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These telescope wires can be useful when we want to get maximum accuracy for specific observations. This typically refers to Lunar distance and accurat AH observations. For traditional Sun, Star, Planet sights, when altitude is measured relative to the natural horizon there is no point in fighting for great accuracy, as the uncertainty of the horizon and approximate (and changing) height of eye can produce larger errors than sextant itself. But for Lunars each tenth of the minute of arc is important and the wires on the high power scopes can be useful.    

From my personal experience with an inverting 7x scope (I use two sextants: SNO-M and SNO-T) I find wires to be useful in following situations:

1) inverting scope (Kepler’s optical system), has a wide field of view and the two vertical wires help to achieve minimum offset from the central vertical  line of the field of view. This is important when observing large angles.

It is convenient that wires are next to the observed objects and can serve as guidelines; and full attention can be focused on contact between the two celestial bodies. The round border of the field of view is somewhere in periphery (usually even partly out of view, because it is practically impossible to comfortably bring an eye so close to the eyepiece to see the full circle of the field of view). Therefore it can not serve as a reference for finding the center (esspecially when sextant is held in uncomfortable position, as it is common during Lunar observations).

In this case we actually use only two vertical wires, because the contact point can be performed anywhere in the central vertical band. It is not important to find geometric center point and try to get contact of the objects excatly in this point. Instead all the central vertical band is suitable (for this reason older sextants had only 2 wires, not 4).

2) telescope colimation control. If it is possible to observe convenient large angle (lunar distance, star/star distance) the two vertical wires allow checking telescope paralelity to the plan of the sextant very fast. It takes only seconds to get very accurate answer to the question: is telescope adjusted correctly. Table top method is convenient for adjusting telescope at any time (of course, if the sextant model has adjustable scope), but for periodical check I prefer lunar distances. It is just a one minute added to the lunar observation sight taking procedure.

3) index error by Stars. When checking index error by Star, it is recomended to adjust sextant mirrors so that there is a little side error. And when adjusting the two images of the Star, it is convenient that there is a horizontal reference system. The horizontal wires can help there.  When the imaginary line connecting the two Star images is paralel to the horizontal wires, I know that an index arm is positioned at zero.

Alex mentioned that the wires help to get objects in focus. I don’t share such an experience. I assume that the peculiarities of individual vision can be observed here.

Besides we must take into the consideration that the wires can be mounted differently on scopes. For example SNO-T inverting scope has different composition than SNO-M scope. At the first glance these scopes seems to be identical (lenses are interchangable), but they are slightly different. SNO-T scope has the wires mounted on objective lens tube, but on SNO-M they are mounted on eypiece lens tube. This means that on SNO-M scope wires are always in focus, but disadvantage is that every time you rotate eyepiece tube you have to adjust the wires to be paralel to instrument’s plane. Contrary in SNO-T scope wires are fixed in place, but when adjusting eyepiece the wires get slightly out of focus. It is a small disadvantage for table top observations for short distances.

If someone has an old sextant (or not so old SNO-M or SNO-T sextant) with adjustable scope that has these wires, it is worth to study how to use the potencial of these wires, esspecially if Lunars and accurate AH sights are planned.

But this does not mean that the wires are obligatory. For low power Gallilean scopes and for traditional altitude sights there is no need for these wires. The lack of these wires does not limit our options in celestial navigation in any way.

In my oppinion “the wire issue” is partly a matter of habit. If observer feels comfortable using these wires, he will miss them if forced to use high power scope without them. And contrary.

The same can be said for telescopes. Some observers don’t like inverting scopes. But I find them excelent (especially for Lunars and AH sights).

Modris Fersters