collimation error

It is generally associated with poorly made optics or collimation errors.

If they separate there is collimation error.

As modern sextants seldom use adjustable telescopes they do not need to be corrected for collimation error.

If not, then a "collimation error" exists.

Index error, horizontal axis error and collimation error are regularly determined by calibration and are removed by mechanical adjustment.

Any discrepancies between eye focus and measured focus which manifest them-selves across a range of distances within a single lens may be collimation error with the lens, but if such discrepancies occur across several lenses, it is more likely to be the flange focal distance or the groundglass (or both) which are mis-set.