Eyepieces, Barlow Lens and Finderscope
Three 0.96" eyepieces are included: the 20mm for low power and the widest view, the 12mm for medium power, and the 4mm for high power. Magnification is the telescope's focal length divided by the eyepiece's focal length, and the 3x Barlow lens triples it. The manual's rule for the highest useful magnification is about 60 power per inch of aperture under average skies, and it notes that most observing is done at 20 to 35 power per inch. It also says the greatly magnified Barlow views should only be used under ideal conditions.
Worked from the focal length, the eyepieces give 30x with the 20mm, 50x with the 12mm, 150x with the 4mm; the 3x Barlow lens turns these into 90x, 150x, 450x. Celestron's highest useful magnification for this aperture is 118x, so the 4mm alone (150x), the 12mm with the Barlow (150x) and the 4mm with the Barlow (450x) go past it: expect dim, soft views there. The 20mm with the Barlow (90x) is the highest combination within it.
The 1.5x image erecting eyepiece is meant mainly for daytime land viewing: the manual says it makes the image both right side up and correct left to right. It goes in like the Barlow and gives 45x with the 20mm, 75x with the 12mm and 225x with the 4mm (the last again beyond 118x); the Barlow cannot be used at the same time.
The manual says to put the Barlow straight into the focuser in place of the diagonal and the eyepiece into the Barlow; you can also put the diagonal into the Barlow, but some eyepieces may then not reach focus. Start with the low-power eyepiece, which is easier to focus.
The 0.96" star diagonal turns the light at a right angle for a more comfortable view. Its prism makes the image right side up but reversed left to right; this right-side-up viewing suits land subjects and also works on celestial objects.
To focus, turn the knob below the eyepiece holder: clockwise for a more distant object, counterclockwise for a closer one. If you wear glasses you may remove them at the eyepiece, but the manual says to keep them on if you have astigmatism.
Align the 5x24 finderscope in daylight on an easily recognizable object such as a streetlight, a license plate or a sign, as far away as possible and at least a quarter mile off. Center it in the 20mm eyepiece, then, without moving the telescope, turn the screws around the finder bracket until its crosshair sits on the same object; the guide suggests loosening one screw half a turn and tightening another by the same amount so the finder stays held. It won't need realigning unless it is bumped, dropped or removed.