Put the rear port cap aside for now (do not lose it) because it is time to install the remainder of the telescope's accessories, just as when preparing for an evening of observing. Go over to the pile of "stuff" and locate three items: the visual back, the star diagonal, and the eyepiece. One end of the visual back is equipped with a rotating threaded ring. Place that up against the lip of the rear port and screw it on. Don't cross-thread and do not force anything. It should go on smoothly. Hand tighten only, of course.
It is possible to insert an eyepiece directly into this visual back. It is just the right size for a 1.25-inch diameter ocular and is equipped with one or two setscrews to hold an eyepiece in place. That would not be a very comfortable way to observe. Imagine the neck bending involved in viewing an object near the zenith. Instead, most amateurs use a star diagonal, a simple device that takes incoming light from the scope, bends it at a 90° angle, and sends it to the eyepiece for comfortable observing. Remove (but save) the plastic caps that seal the diagonal's two barrels and insert the "telescope end" of the diagonal into the visual back (the scope end is usually chrome plated and will not have a setscrew). If it does not seem to want to go in, back off on the visual back's setscrews a bit. Avoid loosening these setscrews too much. They have a tendency to drop into grass or thick carpet and disappear forever. Note that the setscrews can be loosened and the diagonal rotated to various angles for even more comfortable viewing.
We will not be viewing anything yet, of course, but go ahead and insert an eyepiece into the diagonal; this will help balance the scope. Remove the eyepiece's lens caps, slide it into the diagonal, and tighten the setscrew. How is the eyepiece's optical condition? Like the scope corrector, both the eye end (eye lens) and scope end (field lens) should be free of dirt and blemishes, although a little dust is okay and is to be expected.
If this is a GEM-mounted scope, there may be one more item in the box, a polar alignment telescope. Some mounts will have this alignment aid already installed, and some mounts (the Celestron CG5) will not come with one at all (an extra-cost option). The polar scope fits into the hollow "bore" of the mount's RA axis and allows the mount to be accurately aligned on the Celestial Pole via the little telescope's special reticle. Install and align the polar scope as instructed in the manual.
Wow! It is really starting to look like a CAT now. The next item on the agenda is checking it out mechanically, electrically, and electronically. Before that can be done, the mount will, of course, have to be connected to a source of electricity. If, as recommended, the scope will be run off an external battery, go get that. Check to make sure the on-off switch is off and plug the DC power cord into both the telescope and the battery. If internal batteries will be used, install them. If an AC/mains adapter will power the telescope, plug it into the mount and, with the scope's power switch off, into a wall socket or extension cord. Locate the hand control next. Remove any protective plastic covering from the display or keyboard, and double-checking with the manual, plug the HC into the proper receptacle on the mount.
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