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Phillip,
Let's talk about the crosshairs in a sight tube (or combination tool). If you look through the peep hole, you'll see two sets of crosshairs:
--one near your eye, seen directly, and, often, somewhat out of focus for we with older eyes.
--one distant, seen in reflection, and seeming to cross only the reflection of the tool as we see its bottom in the reflection of the secondary.
The one you have to align with the center mark on the primary in order to collimate the focuser axis with the center of the mirror (i.e. by adjusting the tilt of the secondary) is the near-to-your-eye crosshair set, NOT the distant one. [the distant one is made more visible by painting the bottom of the crosshairs white, by the way]
If you cannot focus on the near crosshairs, try backing away from the peephole. To hold your head steady, put a thumb on your face and rest your little finger on the top of the tool. This will allow you to focus on the crosshairs. You don't need to see the whole mirror--only the center mark on the primary and the center crossing of the crosshairs.
The goal is to line up the drawtube of the focuser with a line reflecting off the secondary and hitting the center of the primary. The peep hole, the center of the crosshairs and the primary center mark are 3 points on a line.
Now if the crosshairs are not there, but the front of the tool were painted white, it's conceivable you could line up the peep hole, the center mark on the primary, and the bright ring created by the front of the collimation tool (again, by ONLY adjusting the secondary), but the front of the tool would only be seen in reflection, and that would entail primary collimation accuracy to see it lined up concentric with the peep hole. The crosshairs are there for a reason--so you can line up the focuser axis before adjusting the more critical primary mirror tilt.
When adjusting primary tilt, you obviously adjust the primary to bring the center mark (which you see directly in reflection off the secondary) with the center of the tool, whether its a dark area, as in a cheshire, or simply a large peep hole. Getting this accurate is critical.
But if you wish to adjust the focuser axis/secondary tilt, then you will have to define the center axis of the focuser drawtube. This can be done with a laser, or it can be done with crosshairs in a sight tube.
I've always wondered why sight tube designers didn't use a gun-sight reticle, i.e. a crosshairs with a small circle in the center. You don't have to focus exactly on the crosshairs, then. Other possibilities are double crossed wires, like #, or points that almost meet in the middle, like ><.
Because you certainly aren't the only person who has trouble focusing on the near-to-the-eye crosshairs and the distant centermark on the primary at the same time.
Pulling the eye back helps. Having the center mark be a reflective white ring instead of a dark mark helps. Painting the inside of the wires white can help when there is less light. Note that it is always easier to use a sight tube when the scope is pointed at a bright afternoon sky.
What about the distant reflection of the crosshairs as seen in the reflection of the tool in the center of the secondary's reflection from the primary? They line up behind the near crosshairs when the primary is collimated.
Remember that because collimation builds offset into the secondary, the crosshairs of the spider vanes will NOT intersect the center of the optical axis if the secondary is centered between the walls of the tube/UTA.
Don
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