Advanced CT Laser Collimator Manual (V9.0 =OH=) Page 11 of 20
9.3. The eyepiece drawtube or visual back is not square to your primary mirror
We recommend replacing a poor visual back with a higher quality model
(2-inch backs are generally better in quality than the 1.25-inch models
furnished with SCTs). If the telescope uses a rear-cell focuser, it's possible to
replace a poorly aligned one with a new higher grade eyepiece drawtube or a
focuser that has tip/tilt adjustment to correct the axial error. For example, a
MoonLite CS model
shown here, or a
MoonLite CHL model 2.5”
(see
Step 11.2
) or special modified focuser available from our website. Please
email or call us for availability.
Poor collimation can also result from a focuser or visual back that does not
securely hold eyepieces or accessories. Generally, eyepiece holders using a
compression ring or more than one thumb screw or more than one thumb
screw and a compression ring (as in the figure to the left) do a better job of
maintaining alignment. A higher quality focuser with these features can solve
these problems as well.
10.0. Diagonals, Reducers, and Field Flatteners
If a diagonal, reducer and/or field flattener accessory is machined square with its mirror or lenses square on the optical axis,
and held securely in place, it will not affect collimation. Focus may be in a different position but the collimation will remain
unaffected. (The Celestron Edge series of telescopes have non-removable field flattener lenses as part of its basic design; this
does not change or affect any of the steps or procedures given in this manual.) To check accessory collimation:
a. Achieve Collimation (
Step 7.0 or Step 11.0
) without the accessory. Then, one-at-the-time – Install the component(s),
diagonal, reducer and/or field flattener, and perform collimator & telescope Co-Alignment (
Step 5.0 or Step 11.0
).
b. With the collimator switched to laser mode 2 (or mode 3), check the three collimating lasers’ dots on the collimator target
plate. You may need to re-focus as described in
Step 6.3
.
c. Make the checks in
Steps 10.1 and/or 10.2
as necessary to isolate problem component(s).
10.1 Collimation with a Diagonal Accessory Installed
Two types of collimation problems with attendant hardware problems sometimes manifest with diagonals. Achieve
(
Step 10.0.a
) Collimation and Co-Alignment without the diagonal installed, then, with the diagonal installed – The following two
kinds of problems may be detected observing the reflected collimating lasers’ three dots on the collimator target plate. Check
the following when the diagonal is rotated to different positions relative to the telescope rear cell:
a. Three dots wiggle and walk when rotated, and move to different positions when retaining thumb screws are tightened.
Problem: Most common – This indicates that either or both the focuser or visual back hardware that secures and registers
the diagonal, or the hardware part of the diagonal that secures and registers the final optic is inadequate (the final optic is
the reflector mirror in this case). Tighten and loosen the thumb screws on the focuser or visual back, and the diagonal.
Good hardware that secures and registers properly will maintain collimation before and after rotating the diagonal or final
optic. Replace or repair the focuser, visual back, or diagonal that is not adequately registering and securing its installed
components (for improved focusers
see Step 9.3
).
b. Three dots are not on the same target plate ping and move smoothly in a well-defined circular pattern without wiggling.
Problem: Less common – The mirror inside the diagonal is not correctly aligned. The more the three dots are off a single
concentric target plate ring – with vs. without the diagonal – the more suspect it is that the diagonal’s mirror is out of
alignment. Replace or repair the diagonal with the misaligned mirror.
10.2. Collimation with a Reducer, and/or Field Flattener Accessory Lenses Installed
PRECAUTION – DO NOT adjust the telescope Secondary Mirror Knobs (or screws) with a reducer installed. A reducer
desensitizes adjustments made with the secondary mirror knobs. Instead, collimate the telescope with
Step 11.0
procedures.
To achieve collimation with a reducer or field flattener accessory installed:
a. If the basic collimation is good you will see a good circular pattern of the three collimating laser dots close to the same
target plate ring.
b. If the three collimating laser dots are not on the same target plate ring, the tilt of the rear cell optics axis and/or the
centering of the corrector plate need to be adjusted. To make these checks and adjustments, remove the accessories,
perform
Step 11.0
, and iterate its sub-steps until no further improvement can be made. Then, reinstall the accessory
reducers and field flatteners, and perform
Step 11.0
to check collimation with the accessory. If the collimation is not aligned
the telescope can be collimated by performing
Step 11.0
specifically for the condition of having the accessory installed or
not. To have the best possible optical performance under each of the two conditions (accessory lenses installed or not
installed), the telescope will have to be collimated separately for each condition. If photography is the primary goal, use
Step 11.0
to collimate most accurately for this setup. This may still allow sufficient optical performance to enjoy eyepiece
viewing without having to perform
Step 11.0
collimation specifically for eyepiece use. Because the viewing eye is the final
optical element (eyeball cornea & auto-focus lens), and the ability to move around over the eyepiece to collectively
stacking the best view over different viewing position, this can have the effect of allowing some dynamic fine tuning
collimation, as opposed to camera imaging where the final imaging sensor is fixed.