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I was told that it was "cedar", but that is an unclear term botanically.

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    $\begingroup$ *cedar. What's unclear about it? Wikipedia has a good definition (en.wikipedia.org/wiki/Cedar) of the variety of woods called cedar. It would probably depend on where the observatory was located, although imported cedar is not out of the question. $\endgroup$ – Phil N DeBlanc Nov 10 '17 at 17:47
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    $\begingroup$ @PhilNDeBlanc What's unlear about the variety of woods called cedar is which tree or wood it is exactly, to paraphrase the question, or botanically, to refer to the question details. $\endgroup$ – HannesH Nov 10 '17 at 20:09
  • $\begingroup$ As a chemist it seems highly unlikely that the wood would "absorb moisture" to any appreciable extent. I think Mick's answer is right in that cedar would be naturally more rot resistant than say pine. $\endgroup$ – MaxW Nov 11 '17 at 4:15
  • $\begingroup$ @MaxW I do not think rot is likely on the inside of a copula that also contains delicate instruments, and thus would be taken care of. Still, some buffer to humidity might be useful. Maybe any wood would do, but the use of this red one seems to be quite consistent. When "The great Refractor" near Berlin was restored, they made an effort to obtain just this wood. $\endgroup$ – HannesH Nov 11 '17 at 10:13
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    $\begingroup$ @uhoh This was explained to me when visiting the historic Grosser Refractor in Potsdam (aip.de/highlight_archive/refraktor_einweihung/…) , and also at an historic but active observatory in Bamberg, Germany. When they restored Grosser Refractor, they made some effort obtaining the correct wood. We dit not get past the "cedar" description though. $\endgroup$ – HannesH Nov 12 '17 at 17:39
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Probably western red cedar, thuja plicata, especially if the observatory is in North America. Western red cedar is easily obtainable in North America. It has a light, red, wood that is particularly resistant to decay, and ideal for use in exposed positions.

The soft red-brown timber has a tight, straight grain and few knots. It is valued for its distinct appearance, aroma, and its high natural resistance to decay, being extensively used for outdoor construction in the form of posts, decking, shingles, and siding.[28] It is commonly used for the framing and longwood in lightweight sail boats and kayaks. In larger boats it is often used in sandwich construction between two layers of epoxy resin and/or fibreglass or similar products. Due to its light weight—390 to 400 kg/m3 (24 to 25 lb/cu ft) dried—it is about 30% lighter than common boat building woods, such as mahogany. For its weight it is quite strong but can be brittle.

Wikipedia: Thuja plicata

It may be worth pointing out that while "cedar" refers to a group of Cedar woods, trees of the proper Cedrus genus are rare and localized:

Cedrus (common English name cedar) is a genus of coniferous trees in the plant family Pinaceae. They are native to the mountains of the western Himalayas and the Mediterranean region, occurring at altitudes of 1,500–3,200 m in the Himalayas and 1,000–2,200 m in the Mediterranean.

Wikipedia: Cedrus

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  • $\begingroup$ Nice answer! Hope you don't mind the edit. $\endgroup$ – uhoh Nov 12 '17 at 2:49
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    $\begingroup$ All improvements welcome! $\endgroup$ – Mick Nov 12 '17 at 4:51

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