Meade Instruments LX850 - Manual (Page 24)

AutoStar used during the alignment procedure).
Press the GOTO key to choose that star.
5. The telescope will slew to that star. When slew-
ing has been completed, re-center the star in the
eyepiece or camera. Then hold the “ENTER” key
for 2 seconds. Then press “ENTER” again to
confirm your position.
StarLock will now compensate for the offset.
Test your Collimation
Before collimating an LX850 telescope, you will need
to test your collimation.
To test the collimation, center a bright star that is
overhead, with a medium/high power eyepiece such as
a Meade 12mm eyepiece. Allow the telescope optics
to adjust to the temperature of your observation site
before proceeding; temperature differences between
the optics and the outside air can cause distortion in
the images. Autostar II offers an option in the Utilities
menus that displays the “Ambient Temperature” (the
temperature of the air around mount).
When the star is centered, de-focus the image. You
will notice that the out of focus star image looks like a
ring of light surrounding a dark central spot; the dark
central spot is in fact the shadow of the secondary
mirror. You may notice multiple rings; these are called
diffraction rings. Focus until the diffraction rings fill
about 10% of the eyepiece field-diameter. If the dark
central spot is offset in (i.e., not concentric with) the
diffraction rings, your telescope’s optical system is
misaligned and requires collimation.
Collimation of the optical system:
Meade ACF Models ONLY
a. The only adjustments possible, or necessary,
on the Advanced Coma-Free LX850 telescope
models are from the three screws (Page 8, Fig. 2,
#35) located at the edge of the inner surface of the
secondary mirror housing.
Caution: Do not force the three collimation screws
past their normal travel and do not loosen them more
than two full turns in a counterclockwise direction or
the secondary mirror may come loose from its support.
You will find that the adjustments are very sensitive,
usually requiring only one-half turn or less to produce
the desired result.
b. While looking at the defocused star image, notice
which direction the darker shadow is offset in the
ring of light or notice which part of the ring is the
thinnest
(Fig. 34, #1). Place your index finger in
front of the telescope so that it touches one of the
collimation set screws. You will see the shadow
of your finger in the ring of light. Move your finger
around the edge of the black secondary mirror
support until you see the shadow of the finger
crossing the thinnest part of the ring of light. At this
point, look at the front of the telescope where your
finger is aiming. It will either be pointing directly at
a set screw, or it will be between two set screws
aiming at the set screw on the far side of the black
secondary mirror support. This is the set screw that
you will adjust.
c. Using the AutoStar II’s Arrow keys at the slowest
slew speed, move the defocused image to the
edge of the eyepiece field of view
(Page 23, Fig.
34, #2), in the same direction as the darker shadow
is offset in the ring of light.
d. Turn the set screw that you found with the pointing
exercise while looking in the eyepiece. You will
notice that the star image will move across the field.
If while turning the set screw, the defocused star
image moves out of the eyepiece field, then you
must use the slew keys on the AutoStar to return the
star to the center of the field of view. If the screw you
are turning becomes very loose, tighten the other
two screws by even amounts. If the screw you are
turning gets too tight, un-thread the other two by
even amounts.
f. When you bring the image to center (Page 23, Fig.
34, #3), carefully examine the evenness of the ring
of light (concentricity). If you find that the dark center
is still off in the same direction, continue to make
the adjustment in the original turning direction. If it
is now off in the opposite direction, you have turned
AutoStar #497 HANDBOX
Getting Started
24
(1) (2) (3)
Figure 34 : Collimation