Advanced CT Laser Collimator Manual (V9.0 =OH=) Page 9 of 20
a. This can happen if the telescope was last focused with a diagonal in place (and a diagonal is not installed now), or a focal
reducer was in the optical train the last time the telescope was focused. The focal plane is too far out, and is not at the
normal visual position it would be in if the telescope were focused without a diagonal or focal reducer. If you do not want
to use the diagonal during collimation, you will need to adjust the telescope's focus. Continue to
Step 6.3.b
).
b. Adjust the focus to bring at least two laser dots onto the full view on the target plate. Adjust the focus in one direction first
to see if any of the laser dots are moving toward the center direction of the target. If the laser(s) is moving or expanding
away from the center of the target, reverse the focusing direction to bring at least two laser dots onto the full view of the
target plate. Go to
Step 7
to collimate your telescope.
6.4. Your telescope is grossly out of alignment
When your telescope is grossly out of alignment, the collimating laser dots may be reflected completely away from the target
plate. If you believe this to be the case, begin the collimation process anyway and see if you can bring the three laser dots
onto the target plate.
Mechanical Method to get Collimation into Ballpark – Make a preliminary mechanical adjustment of the secondary mirror
adjustment knobs or screws to achieve a good rough adjustment of the secondary mirror. If you have not replaced the
secondary mirror adjustment screws with knobs, this would be a good time to do so. Since this procedure is essentially the
same as the one used to install knobs in place of the screws and greatly expedites secondary mirror adjustment. From this
point on, the expression “knobs or screws” will be replaced with “knobs”. To make the ballpark adjustment, remove the knobs
turn counterclockwise one at the time (DO NOT REMOVE THE KNOBS ALL AT ONCE). Mark the knobs with index marks.
While unscrewing one, gently pull on it while counting the number of turns and fractions of turns until you see exactly how
many turns and fraction of a turn were made until the threads disengaged. Write down the number of turns and the associated
knob location (which knob). If you are replacing screws with knobs, do it now. Reinstall each knob – making the noted number
of turns so it is back to its original position. Do this, one knob at a time. Now, add up and average the number of turns. And re-
adjust all the knobs, one at the time, to the average number of turns. Begin with the knob threaded the most turns into the
secondary and remove it (CCW). The secondary mirror will be a bit loose at this point, reinstall the knob to the average
number of turns. Go to the next longest knob (always one at the time) and remove and re-turning it to the average number of
turns. Finally set the last knob to the average number of turns. When complete, ensure all the knobs are snug (CW) against the
secondary mirror.
Alternate Method for Gross Alignment – A Mirror Reflections Check for a grossly mis-collimated scope can be done by
observing the reflections of the telescope mirrors by looking directly down the tube. You will have to stop using the collimator.
Turn off the laser before proceeding. Cut a small hole in the center of a white letter size sheet of paper. Holding the paper at a
distance of about 7.5 times the diameter of your telescope, look through the hole as straight as you can into the center of your
Optical Tube Assembly (OTA). Move back and forth until you see white and black (or dark) rings. The white rings are mirror
reflections of the white paper in the primary and secondary mirrors kind of like barbershop mirror reflections where you sit in a
chair with a mirror in front of you and a parallel mirror in back of you, and you see reflections of yourself getting smaller and
smaller off to infinity. The dark rings are the OTA barrel and secondary housing reflections. Find a distance where you can see
three white rings. What you want is to see all the rings, both white and dark to be perfectly concentric. If you are reading this,
the rings are not concentric yet. Now, when you begin to adjust the three secondary mirror collimation knobs always remind
yourself to work with all rings simultaneously. You can’t get just two rings concentric or you could be off the center optical axis
one way or the other. To use this method to achieve gross alignment you must work with all the reflected rings (white and dark)
simultaneously, or the optical axis alignment will be skewed to one side.
7.0. Collimating the Secondary Mirror
7.1. Precautions Before Collimating the Secondary Mirror
PRECAUTIONS – Be aware of the following when adjusting the secondary mirror collimation knobs (or screws) located on the
back of the secondary mirror housing assembly:
a. Never touch the central screw which holds the secondary mirror (usually only older SCTs will have this central screw).
b. The three knobs must be turned in by very small amounts. No screw being over-tightened or totally unscrewed. When
secondary mirror collimation adjustment is complete, all three adjustment knobs must be tight. If collimation knobs are
loose, collimation will not be maintained for long! ALWAYS finish adjustment by tightening knobs ONLY. In the process of
adjusting the knobs too achieve collimation – when turning a screw would over tighten it – loosen the opposite knob(s) to
continue moving the original Knob in the proper direction.
c. As you adjust the collimation knobs, observe the collimating lasers three dots position on the collimator target plate to
determine which knob(s) to turn, and by how much. When collimation is close and the three collimation knobs are snug,
finishing adjustments will be on the order of 1/32 turn and less.
NOTE – You will need to constantly re-check telescope & collimator Basic Co-Alignment during the adjustment process to
make sure proper alignment is maintained.
(Continued Next Page)