Meade Getting Started with Your ETX 90EC Telescope: Master the Viewfinder

8	z
	GETTING STARTED

CHAPTER			THE VIEWFINDER
1	As with most astronomical telescopes, both ETX model
	telescopes present a fairly narrow field of view to the observer.
	As a result it is sometimes difficult to locate and center objects
	in the telescope’s field of view. The viewfinder, by contrast, is a
	low-power, wide-field sighting scope with crosshairs that let you
	easily center objects in the eyepiece of the main telescope’s
	field of view.A telescope’s viewfinder is a tremendous aid in the
	location of faint astronomical objects before observation is
	made through the telescope’s eyepiece.
	ETX-90EC 8 x 21mm Erect-Image Viewfinder:
	Standard equipment with the ETX-90EC, the 8 x 21mm Erect-
	Image Viewfinder has 8-power magnification and an aperture
	of 21mm. This viewfinder presents a correctly oriented image
	both up-and-down and left-to-right.
	ETX-125EC 8 x 25 mm Right-Angle Viewfinder:
	Standard equipment with the ETX-125EC, the 8 x 25mm Right-
	Angle Viewfinder has 8-power magnification and an aperture of
	25mm. This viewfinder presents a correctly oriented image up-
	and-down, but is reversed left-for-right.
	3
	2
				4
		ETX-125EC
	1			5	3

ETX-90EC 2 6

Fig.7: The Viewfinder. (1) Front lens cell; (2) Knurled lock- ring (ETX-90EC) or focus ring (ETX-125EC); (3) Viewfinder eyepiece; (4) Front alignment screws; (5) Viewfinder bracket;

(6) Rear alignment screws.

Focusing the Viewfinder

Both ETX model viewfinders are factory pre-focused to objects located at infinity. Individual eye variations may require that the viewfinder be re-focused. To check the focus look through the viewfinder. Point the viewfinder at a distant terrestrial (land) object such as a telephone pole or light pole. If the viewfinder image is not in sharp focus, follow the procedure for your type of telescope.

ETX-90EC

1.If the viewfinder is mounted in the viewfinder bracket (5,Fig. 7), remove it by slightly unthreading the six alignment screws (4 and 6, Fig. 7) until it can slip easily out of the bracket.

telescope. Gently tighten the six alignment screws. Proceed with Aligning the Viewfinder.

ETX-125EC

Turn the focus ring (2, Fig. 7) at the base of the viewfinder eyepiece (3, Fig. 7) in either direction until a sharp focus is reached.

Aligning the Viewfinder

In order for the viewfinder to be useful, it must first be aligned with the main telescope, so that both the viewfinder and the main telescope are pointing at precisely the same location. To align the viewfinder follow this procedure:

1.The viewfinder bracket (5, Fig. 7) includes six alignment screws (4 and 6, Fig. 7). Turn the three rear alignment screws (6, Fig. 7) so that the viewfinder tube is roughly centered within the viewfinder bracket.

NOTE: Do not overtighten the alignment screws. When tightening one screw it may be necessary to loosen one or both of the two other alignment screws .

2.Using the SP26mm eyepiece, point the main telescope at some easy-to-find, well-defined land object, such as the top of a telephone pole. Center the object precisely in the eyepiece’s field of view, then tighten the vertical and horizontal locks (6 and 10, Fig. 1) so that the object does not move (1, Fig. 8).

	(before
	alignment)
	1
View through the		View through the
viewfinder		telescope eyepiece

(after

alignment)

2

Fig.8: Aligning the Viewfinder.

3.While looking through the viewfinder, turn one or more of the three front viewfinder alignment screws (4, Fig. 7), until the crosshairs of the viewfinder point at precisely the same position as the view through the eyepiece of the main telescope (2, Fig. 8).

Re-check that the viewfinder’s crosshairs and the main telescope are now pointing at precisely the same object. The viewfinder is now aligned to the main telescope. Unless the alignment screws are disturbed or the viewfinder jarred, the viewfinder should remain aligned indefinitely.

2.Loosen the knurled lock-ring (2, Fig. 7) located near the viewfinder’s front lens cell (1, Fig. 7). Unthread this ring (counterclockwise, as seen from the eyepiece-end of the viewfinder) by several turns.

3.Look through the viewfinder and focus on a distant object. Slowly rotate the front lens cell in one direction or the other, until the object appears sharp. One or two rotations of the lens may have a significant effect on image focus.

4.Lock the focus in place by threading the knurled lock-ring clockwise up against the viewfinder’s lens cell.

5.Place the viewfinder into the viewfinder bracket on the main

Using the Viewfinder

To locate any object, terrestrial or astronomical, first center the object in the crosshairs of the viewfinder; the object should also then be centered in the field of view of the main telescope.

Observing Tip: If higher observing magnifications are intended, first locate, center, and focus the object using a low- power eyepiece (e.g., SP 26mm eyepiece). Remove the low- power eyepiece and replace it with a higher-power e yepiece; the object should still be centered in the field of view. Objects are much easier to locate and center at lower powers; higher powers are employed simply by changing eyepieces.