NavList:

Hello, Frank!

I would like to ask you for some coments about telescope parallelism.

Everyone who has experimented with Lunar observations at a slightly deeper level than a beginner  knows that one among the basic rules to get maximum accurate results is to check parallelism of the telescope to the plane of the sextant. If this is ignored the angles will be measured larger than the true value (the greater the angle to be measured—the greater the error).

Usually I take every opportunity to check the parallelism (large lunar distances or Star/Star distances). For such observations I use my 7x power inverting telescope with crosswires in the field of view (wires are not obligatory, but they make the process easier). The procedure is quite simple:

1) for example, choose two Stars which are distant one from other 100°…120°. Make them to be in contact on, let's say, left vertical wire (Fig. 1).

2) Don’t  touch the micrometer and just simply move your sextant slightly so that you can see direct and the reflected image of the star to be on the other vertical wire (in this case—right one) (Fig.2).

If the contact is still maintained, it means the telescope is parallel to the plane of the instrument.

If you will move the instrument so that the two stars are in the center between the two vertical wires, you will see a gap between the stars (Fig 3).

This is the usuall procedure I practise (and the process is described identical in the historic books) . But now I am confused, because accidentally I found some old discussion on this topic in NavList archive. About 20 years ago (August 2004.) [...]

Modris Fersters

[Note from FER: multiple paragraphs from 2004 (!!) deleted. Please do not abuse the archives. NavList is a community for live discussions. If you can't communicate without excavating in the archives, you're not really trying... This applies to everyone...]