NavList:

Marty Lyons, you wrote:
"What is the purpose of the wires in scope view?"

Mostly tradition?

It's a fraught issue. Do we need to take sights in the exact center of the field of view of the sextant scope? Do we need to take sights near the middle of the field of view? Do wires really help materially in locating the center of the scope's field of view? Why are these wires not found in most sextant scopes if they actually improve sight accuracy? What is a poor navigator to do with a scope that has no wires, or for that matter what should you do if you're taking sights with a plain sight tube that has no optics in it? And finally how does this impact sights when there isn't even a sight tube, for example, if you take sights with a "Bris sextant" or any similar barebones "fixed angle" sextant?

The genuinely important condition related to this problem is caused by poor scope "collimation". This is something that we can often see directly in older sextants. You look at the sextant from above the telescope (above the index mirror) at a viewing direction in the plane of the frame of the sextant, and you can see plainly that the scope is tilted. This can be a serious problem, and this tilt may also be present even if it's not obvious.Thus we learn to collimate. Sextant sights are most accurate when the viewing location is on an axis perpendicular to the mirrors --a description, which, while helpful phrasing at first glance, isn't really right. What we want is a line of sight that is parallel to the sextant frame if the mirrors are properly adjusted to be perpendicular to the frame or, if not, then the viewing axis should be parallel to an abstract plane that is perpendicular to the mirrors. If this is not the case, then the "axis of collimation" can be well outside the field of view of the scope, and significant errors in sights follow! So we collimate our scopes... What? You've never done that? In fact, it's only rarely necessary and mostly for instruments that are older than half a century or so. Want to give it a try? There are tabletop methods for this. Details upon request.

Even if your scope is reasonably well collimated, you may still be able to detect some "centering error". This is neither normal nor common, and it may take some effort to make it visible. The best case test to try is a local noon Sun sight off an artificial horizon (mirror or "puddle" of fluid). You may also be able to see it using similar sights of a bright star or planet in place of the Sun. The angle you are observing is stable. Bring the two objects into contact. Then maintaining the contact, slightly rotate the sextant about a "vertical" axis (not really "vertical" but you'll figure it out by "playing" with the motions) so that the images move left and right across the sextant scope's field of view. If you can see this, you should notice that a gap develops between the two Sun (or star) images as you approach the limits of the field of view. Note that there should be no growing gap when you move the images up/down in the field of view (if you see a gap in that direction, your sextant is probably plastic!). What you may well find, however, is that you can't do this at all. The standard optics in many sextants only allow us to see the direct and reflected views relatively close to the center of the field of view in the first place. Also, for nearly all observers, it is "natural" to make contact generally in the middle of the field of view (why would anyone do anything else?), and as long as the scope is properly collimated, there's no issue in that case. The key to all of this is that we get the correct, accurate measurement of the angle when the objects are aligned at the "center of collimation", not normally the center of the field of view but very close if the scope is collimated.

Can you find the center of the field of view better if you have wires in there? This part I have never understood. You look through the scope. The center is obvious. Perhaps the wires are intended to encourage more careful behavior, as a sort of "reminder", but you don't need them. Certainly one can take high-accuracy sextant sights with a scope that has no wires or with a simple sight tube. The real driver of accuracy in these cases is right "on the label": the magnification of the scope. And this is where the so-called inverting scope usually wins. By tradition, it was the highest magnification scope in the case. Wires needed? No. I can't see how (except for Alex's interesting suggestion that they may help with accurate focus...). Magnification recommended? Yes, if you want better than a minute of arc accuracy in your observations.

Should we add wires to our "Bris sextants"? (that's meant to be funny, but if you don't know what a Bris sextant is yet, it may not be!) A Bris sextant consists of several glass plates (microscope slides were recommended in the original) that are glued together in a fanned out layout so that we can see multiple reflections of the Sun when it's pointed at the horizon. This is a true double-reflecting instrument applying the same optical principle of the earliest reflecting quadrants three hundred years ago. This is the ultimate minimalist sextant. It's a "fixed angle" sextant, meaning that it only measures a handful of angles, which have to be determined by practical calibration with known angles (easy with my app, by the way). Since it measures only fixed angles, there is no index arm and no moving parts at all. The Bris sextant goes ultimate minimalist by dispensing with the sextant frame itself. It's just the "mirrors" of a standard sextant "free-floating", and it's held by the edges of those mirrors. When taking sights with a Bris sextant, it must be rocked, "swinging the arc" just as any sextant, and in addition it should be rocked a bit left and right to make sure that the sight is being taken near the center of collimation. If this isn't done, it is quite easy to be many degrees off-axis with a Bris sextant yielding very poor sights! So you rock it in a two-dimensional fashion seeking the direction and orientation that gives minimal contact between Sun and horizon. And then "Mark".

One case to watch out for: I have met two navigation enthusiasts who became comfortable getting contact on the extreme left of a sextant scope's field of view. This is not even possible with the optics of most sextants, but it can happen. They had become accustomed to this approach because it served as another way to reduce the brightness of the Sun. Generally, this would qualify as a bad habit. But even here, there is an exception ... an "out", an escape clause. The sextant scope can be collimated to place the center of collimation wherever we want. And if the left side of the field of view is attractive for some other reason (like those two navigation enthusiasts thought they had discovered), then it is possible to move that center of collimation to that favored location. Then it's all good. You have to remember, though, if this has been done, that all sights should be taken somewhere near that side of the field of view.

Collimation error grows quadratically across the field of view with angular distance from the center of collimation (which is normally very close to the center of the field of view). If it's normal for you, as it is for the vast majority of navigators, to take your sights with the body "more or less" in the middle of the field of view, then the error is essentially zero. This requires no special procedure. Do you need wires to achieve this? No, but perhaps they're a nice reminder. If the scope is misaligned, and the center of collimation is actually outside the field of view, then you will get large errors, growing steadily across the field of view, which are greater at larger angles. Collimate that scope! :)

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA

PS: Modris Fersters went to rather great lengths back in February to take sights off-axis and get some numbers on errors that can result from this. You really have to force the matter to get these sorts of errors, but it's certainly worth seeing that if we do force our sights off-center, then there are measurable errors. Here's that post:
https://navlist.net/Telescope-parallelism-lunars-Fersters-feb-2024-g55508.