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Collimation
Yep, the entire secondary mirror assemby - the cylindrical base in which the secondary mirror is clipped - attaches to the spider via that center bolt, which is tightened or loosened from above the spider - that's the bolt. This bolt has a spring around it, which serves a purpose in assisting with collimation (below). But the bolt also holds that secondary mirror assembly on the spider, so don't be loosening that unless you are prepared to catch the assembly if you loosen too much. Best to do this with tube horizontal!
As you turn that bolt, the spider will move up or down. This movement gives you freedom to position the secondary mirror under the focuser drawtube correctly. Bintel does a good job in dialing that in for you, but it's not uncommon for that to be way off for other "out of box" purchases. This centering the secondary under the focuser is part of collimating a dob. If it's off you will be losing some of your mirror surface area: vignetting.
That center screw atop the spider, when turned, will either move the spider "up" and thereby compressing the spring, or "down" and thereby loosening the spring. If you removed that spring, you'd notice that the secondary mirror assembly actually pivots or wobbles a bit on the end of the bolt; it isn't fixed solid with the bolt. That wobble is intentional. When compressed the right amount, that spring provides dynamic tension in the assembly, against which the OTHER three smaller bolts - the three collimation bolts - push in the opposite direction. They push on the top of the secondary mirror assembly to fine tune the 'tilt' of the spider. There should be a metal washer (a fender washer) against which those three bolt ends push.
My spring was too lax, actually. I had to add a washer to take up space, make it more compressed, to achieve the fine tuning of the secondary mirror.
Here is a cut-and-paste of my description on the skyquest_telescopes Yahoo group regarding a related question.
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I have a GSO dob, and the secondary looks identical. This GSO came
with a fender washer.
I switched the stock secondary collimation bolts with Bob's Knobs,
which, I think, are similar to the Meade Lightbridge specs. Much easier
than dangling a screwdriver over that end...
I also experienced that erratic secondary collimation. I finally
figured it out in my particular situation. At least I think so, and
I'll try to explain it. I obviously can't confirm that is what you are
experiencing.
Smooth collimation of the the 2nd mirror depends on the right tension
balance between the spring-loaded center bolt and the three collimation
screws. The center bolt at its top isn't fixed to the vane, but rather
wiggles and pivots a bit, thereby allowing the secondary to "wiggle"
when all three collimation bolts are loose. This wiggly center bolt is
holding the secondary to the vane, but the spring around it is pushing
away. This degree of wiggle is determined by the tension of the
spring of the center bolt. The tighter the secondary bolt, the more
tightly compressed is this spring. If the spring is too loose then the
three collimation bolts, as they move down to push against the fender
washer, don't have much to move before they "push" the secondary beyond
the tension of the spring. The result is clunking of the secondary when
you try to adjust the collilmation bolts. You need a longer spring,
or...
My solution was to increase the tension of the spring by inserting
several washers over the "upper" end of the center bolt, thereby
increasing the tension of the spring when the collimation bolts would
make contact with the fender washer. I also took the liberty of
putting small thin rubber caps on the business ends of the collimation
bolts so the bare metal end of the bolts doesn't dig grooves into the
fender washer, thereby causing erratic jumps as the bolt rides over,
then off, one of these grooves in the washer. I rotated the fender
washer after apply these caps so the capped bolt ends are making
contact with smooth unmarred metal.
Now it collimates with wonderful predictability, not the erratic
jumping of earlier.
After you make any changes, re-confirm that your secondary mirror is
correctly centered under the focuser tube. Lots of back and forth
adjustments of the center bolt might leave you unexpectedly off by a
significant amount.
Hope this makes sense and helps.
Scott
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