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Orientation of the NRAO 140 Foot Telescope Axis

-- | July 21, 2015

Question: I was at the NRAO two days ago. Magnificent! But in walking by the 140′ scope (by “Neptune”) it looked as though there was a mistake in the labeling on the informational sign. On the diagram that explains how an equatorial mount works it purports to point to be pointing with a red arrow to the telescope axis parallel to the Earth’s axis. But the arrow is actually pointing to an earth radius at the base of the telescope (vertical axis to the center of the Earth). Was my understanding of the sign wrong or is the sign itself wrong. Sorry I could not take a digital photo of the sign in question to send (-; By the way, I also found it unusual for scientists to be mixing their Imperial measurements up with the Metric, with a 140 foot telescope being located so close to a 30 meter one, for instance. Wasn’t a space probe or space telescope destroyed by such confusion in its manufacture? — Richard

Answer: I believe that your interpretation of the 140 foot telescope mount is correct, and that the diagram showing the arrow indicating the axis of the antenna pointing to an earth radius at the base of the telescope axis parallel to the Earth’s axis is not exactly correct. It is the actual axis of the moving part of the antenna (the “polar axis”) that is parallel to the Earth’s axis. Regarding the name of this telescope and its use of an Imperial unit, this is simply history and a desire not to rename. We do work exclusively in a metric-based system, with few exceptions. The one famous exception that you refer to, where a misunderstanding as to whether a calculation was done in metric or imperial units in a critical piece of software, caused the Mars Climate Orbiter to dip too deeply into the atmosphere of Mars and burn-up in 1999.

Jeff Mangum

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