Orion 9843 instruction manual Star-Testing, Telescope, Adjusting the Primary, Mirror

the center screw with a larger					the flush screw one full turn,
Phillips head screwdriver (see				drawtube	and then tighten the adjacent
Figure 9). Turning the screw					raised screw until it is tight
clockwise will move the sec-				Reflection	as in Figure 11 (do not over-
ondary mirror toward the front					tighten). Look into the focuser
				of primary
opening of the optical tube,				mirror clip	and see if the secondary
while turning the screw coun-					mirror reflection has moved
ter-clockwise will move the					closer to the center of the pri-
secondary mirror toward the					mary. You can tell this easily
primary mirror.					with the collimation cap and
When the secondary mirror is					mirror center mark by simply
					watching to see if the “dot” of
centered in the focuser draw-
					the collimation cap is moving
tube, rotate the secondary
					closer or farther away from
mirror holder until the reflec-
					the ring on the center of the
tion of the primary mirror is
					primary mirror. Repeat this
as centered in the second-
					process on the other two
ary mirror as possible. It may
					sets of collimation screws, if
not be perfectly centered, but		a.
					necessary. It will take a little
that is OK. Now tighten the
					trial and error to get a feel for
three small alignment screws
					how to tilt the mirror in this
equally to secure the second-
					way. When you have the dot
ary mirror in that position.
					centered as much as pos-
If the entire primary mirror
					sible in the ring, your primary
reflection is not visible in the
					mirror is collimated. The view
secondary mirror, as in Figure
					through the collimation cap
8c, you will need to adjust
					should resemble Figure 8e.
the tilt of the secondary mir-
					Make sure all the collimation
ror. This is done by alternately		b.	c.
					screws are tight (but do not
loosening one of the three
					overtighten), to secure the
alignment screws while tight-
					mirror tilt.
ening the other two, as depict-
					A simple star test will tell you
ed in Figure 10. The goal is
					whether the optics are accu-
to center the primary mirror
					rately collimated.
reflection in the secondary
mirror, as in Figure 8d. Don’t					Star-Testing the
worry that the reflection of the
					Telescope
secondary mirror (the small-
			e.		When it is dark, point the
est circle, with the collimation		d.
					telescope at a bright star and
cap “dot” in the center) is off-
	Figure 8. Collimating the optics. (a) When the mirrors are				accurately center it in the eye-
center. You will fix that in the
next step.	properly aligned, the view down the focuser drawtube should look				piece’s field of view. Slowly
	like this. (b) With the collimation cap in place, if the optics are out				de-focus the image with the
Adjusting the Primary	of alignment, the view might look something like this. (c) Here, the				focusing knob. If the tele-
Mirror	secondary mirror is centered under the focuser, but it needs to be				scope is correctly collimated,
The final adjustment is made	adjusted (tilted) so that the entire primary mirror is visible. (d) The				the expanding disk should be
	secondary mirror is correctly aligned, but the primary mirror still				a perfect circle (Figure 12). If
to the primary mirror. It will
	needs adjustment. When the primary mirror is correctly aligned, the
					the image is unsymmetrical,
need adjustment if, as in	“dot” will be centered, as in (e).
					the scope is out of collima-
Figure 8d, the secondary

mirror is centered under the	tion. The dark shadow cast
	by the secondary mirror should appear in the very center of
focuser and the reflection of the primary mirror is centered in
	the out-of-focus circle, like the hole in a donut. If the “hole”
the secondary mirror, but the small reflection of the secondary
	appears off-center, the telescope is out of collimation.
mirror (with the “dot” of the collimation cap) is off-center.
The tilt of the primary mirror is adjusted using the three sets	If you try the star test and the bright star you have selected is
	not accurately centered in the eyepiece, the optics will always
of two collimation screws on the back end of the optical tube.
	appear out of collimation, even though they may be perfectly
Adjusting the tilt of the mirror requires a “push-pull” technique
	aligned. It is critical to keep the star centered, so over time you
involving adjustment of each set of collimation screws. Loosen

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