good morning,
i am currently writing and researching for an article, hopefully to be published soon.
one significant element of this concerns astrometry and the use of a zenith sector. this is an instrument that i feel i don't know enough about.
first, the details: the zenith sector that i am researching belonged to Andrew Ellicott. the scope was about 6 feet long, and the focal length about the same. it is stored at the Smithsonian, but to date they can't answer my questions. their website does have a couple of good photos, it is catalog number PH*152078
having said that, here are some questions:
What is the scale range? An associate of mine thinks it could be seven degrees each way from zenith. Please clarify.
What is the resolution of the scale? Based on NMAH's photos, it appears to be one half arc second, but again I am making an estimated, educated guess.
Are there two separate eyepieces? One appears axial, the other appears to be right angle.
i have read claim that this instrument was able to measure second and third magnitude stars any time of day or night. is this possible? would a large sun shade have been necessary, or would the observation tent been enough to filter the light? i have read some claims that it (third magnitude stars in daylight) is possible with modern scopes, but i do not know about an old zenith sector
You might want to contact the reenacters of the Corps of Topographical Engineers, particularly Don Erickson. They have a zenith sector that they demonstrate at various events.
They were at the 2013 Surveyors Historical Society rendezvous, and there is a picture somewhere on that site.
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