LBT 8.4 meter Honeycomb Mirror Casting #1

by: A.R. Lampis, Y.Y. Shlemenzon, J. M. Hill and J. A. Rigoli

On September 10, 1997, the Mirror Lab personnel opened the furnace to reveal a very beautiful looking 8.4m honeycomb mirror! This is the world's largest optical telescope mirror blank which is made from a single piece of glass.
Casting of the mirror took place in mid-January 1997. The mirror underwent a slow annealing and cooling process and was inspected in early April. After estensive analysis it was confirmed that the leakage of glass noted during the casting resulted in some thinning of the mirror's faceplate. An area consisting of approximately 10% of the total surface of the mirror was at a less than optimum thickness. Apparently the hydrostatic forces from the liquid glass caused several of the tub wall segments to shift and open the joints to allow glass leaks. Two tons of glass were added to the mold in late April and a slow heating process was started. The highest temperature for flash melting of the new glass occured on June 10. After a cool down period of three months the mirror was ready for inspection and the news are good: the face plate has been found to be pretty uniform with an average thickness of 1.65 inches . There are very few small bubbles, the majority of which will be removed during the generating/polishing process.
The furnace wall sections and the inconel bands encircling the mold have been removed. On February 23, 1998 the mirror was lifted and positioned in the cleaning station where the refractory material will be removed with high pressure water jets.
As of July 24, 1998 all of the mold material had been removed from the inside of the glass honeycomb. The 8.4 meter mirror had to wait a few months for completion of polishing on the Magellan 6.5 meter mirror.
The 8.4 meter mirror blank has now moved into the polishing lab in the nadir-pointing position. Grinding (generating) and polishing of the flat back surface of the blank began in January 1999 and was completed in June 1999. The permanent load spreaders were bonded to the back of the mirror with RTV during the last few months of 2000. In parallel, the 8.4m polishing cell was prepared to accept the mirror. The 8.4m mirror was installed onto the polishing cell in March 2001.
Grinding (generating) on the parabolic front side of the 8.4 meter mirror began in August 2001 and was completed in October 2001. After converting the LOG to the stressed lap configuration, loose abrasive lapping of the generated surface began in January 2002. At the same time, the infrared (10 micron) null corrector was installed in the test tower to measure the ground surface.

The 8.4m honeycomb mirror on the LOG for grinding the front side of the mirror blank. Kirk O'Laughlin is seen inspecting the outer diameter of the faceplate. The grinding head is just out of the picture on the right side. (photo by J. Hill)
This diamond-impregnated wheel is used to edge the frontplate of the honeycomb mirror blank to the proper diameter. A water-based coolant is sprayed on the cutting surface to keep the wheel and the glass from overheating. (more photos of the frontside generating process) (photo by J. Hill)

Final cleaning of the loadspreaders on the back side of the first 8.4m mirror. (photo by J. Hill)
Lowering the first 8.4m mirror onto the polishing cell. (more photos of the installation of the mirror into the polishing cell) (photo by J. Hill)

Loadspreaders are bonded onto the back of the 8.4m mirror with RTV. These loadspreaders are the permanent attachment between the glass and the support actuators. (photo by J. Hill)
G. Weir installing static supports on the 8.4m polishing cell. The static supports protect the mirror in case the support acutators have a failure. (photo by J. Hill)

Engineering intern J. Fales carefully explores the polished back side of the 8.4m mirror. (photo by J. Hill)
The 8.4m polishing cell arrives at the Mirror Lab. (more photos of the arrival of the polishing cell) (photo by J. Waack)

The LOG (with a tiny grinding head) is used to mark the positions on the backplate of the first 8.4m mirror where the loadspreaders will be glued in place. (photo by J. Hill)
The first LBT 8.4m mirror waits face down in the polishing lab while the second Magellan 6.5m mirror is generated in the background. (photo by J. Hill)

LOG operator John Ray monitoring the polishing of the backplate. (photo by J. Hill)
Polishing lap overhanging the edge of the backplate. (photo by J. Hill)

More Backplate Grinding Pictures

Casting Updates

Casting Pictures

Casting Movies

Mold Construction

Casting Process

Glass

Furnace

Advantages of Spin-Casting

8.4m LBT mirror with J. Hill and W. Davison
Photo by: Lori Stiles
(click here for the 0.7Mb version.)
(click here for the 2.3Mb version.)
(click here for the 6.6Mb version.)

8.4m LBT mirror and a few close friends
Photo by: Lori Stiles and John Florence
(click here for the 1.9Mb version.)
Be sure to see the article in Scientific American Quarterly:"Magnificent Cosmos" by J. R. P. Angel and N. J. Woolf about how this mirror may be used to search for life in other solar systems (March 1998).
See also the article about the LBT mirror and other large telescopes in the May 1999 issue of Discover Magazine .

