What kind of reddish wood is it exactly that was historically used to line the inside of observatory cupolas (domes), in order to absorb moisture? I was told that it was "cedar", but that is an unclear term botanically.
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