mirror and the tilt of the primary mirror are the two collimation adjustments that will be done most often.
If the entire primary mirror reflection is not visible in the sec- ondary mirror, as in Figure 25c, you will need to adjust the tilt of the secondary mirror. This is done by alternately loosen- ing one of the three secondary mirror alignment set screws while tightening the other two, as depicted in Figure 28. Do not make excessive turns of these set screws or force them past their normal travel. A simple 1/2 turn of the screw can dramatically change the tilt of the secondary mirror. The goal is to center the primary mirror reflection in the secondary mir- ror, as in Figure 25d. Don’t worry that the reflection of the secondary mirror (the smallest circle, with the collimation cap “dot” in the center) is off-center. You will fix that in the next step.
Aligning the Primary Mirror
The final adjustment is made to the tilt of the primary mirror. It will need adjustment if, as in Figure 25d, the secondary mirror is centered under the focuser and the reflection of the primary mirror is centered in the secondary mirror, but the small reflection of the secondary mirror (with the “dot” of the collimation cap) is off-center.
The tilt of the primary mirror is adjusted with the three large spring-loaded collimation knobs on the rear end of the opti- cal tube (bottom of the primary mirror cell). The three smaller thumb screws lock the mirror’s position in place. These thumb screws must be loosened before any collimation adjustments can be made to the primary mirror (Figure 29).
To start, turn the smaller thumb screws counterclockwise a few turns each. Use a screwdriver in the slots, if necessary.
Now, try tightening or loosening one of the collimation knobs (Figure 30). Look into the focuser and see if the secondary mirror reflection has moved closer to the center of the primary mirror. You can easily determine this with the collimation cap and mirror center mark by simply watching to see if the “dot” of the collimation cap is moving closer or further away from the “ring” on the center of the primary mirror. If turning the one knob does not seem to bring the dot closer to the ring, try using one of the other collimation knobs. It will take some trial- and-error using all three knobs to properly align the primary mirror. Over time you will get the feel for which collimation screws to turn to move the image in a given direction.
When you have the dot centered as much as is possible in the ring, your primary mirror is collimated. The view through the collimation cap should resemble Figure 25e. Re-tighten the locking thumb screws in the bottom of the mirror cell.
A simple star test will tell you whether the optics are accu- rately collimated.
Star-Testing the Telescope
When it is dark, point the telescope at a bright star high in the sky and center it in the eyepiece’s field of view. Slowly defocus the image with the focusing knob. If the telescope is correctly collimated, the expanding disk should be a perfect circle (Figure 31). If the image is unsymmetrical, the tele-
scope is out of collimation. The dark shadow cast by the sec- ondary mirror should appear in the very center of the out-of- focus circle, like the hole in a doughnut. If the “hole” appears off center, the telescope is out of collimation.
If you try the star test and the bright star you have selected is not accurately centered in the eyepiece, then the optics will always appear out of collimation, even though they may be perfectly aligned. It is critical to keep the star centered, so over time you will need to make slight corrections to the telescope’s position in order to account for the sky’s apparent motion.
9. Using Your Telescope
Focusing the Telescope
The SkyQuest XTg Dobsonians come standard with a 2" dual-speed (11:1) Crayford focuser (Figure 21). The large 2" format focuser allows use of 2" or 1.25" eyepieces and the Crayford design prevents imaging shifting while focusing. The focuser has coarse focus knobs and a fine focus knob for most precise focusing.
With the 28mm Deep View eyepiece in the focuser and secured with the thumb screws, move the telescope so the front end is pointing in the general direction of an object at least 1/4-mile away. Now, with your fingers, slowly rotate one of the coarse focus knobs until the object comes into sharp focus. Go a little bit beyond sharp focus until the image just starts to blur again, then reverse the rotation of the knob, just to make sure you’re close to the focus point.
Now, use the fine focus knob to achieve precise focus. Eleven turns of the fine focus knob is equivalent to one turn of the coarse focus knobs, so much finer adjustment is possible than with just the coarse focus knobs alone. You’ll find this is a great convenience, especially when attempting to focus at high magnifications.
If you have trouble focusing, rotate the coarse focusing knob so the drawtube is inward as far as it will go. Now look through the eyepiece while slowly rotating the focusing knob in the opposite direction. You should soon see the point at which focus is reached.
The thumb screw on the bottom of the body of the focuser (Figure 21) will lock the focuser drawtube in place once the telescope is properly focused. Before focusing, remember to first loosen this thumb screw.
If you find the drawtube tension when focusing is either too tight (i.e., focus knob is difficult to turn) or too loose (i.e., draw- tube moves by itself under the weight of the eyepiece), you can adjust it by tightening or loosening the drawtube tension- ing set screw on the focuser, which is located just below the focus lock thumb screw. Adjust this set screw with the includ- ed 2.5mm hex key. Do not loosen the set screw too much as there must be some tension to keep the drawtube secure within the focuser. The other set screw below the drawtube tensioning set screw does not affect drawtube tension and should not be adjusted.
