Double Arch 18 inch pyrex fused F/2.8 Cass mirror. Made in 1988. This "ribbed "version/layout is my own invention .
The 40 inch Gemini M2 secondary mirror, milled of Zerodur,has a triangular open cell open back double arch design with a 2 inch or so faceplate with 5mm thick ribs.Numerous other professional solid double arch mirrors have been made in various sizes since.
Internal mounting supports - three evenly spaced plus a solid base ring support.This type of mirror only properly works with internal support points very close to the centre of gravity of the blank.See articles in PDF below.
On test stand.Internal cross braces clearly seen.
Pre fused/slumped pyrex elements - no bottom ring yet installed.5/8 inch thick faceplate.All ribs are 3/8 inch. No Frit was used -strickly fused at temp over a slumping convex mold in one firing/anneal cycle.
The double arch concept was expanded on in a design study outlined in the Aug 1980 issue of Applied Optics (Vol 19 #16 pp2670-79) for a solid cast 4 meter mirror in a Alta-Az mount.This mirror was to have eight internal support points and also sit on air bags that were between the supports.The first mention of using arches for mirror design came in a 1975 paper in Optica Acta vol 22 #9 pp745-59 by D.C.Talapatra.
The shape of the arch - on the ribs edge - can be varied and this influences the base ring location/radius (or the number of internal supports) plus the overall optical performance.Read the PDF articles carefully.A solid bottom ring is not necessary as long as enough internal cross braces are used between the ribs.
The "ribbed" cellular version shown above can be scaled up to any size one desires.Schott Borofloat 33 can be had up to a sheet size of 66 inches square.
Another view of finished scope.
