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Bresser AR-90 Manual

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Looking at or near the Sun will cause instant and irreversible damage to your eye!
21
ma I n t e n a n c e a n d se r v I c e
IMPORTANT NOTE:
Do not force the 4 screws past their normal travel, and do not rotate any
screw or screws more than 2 full turns in a counterclockwise direction (i.e.,
not more than 2 full turns in their „loosening“ direction), or else the  diagonal 
mirror may become loosened from its support. Note that the diagonal mirror
collimation adjustments are very sensitive: generally turning a collimation
screw 1/2-turn will have a dramatic effect on collimation.
3. If the reflection of the diagonal mirror is not centered within the reflection
of the primary mirror, adjust the 3 collimation screws located on the rear of
the primary mirror cell.
NOTE:
There are 6 screws (Fig. 31c) on the primary mirror cell. The 3 knurled
knobs are the collimation screws, and the 3 smaller thumb screws are lock-
ing screws. The locking screws must be loosened slightly in order to adjust
the collimation screws.
Proceed by „trial and error“ until you develop a feel for which collimation 
screw to turn in order to change the image in any given way.
4. Perform an actual star test to confirm the accuracy of steps 1 through
3. Using the 25mm eyepiece, point the telescope at a moderately bright
(second or third magnitude) star, and center the image in the main
telescope‘s field of view.
5.  Bring the star‘s image slowly in and out of focus until you see several 
disks surrounding the star‘s center. If steps 1 through 3 were done
correctly, you will see concentric (centered with respect to each other)
circles (1, Fig. 32).
An improperly collimated instrument will reveal oblong or elongated circles
(2, Fig. 32). Adjust the 3 collimating screws on the primary mirror housing
until the circles are concentric on either side of the focus.
In summary, the 4 adjustment screws on the plastic diagonal mirror housing
change the tilt of the secondary mirror so that it is correctly centered in the
focuser drawtube, and so that the primary mirror appears centered when
looking into the focuser. The 3 collimating knobs on the primary mirror
change the tilt of the primary mirror so that it reflects the light directly up the
center of the drawtube.
Inspecting the Optics
A Note About the “Flashlight Test: If a flashlight or other high-intensity light 
source is pointed down the main telescope tube, the view (depending upon
the observer’s line of sight and the angle of the light) may reveal what
appears to be scratches, dark or bright spots, or just generally uneven coat-
ings, giving the appearance of poor quality optics. These items are only
seen when a high intensity light is transmitted through lenses or reflected
off the mirrors, and can be seen on any high quality optical system, includ-
ing giant research telescopes.The optical quality of a telescope cannot be
judged by the “flashlight test;“ the true test of optical quality can only be 
conducted through careful star testing.
Laser collimator:
To make the collimation easier, a laser can be used to align the secondary
mirror and the main mirror. Especially fast Newtonians (f/5 or f/4 systems)
require precise collimation to deliver the best possible optical performance.
It is recommended, to check the collimation prior to every observing or
imaging session.A suitable laser collimator can be found with item number
4910200 in our product range. All Bresser Messier Newtonian telescopes 
have a center marking on the main mirror so a laser can be used for col-
limation.
Fig 31b: The four collimation
screws on the secondary mirror
housing
Fig. 31c: The six collimation
screws on the rear of the primary
mirror cell.
knurled knob
thumbscrew
Fig. 32: Correct (1) and incorrect
(2) collimation viewed during a star
test.
1 2
… 21 …


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