Observing Tips And Techniques - Orion 9931 Instruction Manual

Figure 5. Primary mirror correctly aligned. The telescope's
optical system is now collimated.
with your eye in it) and spider are off-center (as also is the
case in Figure 4); you will fix that in the next step.
adjusting the tilt of the Primary Mirror
The final adjustment is made to the primary mirror. It will need
adjustment if, as in Figure 4, 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 your eye inside) is off-center.
The tilt of the primary is adjusted with the three Allen-head
collimation bolts at the bottom of the optical tube, behind the
primary mirror. With the large Allen wrench provided, turn one
bolt at time, no more than one turn at a time, then look into
the focuser again and see if the secondary mirror reflection
has moved closer to the center of the primary mirror reflec-
tion. You will soon get a feel for which bolts to turn in which
direction and how far, until finally the secondary mirror reflec-
tion is dead center. (It helps to have two people for primary
mirror collimation, one to look in the focuser while the other
adjusts the collimation bolts.)
The collimation bolts are spring-loaded, so once you achieve
the correct mirror tilt, just leave the bolts as they are; the mir-
ror will remain in the adjusted position.
The view through the Collimating Eyepiece should now
resemble Figure 5. The secondary mirror is centered in the
focuser; the reflection of the primary mirror is centered in the
secondary mirror, and the reflection of the secondary mirror
is centered in the reflection of the primary mirror.
Your telescope is in precise collimation—optically primed for
peak performance! Once again, confirm the collimation by
conducting a star test.
6. observing tips &
techniques
Pick a location away from street lights and bright yard lighting.
Avoid viewing over rooftops and chimneys, as they often have
warm air currents rising from them, which distorts the image
seen in the eyepiece. Similarly, you should not observe
through an open window from indoors.
"Seeing" and transparency
Atmospheric conditions vary significantly from night to night.
"Seeing" refers to the steadiness of the Earth's atmosphere at
a given time. In conditions of poor seeing, atmospheric turbu-
lence causes objects viewed through the telescope to "boil."
If, when you look up at the sky with just your eyes, the stars
are twinkling noticeably, seeing is bad and you will be limited
to viewing with low powers (bad seeing affects images at high
powers more severely). Planetary observing may also be
poor. Make sure you are not looking over buildings or any
other source of heat; that will also cause image degradation.
In conditions of good seeing, star twinkling is minimal and
images appear steady in the eyepiece. Seeing is best over-
head, worst at the horizon. Also, seeing generally gets better
after midnight, when much of the heat absorbed by the Earth
during the day has radiated off into space.
Especially important for observing faint objects is good transpar-
ency—air free of moisture, smoke, and dust. All tend to scatter
light, which reduces an object's brightness. Transparency is
judged by the magnitude of the faintest stars you can see with the
unaided eye (6th magnitude or fainter is desirable).
Cooling the telescope
All optical instruments need time to reach "thermal equilibri-
um" to achieve maximum stability of the lenses and mirrors,
which is essential for peak performance. When moved from a
warm indoor location into the cooler outdoor air, a telescope
needs time to cool to the outdoor temperature. The bigger the
instrument and the larger the temperature change, the more
time is needed.
For your DSE, allow at least one hour for it to equilibrate. If the
scope has to adjust to more than a 40° temperature change,
allow two to four hours. In the winter, storing the telescope
outdoors in a shed or garage greatly reduces the amount of
time needed for the optics to stabilize.
Do You Wear Eyeglasses?
If you wear eyeglasses, you may be able to keep them on
while you observe, if your eyepieces have enough "eye relief"
to allow you to see the whole field of view. You can try this by
looking through the eyepiece first with your glasses on and
then with them off, and see if the glasses restrict the view to
only a portion of the full field. If they do, you can easily
observe with your glasses off by just refocusing the telescope
the needed amount.
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