Hi Dave, 1. Some refractors require periodic collimation, some don't. It depends on the robustness of the build. The last decade has seen a rise in the popularity of the collimateable objective cell; that is, it has 3 adjustment screws (and 3 locking screws), much like the primary mirror on a Newtonian. 2. It can be either the objective, the focuser, or both. 3. Too great a load on the focuser can cause the drawtube to deflect off-axis slightly, especially when close to fully extended. A heavy focuser load may require a more heavy-duty focuser. Crayford focusers are falling out of favor among refractor users because of slippage under load. Not really a collimation issue, but it's something to be aware of, and it's cause is the same as deflection of the drawtube. Too much load, not enough beef. 4. If the focuser is deflecting, see my answer to question 3. Get a beefier focuser. If it's not a drawtube deflection problem, it may be that the focuser got whacked and is just off-center. A laser collimator inserted into the drawtube should show you if it's off-axis. The dot should be in the precise center of the objective lens. Be careful using a laser on a refractor, don't let the beam enter your eye. You're a scientist so I know that you know the ropes. Focusers that are bolted on can be re-squared by loosening the screws and re-tightening when aligned. Thread-on focusers out of alignment may be damaged and require replacement. Or the main tube could be dinged close to the focuser attachment point. This can cause a mis-alignment in some cases. The returning beam will also tell you if the objective needs collimating. If the objective is tilted, the beam will not return along it's path exactly. If that's the case, you will need to recollimate your objective. If the cell has the three adjustment screws, it's easy. If not, then your options are more limited. If it's a thread-on cell, you may need to shim the objective in the cell. If it's bolted or screwed-on, you may need to loosen the fastening screws or bolts and manhandle the cell until the beam is centered, then re-tighten. If it's a commercial scope and you don't want to perform the surgery yourself, send it back for a factory re-collimation. The laser will only get you 95% of the way there. You'll have to check the diffraction pattern of a star to be sure of perfect collimation (the "star test"). You might get lucky and have it nailed, but you might need to do some tweaking. Does any of this help? I'm off to bed. 3:30 AM gets here all too quickly. BTW, I'm amazed at how bright the eastern sky is at 5 AM these days! C. On Wed, May 21, 2014 at 4:46 PM, Dave Gary <davegary@me.com> wrote:
Don, Joe, Steve, Chuck, Sig, Patrick, anyone who knows:
My ignorance knows no bounds. I’ve been reading your posts regarding Steve’s collimation/tracking problems.
1. Do refractors usually require collimation after a period of time?
2. Does it, usually, involve the focuser and not the objective?
3. If it’s the focuser-end of the scope, can it occur because you have put too equipment (weight) on that end and whacked it out?
4. How does one fix it?
I don’t have any problems with my scope. I’m just ignorant of the process. I, honestly, didn’t think you collimated refractors.
Thanks in advance for any input.
Dave