Tuesday, May 12, 2015

Evolution of the LBT

I've been looking at so many images of the Large Binocular Telescope (LBT) the last few weeks that I'm going cross-eyed! You all likely know by now that I had a hand in polishing both of the 8.4 meter mirrors (about 28 feet across each!) for the scope, and that was back a decade ago. Once both mirrors were installed in the telescope, I sort of lost track of it, but looking at those images now on my rare visits, the telescope never looks the same two consecutive trips! On a huge project like this, evolution past the basic telescope structure takes time, so even now, new instruments are coming on line and making new discoveries. Just last week, the Steward Observatory director notified us of an early use of the LBT imaging interferometer to image volcanos on Jupiter's moon Io. Be sure to visit the page, and scroll to the bottom where there is a movie clip showing Io being eclipsed by another moon Europa!

So let's look at some of the major changes in LBT over the years... From my first trip there in April, 2007, the basic telescope was complete.  The only instrument commissioned at the time were the prime focus cameras. Each of the big mirrors had it's own camera - the right mirror (called the SX side) had a blue-optimized camera, the left mirror (DX) had a red-optimized, both co-aligned precisely to the same field. Compared to recent images, the scope looks like a basic framework, which is pretty true...

On my next trip there a year later, May of 2008, it looked pretty much the same, but some major changes can be pointed out. Shown at labeled at right, perhaps the most obvious is a large tank between the elevation bearings.  The explanation is simpler than you can imagine - liquid is used, pumped between several storage tanks around the structure to balance the telescope as instruments and observing configurations are changed.  Of course, antifreeze is required as temperatures much below freezing is frequent.  More importantly, in preparation for new instrumentation, a Gregorian secondary mirror has been installed atop the SX telescope, and a tertiary mirror is also mounted now, where just the mounting jigs were in place a year before.  The LBT is designed for many instruments to be mounted simultaneously and the light can be brought to each in a few minutes with these auxiliary optics.  More on that in a paragraph or two.

Shown here at left is a close-up of the Gregorian secondary (taken in May, 2008).  When inserted into the telescope beam, it re-directs the light down to instruments either below the primary mirror, of via the tertiary to instruments located between the two primaries.  In this shot, the mirror wasn't in the beam, but nearly the entire primary mirror can be seen reflected in it.

Many modern telescopes use a convex secondary mirror, but LBT uses a concave for a couple reasons - one, it is easier to test a concave mirror, and in this optical configuration, it clears the prime focus camera, as shown at right.  If a convex mirror were used, it would interfere with the camera mounted closer to the primary mirror.  There is a clip at the end of this post that will show the change between camera and Gregorian secondary - it is really cool!

Also from May, 2008 is this shot in twilight of the space between the two mirrors from the "rear" of the telescope. Looking out the open enclosure, the "peak" of Mount Graham can be seen. At left is the "SX" mirror, the "DX" mirror at very far right. The thing to note is that the big mounting rings and the walkway between them is... empty!

Now check out the same space on our visit last month, April, 2015 at right. It is now packed with different instruments - I think the main one in center is the imaging interferometer, referenced in the link in the first paragraph above. PEPSI is an echelle spectrograph at lower left, LINC-NIRVANA at right, and LUCIFER (now LUCI) is mounted somewhere in there too...

A wider shot (also last month) at left shows how the light is fed to the various instruments. Though not in the beam here, the tertiary mirror is shown at left, and that feeds the light to the desired instrument (mounted in the ring flanges) when in place, and rotated to the proper alignment. This particular night, the observers were using the MODS spectrograph located below the primary, so the light went through the hole in the big mirror. The huge spectrograph is permanently mounted below the primary cell, shown at the bottom of the SX mirror at right. Since I've mentioned the ARGOS lasers in previous posts, I'm also pointing out some of the ARGOS optical system in the image at upper left. ARGOS uses 18 watt (!) lasers to project spots about 10km up in the atmosphere to partially correct turbulence that would blur the image.

The wide variety of instruments available and the ease of switching between them allows for most efficient use of telescope time should observing conditions change.

Finally, my favorite time-lapse clips are collected here in one sequence. Labels introduce each one, and should be self-explanatory. Full screen and HD should be used if you have the bandwidth. Any questions should be asked in the comments...

I think after recent posts I'm about LBT'ed out! Until my next trip up there, anyway!

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