Giant Magellan Telescope mirror 3

Image Credit Dean Ketelsen

Working in education and public outreach in the Department of Astronomy's Steward Observatory at the University of Arizona, I try to keep abreast of the remarkable work that happens at the department's Mirror Lab (or SOML as they call it).  The mirror lab is the only facility of its kind in the world, where we cast and polish mirrors for the worlds largest (such as the Large Binocular Telescope - LBT) and next generation telescopes (such as the Giant Magellan Telescope - GMT).  Currently the 3rd off-axis mirror for the GMT is in the oven, and the glass is approaching/passing melt-point as I write this.  (EDIT: Glass is now melted and smoothing.)  Each of the mirrors for the GMT is 8.4 meters across and in total seven mirrors will work together forming a single mirror over 25 meters in diameter!

After extremely careful inspection, the glass chunks were loaded into the oven a couple weeks ago around the custom built mold, and this past Monday the oven was turned on.  Long time SOML employee and the inspiration for this blog, Dean Ketelsen published a report documenting the casting preparations which I would recommend and can be seen here.  Temperatures have been ramping up over night and maximum temperature of 1165 degrees C will be between 10 and 11 PM local time tonight.  Already, the glass has melted, acquiring the consistency of honey and running down into the mold.  As the oven continues to heat up and rotate at almost 5 rpm the surface will slowly smooth out and the glass will become clear.  Early in the hours of Sunday morning the furnace will begin a slow cool down (continuing to rotate) and around Thanksgiving the mirror will be revealed.

There are several cameras inside the oven (remarkable in and of itself!) that are taking pictures in order that the process can be closely monitored.  Below are images from various temperatures which give you peek into the formation of the mirror, and the changes that occur at increasing temperatures.  You can see subtle changes even between the images from 892 degrees and 900 degrees C as the glass is noticeably transforming from its slumped state to melting.  I'll post a couple updates later today if I have time with pictures later in the process.

Camera 7 showing the edge of the mirror - click the images to enlarge them:

499 C

750 C

892 C

900 C

922 C

934 C

Shifting to camera 5 as the image is clearer...and the glass is now smoothing

990 C

Camera 3 showing a wide angle view at similar temps to above - click the images to enlarge them:

499 C

750 C

892 C

900 C

922 C

934 C

At this point the glass is now smoothing and you can see the reflection of the furnace top in the surface, as well as the very nice honeycomb structure of the mirror mold.

990 C

1012 C

At 1028 degrees C now, you can see the air bubbles that are rising up through the glass as it continues to smooth.

1028 C

UPDATE- 10:25 PM local time (MST) the furnace is now nearly at "high fire" maximum temperature.  You can see in both the edge view and wide view that the glass is clear and smooth.  At 1160 degrees Celsius, the furnace is only 5 degrees from its top temperature.

1160 C

1160 C

This is a short video I shot last year while the furnace was rotating during the high fire day for the second GMT mirror.