Primary Mirror Collimation

[shane.mcneil]

Hi everyone,

Just a question about my ongoing quest for collimation perfection. I noticed that there is a lot of travel on the collimating screws for the primary mirror. I can for example wind the mirror down close to the end of the tube and collimate it there. Or I can wind the mirror away from the end of the tube as far as the screws allow and collimate it there.

Does it matter where the primary mirror is positioned along the length of the tube before collimating?

Hope that makes sense.

Shane

[mswhin63]

I find that may provide a bit of length sorter or longer for focus but i can't imagine it would make any difference. I may do this for camera as inserted only leave me about 1mm of focal travel left before runnout. Most lenses wouldn't matter.

[erick]

Most with GSOs prefer to pull the mirror down a fair way so the springs are well compressed. Some have found the springs to be weak, so if they are not compressed the primary mirror may "flop" about a bit - so you lose collimation as you adjust scope altitude.

Suggest that you collimate at about the altitude where you want to observe. If it is everywhere ( :D ) then collimate at around 45 deg. Then move the tube unit vertical and check collimation, then move it horizontal and check collimation. The is easily done with a laser collimator since you can watch the return dot through the laser window as you move the OTA. If you are getting a fair amount of movement in collimation, pull the primary down further (springs more compressed) and try again.

[shane.mcneil]

Thank you for the advice. I had heard about the GSO springs not being that great. That's why I tried it. But I assume changing the distance between the primary and secondary mirrors also changes the focal point. It doesn't seem that it matters too much then. Many thanks again.

Shane

[erick]

Yes, the focal point will remain the same distance from the primary mirror - so when you move mirror up the tube, the focal point will move further out of the focusser draw tube. So you don't wind in as far to reach focus. The only problem (apart from perhaps hitting the max out travel on the focusser for some eyepieces) is that the light cone is a bit broader at the secondary. Depending on secondary size, perhaps you lose a little of the converging light around its sides? Pulling the mirror down the tube just leads to possibly hitting max in travel before achieving focus.