Explore Scientific Explore FirstLight 10" f/5 Dobsonian, Instruction Manual

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VI

 NOTE!

Do not force the 3 screws (Fig. 22, 2) past their normal travel. 
Do not rotate the collimation screws more than two full turns in a counterclockwise direction 
(i.e. not more than two full turns in their "loosening" direction), or else the diagonal mirror 
may become loosened from its support. NOTE: The diagonal mirror collimation adjustments 
are very sensitive. In general, turning a collimation screw a half 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 adjustment screws located on the rear of the primary mirror 
cell.

 NOTE!

The primary mirror housing (Fig. 23) is equipped with 6 screws. The 3 large knurled screws  
are collimation screws (Fig. 23, 21) and the 3 smaller knurled screws are lock screws 
(Fig. 23, 20). These lock screws must be loosened before the collimation screws can be 
turned. 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 (Fig. 25, 1). An improperly collimated instrument will 
reveal oblong or elongated circles (Fig. 25, 2). Adjust the 3 collimating screws on the 
primary mirror housing until the circles are concentric on either side of the focus.

In summary, the adjustment screws on the 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.

20

21

20

21

21 20

Fig. 23: Underside of rear mirror tube (primary mirror 
housing). Collimation screws (20); Lock screws (21).

Fig. 22: Secondary Mirror Assembly (front and side view). 
Spider vanes (1); tilt screws (2); secondary mirror holder (3).

Fig. 24: View you see while collimating the telescope 
(without eyepiece inserted). Focuser drawtube (1); 
diagonal mirror (2); reflection of primary mirror (3); 
reflection of secondary mirror (darkened due to back 
lighting) (4); primary mirror holder (5); reflection of 
observer's eye (6)

Fig. 25: Correct (1) and incorrect (2) collimation as viewed 
during a star test.

Maintenance and Cleaning