- 7 -
Fig. 8. (Note that the secondary mirror is misaligned.)
Secondary Mirror Adjustments
If the secondary mirror (1, Fig. 8) is centered in the drawtube (2, Fig. 8) but the primary mirror is only partially visible in the reflection (3, Fig. 8) the 3 hex screws located on the secondary mirror assembly (2, Fig. 5) must be unthreaded slightly to refine the tilt-angle of the secondary mirror until the entire primary mirror can be seen centered within the secondary mirror reflection. When the secondary mirror is correctly aligned, it will look like Fig. 9. (Note that the primary mirror is misaligned.)
Primary Mirror Adjustments
If the secondary mirror and the reflection of the primary mirror (1, Fig. 9) appear centered within the drawtube (2, Fig. 9) but the reflection of your eye (3, Fig. 9) appears off-center, you will need to adjust one or more of the three primary mirror hex screws of the primary mirror cell. These primary hex screws are located behind the primary mirror, at the lower end of the main tube. Adjust the primary mirror alignment by slightly turning one hex screw at a time, looking through the focuser after each adjustment to determine if the mirror is moving in the correct direction.
Star Testing the Collimation
With the collimation performed, the next step is to test the accuracy of the alignment on a star. Use the 25mm eyepiece and point the telescope at a moderately bright (second or third magnitude) star, then center the star image in the telescope’s field-of-view. With the star centered, follow the method below:
1.Bring the star image slowly out of focus until one or more rings are visible around the central disc. If the collimation was performed correctly, the central star disk and rings will be concentric circles, with a dark spot dead center within the out-of-focus star disk (this is the shadow of the secondary mirror), as shown in Fig. 10A. (An improperly aligned telescope will reveal elongated circles, Fig. 10B, with an off-center dark shadow.)
2.If the out-of-focus star disk appears elongated (Fig. 10B), you will need to adjust the primary mirror tilt hex screws of the primary mirror cell. Adjust the hex screw on the mirror cell until the circles are concentric on either side of focus.
TIPS ON USING A
DOBSONIAN TELESCOPE
1.Never lubricate the Teflon pads on the ground plate. The Meade Starfinder Dobsonian has been designed with some inherent friction. You want the telescope to move easily when you position it, but you also want it to stay in the position you place it. Using any kind of oil, silicone spray, wax, or grease will ruin the performance by causing the telescope to move too easily. Just keep these bearing surfaces clean; that’s all the maintenance required.
2.The altitude bearing surfaces (11, Fig 1) of the telescope are lightly lubricated at the factory for optimum performance. Over a period of time, these surfaces may become dry or dirty. Simply clean off the bearing surfaces with a dry cloth or paper towel and reapply a thin coating of silicone grease or spray to the surfaces to maintain peak performance. Do not use solvents or alcohol-based
cleaning solutions as this may damage the bearings or the painted surfaces of the telescope.
3.You will notice that your telescope will move in altitude by raising and lowering the tube, and in azimuth by rotating the base. As you observe objects in the night sky they will appear to drift out of the field of view due to the Earth’s rotation. To keep an object centered in the field of view, just lightly nudge the telescope in the proper direction. This may take a little practice at first, but you’ll soon get the hang of it.
4.Be sure the Mount is placed on a relatively level surface to allow proper operation. Each of the three feet should be in firm contact and not wobble. If you are in an area with particularly rough or soft ground, it may be helpful to place the Mount on a thick piece of plywood.
5.Part of the fun of using a Dobsonian type of telescope is the challenge of hunting for objects in the night sky. Invest is some simple star charts and books that tell you how to locate objects using a technique called “star hopping.” Once you begin learning the star patterns and constellations, you’re well on you way to finding many amazing sights.
MAGNIFICATION
The magnification, or power, at which a telescope is operating is determined by two factors: the focal length of the eyepiece employed and the focal length of the telescope. The Meade Starfinder Dobsonian telescope is supplied with one eyepiece as standard equipment. The focal length of the eyepiece, 25mm, is printed on its side.
Telescope focal length is, roughly speaking, the distance that light travels inside the telescope before reaching a focus.
The focal length of the Dobsonian 6" f/8 = 1220mm.
The focal length of the Dobsonian 8" f/6 = 1220mm.
The focal length of the Dobsonian 10" f/4.5 = 1140mm
The focal length of the Dobsonian 12.5" f/4.8= 1525mm
On a given telescope, such as the Starfinder Dobsonian, different eyepiece focal lengths are used to achieve different magnifications, from low to high.
To calculate the magnification obtained with a given eyepiece, use this formula:
Power = Telescope Focal Length
___________________
Eyepiece Focal Length
Example: Using the 25mm eyepiece supplied with the 8" f/6, the power is:
Power = 1220mm
________ = 49x
25mm
The type of eyepiece, whether Modified Achromatic, Plössl, or Super Plössl, has no effect on magnification, but does have a bearing on such optical characteristics as field of view, flatness of field, and color correction.
Maximum practical magnification is about 50X per inch of aperture. Generally, however, lower powers will produce higher image resolution. When unsteady air conditions prevail (as witnessed by rapid “twinkling” of the stars), extremely high powers result in “empty” magnification, where the object detail observed is actually diminished by the excessive power.
When beginning observations on a particular object, start with a low power eyepiece; get the object well-centered in the field of view and sharply focused. Then try the next step up in magnification. If the image starts to become fuzzy as you work up into higher magnifications, then back down to a lower power: the atmospheric steadiness is not sufficient to support high powers at the time you are observing. Keep in mind that a bright, clearly resolved, but smaller, image will show far more
