Joe, refractors have an optical axis, like any other telescope, and it must be aligned with the focuser for the telescope to perform properly. By their construction nature, well-built refractors don't require periodic adjustment unless dropped or damaged. As Siegfried noted, no need to check unless you suspect a problem. Most of the time, the problem is at the focuser end. If the tube end has been machined truly square, and the objective cell has been machined correctly and attached robustly, a collimated objective will remain that way. Focuser deflection probably accounts for most refractor collimation issues. This can be a problem with both Crayford and rack-and-pinion focusers. Many amateur refractor builders use an adjustable cell because they lack the tools or skill to produce a tube with a truly square tube end, or use unstable materials such as PVC for the main tube. In those cases, the objective will require collimation. But commercially made refractors are typically built pretty well (except the low-end stuff) and will probably not require any adjustment during the instrument's lifetime. Collimation errors can be introduced if an objective is removed for cleaning, and not re-installed correctly. A good reason to practice good housekeeping when using your telescope, try to not introduce foreign material such as dust into the OTA interior. Cap the focuser when not in use. Like reflectors, the faster the f-ratio, the more critical the collimation requirements with a refractor. Longer f-ratios can be off a tad and still produce a good image. Short f-ratios have to be dead-on. On Thu, May 22, 2014 at 12:09 AM, Joe Bauman <josephmbauman@yahoo.com>wrote:
I haven't heard of collimation problems with refractors, but I suppose almost anything might go wrong with a telescope.