Collimation steps for Newtonian telescope, long to medium f/ratio
This is a rough guide to collimation, but should serve for a quick fix, and may be enough for
long focus scopes.
The essential features of the telescope:
* adjusting screws on primary and secondary mirrors to change
#tilt of both and
#position of the secondary in the tube up/down l/r, rotate
* focuser/ eyepiece holder
Stage one: daylight/ bright room. No eyepiece in focuser
The focuser should be perpendicular to the axis of the tube, and the axis of the focuser should
intersect the axis of the tube. This is usually OK, but to check you can remove both mirrors
and stretch a string between two boards on either end of the tube to locate the centre line, and
then sight through the focuser.
The secondary mirror should be centred on the axis (well, not quite, but centred is good enough
for a small scope). Once the focuser is OK, and with both mirrors in place, look through the
focuser. You'll see :
*the bottom edge (circular) of the focuser
*the secondary mirror
*the reflection of the primary mirror in the secondary mirror
*the reflection of the secondary mirror and your eye in the primary mirror
All these circles should be concentric. Start by adjusting the secondary mirror so that its outline
is completely circular and concentric with the bottom of the focusing tube.
Once the secondary is OK, look at the reflection of the primary. It will probably be off-centre.
Best to work with someone to turn collimating screws at the back of the primary- trial and error,
with no more that 1/2 turn at a time on any screw, and you'll eventually get things lined up.
Useful guides are the reflections of the supports for the secondary mirror, and the shape and
area of the sections that they divide the reflection into.
Step 2: Fine adjustment at night, using a star, eyepiece in the tube
Adjust the focus so that the star (telescope aperture illuminated by star) appears as quite a large
disk with the silhouette of the secondary mirror and supports superimposed.
Adjust the primary screws until all is lined up.
Refocus the eyepiece through the focal plane and out of focus on the other side. If all is well,
the disc will be identical. You may need to tweak the screws a bit to line things up perfectly.
Simon Walsh
Oct 1996
