper’s handle to it's “pan”. It is a good guide to how conditions are. If you can not see Megrez (a 3.4 mag star) then conditions are poor.

matter.Your eyes take about 30 minutes to reach perhaps 80% of their full dark-adapted sensitivity. Many observers notice improvements after several hours of total darkness. As your eyes become dark-adapted, more stars will glimmer into view and you will be able to see fainter details in objects you view in your telescope. So give yourself at least a little while to get used to the dark before you begin observing.

To see what you are doing in the darkness, use a red light flashlight rather than a white light. Red light does not spoil your eyes’ dark adaptation like white light does. A flashlight with a red LED light is ideal, or you can cover the front of a regular flashlight with red cellophane or paper. Beware, too, that nearby porch and streetlights and automobile headlights will spoil your night vision.

Tracking Celestial Objects

The Earth is constantly rotating about its polar axis, complet- ing one full rotation every 24 hours; this is what defines a “day.” We do not feel the Earth rotating, but we can tell that it is at night by seeing the apparent movement of stars from east to west. This movement translates into a rate of approximately

.25° per minute, or 15 arc-seconds per second. (There are 60 arc-minutes in 1°, and 60 arc-seconds in one arc-minute.) This is called the sidereal rate.

When you observe any astronomical object, you are watching a moving target. This means the telescope’s position must be continuously updated over time to keep an object in the field of view. This is easy to do with the SkyQuest XT because of its smooth motions on both axes. As the object moves off toward the edge of the field of view, you just lightly nudge the tele- scope to bring it back to the center.

You will notice that it is more difficult to “track” objects when the telescope tube is aimed nearly straight up. This is inherent to the basic design of the Dobsonian, and stems from the fact that there is very little mechanical leverage to move in azimuth when the tube is in a near vertical position. To gain more lever- age, try grasping the tube close to the altitude side bearings with both hands.

Remember that objects appear to move across the field of view faster at higher magnifications. This is because the field of view becomes narrower.

which shows a larger area of sky in the eyepiece. This makes acquiring and centering an object much easier. If you try to find and center objects with high power (narrow field of view), it’s like trying to find a needle in a haystack!

Once you’ve centered the object in the eyepiece, you can switch to higher magnification (shorter focal length eyepiece), if you wish. This is especially recommended for small and bright objects, like planets and double stars. The Moon also takes higher magnifications well.

Deep-sky objects, however, typically look better at medium or low magnifications. This is because many of them are quite faint, yet have some extent (apparent width). Deep-sky objects will often disappear at higher magnifications, since greater magnification inherently yields dimmer images. This is not the case for all deep-sky objects, however. Many galaxies are quite small, yet are somewhat bright, so higher power may show more detail.

The best rule of thumb with eyepiece selection is to start with a low power, wide field, and then work your way up in magnifica- tion. If the object looks better, try an even higher magnification. If the object looks worse, then back off the magnification a little by using a lower-power eyepiece.

Using 2" Eyepieces (XT8 and XT10)

The SkyQuest XT8 and XT10 Crayford focusers are capable of accepting optional 2" eyepieces. To use 2" eyepieces you must remove the 1.25" adapter from the focuser by turning the two thumbscrews that hold it in place. Once this adapter is removed, insert a 2” eyepiece directly into the eyepiece holder and use the same thumbscrews to secure the larger eyepiece.

2" eyepieces are desirable because they provide a wider field of view than 1.25" eyepiece. Many observers own at least one 2" eyepiece to yield the largest field of view possible for looking at wide deep-sky objects, such as open star clusters or gaseous nebulae.You’ll be stunned at the large fields of view that 2" eye- pieces deliver. You’ll feel like you’re floating in space!

Now that you are all set up and ready to go, one critical decision must be made: what to look at?

A. The Moon

With is rocky and cratered surface, the Moon is one of the most interesting and easy subjects to view with your scope. The best time to view it is during its partial phases when

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