For more information on the Large Binocular Telescope visit the project home page at:
"http://mirrorlab.as.arizona.edu/lbtwww/"

Back to Mirror Lab Homepage

Last modified: Wed Jan 16 13:54:45 2002

On September 10, 1997, the Mirror Lab personnel opened the furnace to reveal a very beautiful looking 8.4m honeycomb mirror! This is the world's largest optical telescope mirror blank which is made from a single piece of glass. Casting of the mirror took place in mid-January 1997. The mirror underwent a slow annealing and cooling process and was inspected in early April. After estensive analysis it was confirmed that the leakage of glass noted during the casting resulted in some thinning of the mirror's faceplate. An area consisting of approximately 10% of the total surface of the mirror was at a less than optimum thickness. Apparently the hydrostatic forces from the liquid glass caused several of the tub wall segments to shift and open the joints to allow glass leaks. Two tons of glass were added to the mold in late April and a slow heating process was started. The highest temperature for flash melting of the new glass occured on June 10. After a cool down period of three months the mirror was ready for inspection and the news are good: the face plate has been found to be pretty uniform with an average thickness of 1.65 inches . There are very few small bubbles, the majority of which will be removed during the generating/polishing process. The furnace wall sections and the inconel bands encircling the mold have been removed. On February 23, 1998 the mirror was lifted and positioned in the cleaning station where the refractory material will be removed with high pressure water jets. As of July 24, 1998 all of the mold material had been removed from the inside of the glass honeycomb. The 8.4 meter mirror had to wait a few months for completion of polishing on the Magellan 6.5 meter mirror. The 8.4 meter mirror blank has now moved into the polishing lab in the nadir-pointing position. Grinding (generating) and polishing of the flat back surface of the blank began in January 1999 and was completed in June 1999. The permanent load spreaders were bonded to the back of the mirror with RTV during the last few months of 2000. In parallel, the 8.4m polishing cell was prepared to accept the mirror. The 8.4m mirror was installed onto the polishing cell in March 2001. Grinding (generating) on the parabolic front side of the 8.4 meter mirror began in August 2001 and was completed in October 2001. After converting the LOG to the stressed lap configuration, loose abrasive lapping of the generated surface began in January 2002. At the same time, the infrared (10 micron) null corrector was installed in the test tower to measure the ground surface.
The 8.4m honeycomb mirror on the LOG for grinding the front side of the mirror blank. Kirk O'Laughlin is seen inspecting the outer diameter of the faceplate. The grinding head is just out of the picture on the right side. (photo by J. Hill)	This diamond-impregnated wheel is used to edge the frontplate of the honeycomb mirror blank to the proper diameter. A water-based coolant is sprayed on the cutting surface to keep the wheel and the glass from overheating. (more photos of the frontside generating process) (photo by J. Hill)
Final cleaning of the loadspreaders on the back side of the first 8.4m mirror. (photo by J. Hill)	Lowering the first 8.4m mirror onto the polishing cell. (more photos of the installation of the mirror into the polishing cell) (photo by J. Hill)
Loadspreaders are bonded onto the back of the 8.4m mirror with RTV. These loadspreaders are the permanent attachment between the glass and the support actuators. (photo by J. Hill)	G. Weir installing static supports on the 8.4m polishing cell. The static supports protect the mirror in case the support acutators have a failure. (photo by J. Hill)
Engineering intern J. Fales carefully explores the polished back side of the 8.4m mirror. (photo by J. Hill)	The 8.4m polishing cell arrives at the Mirror Lab. (more photos of the arrival of the polishing cell) (photo by J. Waack)
The LOG (with a tiny grinding head) is used to mark the positions on the backplate of the first 8.4m mirror where the loadspreaders will be glued in place. (photo by J. Hill)	The first LBT 8.4m mirror waits face down in the polishing lab while the second Magellan 6.5m mirror is generated in the background. (photo by J. Hill)
LOG operator John Ray monitoring the polishing of the backplate. (photo by J. Hill)	Polishing lap overhanging the edge of the backplate. (photo by J. Hill)
More Backplate Grinding Pictures Casting Updates Casting Pictures Casting Movies Mold Construction Casting Process Glass Furnace Advantages of Spin-Casting	8.4m LBT mirror with J. Hill and W. Davison Photo by: Lori Stiles (click here for the 0.7Mb version.) (click here for the 2.3Mb version.) (click here for the 6.6Mb version.)
8.4m LBT mirror and a few close friends Photo by: Lori Stiles and John Florence (click here for the 1.9Mb version.) Be sure to see the article in Scientific American Quarterly:"Magnificent Cosmos" by J. R. P. Angel and N. J. Woolf about how this mirror may be used to search for life in other solar systems (March 1998). See also the article about the LBT mirror and other large telescopes in the May 1999 issue of Discover Magazine .