|
'n Kunsmatige horison is nuttig wanneer die horison nie sigbaar is nie. Proffesionele sekstante het 'n kunsmatige horison hegstuk wat aangeheg kan word in die plek van die horisonspieël samestelling. 'n Kunsmatige horison bestaan gewoonlik uit 'n spieël wat 'n vloeistof gevulde buisie met 'n borrel in weerspieël. Die meeste sekstante het ook filters wat gebruik word wanneer die son besigtig word of om die effek van wasigheid te verminder.
Die meeste sekstante het 'n enkellens teleskoop van 1 tot 3 maal vergroting vir besigtiging. Baie gebruikers verkies 'n eenvoudige sigbuis wat 'n wyer en helder sigveld verleen en makliker is om snags te gebruik. Sommige navigators bring 'n ligversterkende teleskoop aan om die horison op maanlose nagte makliker sigbaar te maak. Ander weer verkies die gebruik van 'n beligte kunsmatige horison.
<!-- Moet nog vertaal word ▼
Proffesionele sekstante gebruik 'n stopmeganisme op die gradeboog en 'n wurmwiel om verstellings te doen tot op die naaste [[minuut van grade|minuut]], oftewel 1/60<sup>ste</sup> van 'n [[graad]]. Die meeste sekstante sluit ook 'n [[vernier]] op die wurmwiel in wat dit moontlik maak om die hoek tot die naaste 0.2 minute af te lees. Aangesien 'n 1 minuut fout ongeveer 'n [[seemyl]] is, is die beste moontlike akkuraatheid wat verkry kan word ongeveer 0.1 seemyl. Dit is ongeveer 200 meter. Op die oop see is 'n resultaat wat akkurater is as 'n paar seemyl aanvaarbaar aangesien dit gewoonlik binne sigafstand is.
Most sextants mount a 1 or 3 power [[monocular]] for viewing. Many users prefer a simple sighting tube, which has a wider, brighter field of view and is easier to use at night. Some navigators mount a light-amplifying monocular to help see the horizon on moonless nights. Others prefer to use a lighted artificial horizon.
'n Verandering in temperatuur kan die gradeboog se dimensies beïnvloed en onakuraathede teweeg bring. Baie navigators koop gewoonlik weerbestande tasse om hulle sekstant in te bewaar sodat die sekstant buite die kajuit gestoor kan word en die gradeboog se temperatuur dieselfde is as die temperatuur buite. Maatreëls word dikwels ook getref in die keuse van die materiale van sommige sekstante om die termiese uitsetting van die gradeboog so laag as moontlik te hou.
Professional sextants use a click-stop degree measure, and a worm adjustment that reads to a [[minute of arc|minute]], 1/60 of a [[degree (angle)|degree]]. Most sextants also include a [[vernier]] on the worm dial that reads to 0.2 minute. Since 1 minute of error is about a [[nautical mile]], the best possible accuracy of celestial navigation is about 0.1 nautical miles. This is about 200 [[yard]]s. At sea, results within several miles, well within visual range, are acceptable.
A change in temperature can warp the arc, creating inaccuracies. Many navigators purchase [[weatherproof]] cases so their sextant can be placed outside the cabin to come to equilibrium with outside temperatures. The standard frame designs (see illustration) are supposed to equalize differential angular error from temperature changes. The handle is separated from the arc and frame so body heat does not warp the frame. Sextants for tropical use are often painted white to reflect sunlight and remain relatively cool. High-precision sextants have an [[invar]] (a special low-expansion steel) frame and arc. Some scientific sextants have been constructed of quartz or ceramics with even lower expansions. Many commercial sextants use low expansion brass or aluminum. Brass is lower-expansion than aluminum, but aluminum sextants are lighter and less tiring to use. Some say they are more accurate because one's hand trembles less.
▲<!-- Moet nog vertaal word
[[Aircraft]] sextants are now out of production, but had special features. Most had artificial horizons to permit taking a sight through a flush overhead window. Some also had mechanical averagers to make hundreds of measurements per sight, to compensate for random accelerations in the artificial horizon's fluid. Older aircraft sextants had two visual paths, one standard, another designed for use in open-cockpit aircraft that let one view from directly over the sextant in one's lap. More modern aircraft sextants were periscopic with only a small projection above the fuselage. With these, the navigator pre-computed his sight and then noted the difference in observed versus predicted height of the body to determine his position.
A sextant is a delicate instrument. If dropped, the arc might bend. After one has been dropped, its accuracy is suspect. Recertification is possible with surveying instruments and a large field, or with precision optical instruments. Repair is not possible.
To avoid worries about bent arcs, serious navigators traditionally buy their sextants new. Common wisdom is that a used sextant is probably bent. Many navigators refuse to share their sextant, to assure that its integrity is traceable. A used sextant lacking a case is very likely to have a bent arc.
Most sextants come with a neck-lanyard, and all but the cheapest come with a case. Traditional care is to put on the neck lanyard before removing the sextant from its case, and to always case the sextant between sights.
After a sight is taken, it is reduced to a position by following any of several mathematical procedures. The simplest sight reduction is to draw the equal-elevation circle of the sighted celestial object on a globe. The intersection of that circle with a dead-reckoning track, or another sighting gives a more precise location.
| 