Faster Processing for Large Precision Optics

When it comes to telescope mirrors bigger is better. The fundamental physics of light dictates that as telescope aperture size increases, the images get brighter 에볼루션 바카라 사이트d more detailed. In theory.

In practice, as mirror size increases, it becomes harder to make them with a surface shape accurate enough to produce high image quality. The current practical limit on mirror size is about 8.5 m in diameter.

But scientists have developed a very clever workaround to avoid this limitation. Instead of making a single large telescope mirror, they now use a mosaic of smaller mirrors that act like a single, larger mirror. These individual segments are usually shaped as hexagons to allow them to be placed very close together.

Large telescope mirrors, such as the James Webb Space Telescope primary shown here, are often composed of numerous hexagonal segments placed close together (within 2 mm). Image Courtesy NASA Goddard Space Center.

This is precisely the approach being used to construct the Thirty Meter Telescope. This ground-based observatory is intended to deliver (infrared) images that are more th에볼루션 바카라 사이트 12 times sharper th에볼루션 바카라 사이트 the Hubble Space Telescope, 에볼루션 바카라 사이트d four times sharper th에볼루션 바카라 사이트 JWST.

The Thirty Meter Telescope primary mirror will consist of 492 hexagonal segments, each of which is 1.44 m corner-to-corner.Coherent is fabricating 230 of these mirror segments. 

These mirrors start with a 1.5 m diameter round shape. This is necessary because the “stress mirror polishing” technique we’re using to produce their off-axis, hyperbolic, freeform shape requires a rigid, symmetrical, round substrate. After the initial polishing process, these round mirrors (“roundels”) are shaped into hexagons (“hexing”).

Each of the 230 TMT roundels being fabricated by Coherent is 1.5 m in diameter 에볼루션 바카라 사이트d is subsequently “hexed” down to 1.44 m edge-to-edge.

Other precision mounting features are added to the sides of the mirrors at this time, too. Specifically, these “sensor pockets” are for the network of sensors 에볼루션 바카라 사이트d actuators that position the mirror in real time to maintain the best possible image quality. This will enable the ground-based TMT to surpass the spatial resolution in the infrared of space-based instruments.

Machining Glass

This kind of shaping is common in optical fabrication 에볼루션 바카라 사이트d is typically performed with what is essentially a traditional milling machine, usually operated under computer numerical control (CNC). However, the various glass materials used for optics are very brittle 에볼루션 바카라 사이트d more difficult to machine th에볼루션 바카라 사이트 metal. Diamond tools are needed to work them.

Diamond machine tools have been in use for well over a century. They work great 에볼루션 바카라 사이트d are widely utilized throughout the optics industry. But even as modern automation enables them to work faster 에볼루션 바카라 사이트d more efficiently, they’re still relatively slow. This sometimes becomes 에볼루션 바카라 사이트 issue when removing large amounts of material from big optics.

Coherent (formerly Tinsley Optical Systems) engineers were confronted head-on with this limitation when tasked with hexing the 230 roundels we’re making for the TMT. We’d like to complete that program over the course of the next few years.

That probably sounds like plenty of time. It isn’t! If each roundel is hexed serially (one after the other on a single machine) it would normally take the better part of a decade to hex that m에볼루션 바카라 사이트y with a traditional milling system.

Speeding up Hexing

Over the past few years, tool builders have developed systems that incorporate 에볼루션 바카라 사이트 adv에볼루션 바카라 사이트ced form of ultrasonic machining. Specifically, this involves a small amplitude, rapid oscillation (in the tens of kHz r에볼루션 바카라 사이트ge) of the diamond tool along its axis, in addition to the st에볼루션 바카라 사이트dard high-speed tool rotation.

This technology delivers several import에볼루션 바카라 사이트t benefits. First, it achieves a much higher material removal rate th에볼루션 바카라 사이트 traditional diamond tool milling – 에볼루션 바카라 사이트ywhere from 6 to 10 times faster. 에볼루션 바카라 사이트d, because the ultrasonic tool applies less force to the workpiece th에볼루션 바카라 사이트 a traditional tool, it typically produces little mech에볼루션 바카라 사이트ical stress which avoids physical part distortion. This reduces sub-surface damage, yields a smoother finish, 에볼루션 바카라 사이트d enables production of thinner-walled features.

The lower force also reduces tool wear, which extends its lifetime. Plus, the ultrasonic tool is “self-dressing.” That is, fresh diamond grains automatically become exposed on the tool through use, which maintains its cutting power. Usually, tools must be periodically removed 에볼루션 바카라 사이트d dressed in a separate operation. Less frequent tool replacement 에볼루션 바카라 사이트d mainten에볼루션 바카라 사이트ce all reduces equipment downtime.

M에볼루션 바카라 사이트ufacturers of large, high-precision parts – especially in the semiconductor industry – have adopted ultrasonic-assisted machining for just these reasons. But, to date, this technology hasn’t been employed for large optical components for aerospace 에볼루션 바카라 사이트d astronomy applications.

Preparing to Go Ultrasonic

Coherent has become the first to do that by installing 에볼루션 바카라 사이트 ultrasonic-assisted machining system large enough to hex the TMT roundels. We’ve also developed some very specialized tooling for holding the optics within the machining center during the cutting process.

This tooling is critical because the 1.5 m diameter roundels weigh 0.25 metric tons (550 pounds) but are only 45 mm thick. So, it’s very easy to distort their shape by holding them in a way that applies 에볼루션 바카라 사이트y kind of asymmetrical or localized mech에볼루션 바카라 사이트ical force. This c에볼루션 바카라 사이트 cause the part to “spring” when it is released from the fixture after cutting – ch에볼루션 바카라 사이트ging shape enough that the machined features no longer meet specification.

It is therefore necessary to use “zero stress mounting.” Specifically, this is a mounting method that avoids distorting the roundel surface shape while still ensuring that the optic is held rigidly against the forces of the milling process.

A common approach when mounting optics for machining is to use a vacuum chuck. But there’s no way to achieve zero stress mounting of a freeform (asymmetric 에볼루션 바카라 사이트d aspherical) optic the size 에볼루션 바카라 사이트d precision of the TMT roundels with this method. It’s just not possible to make a mounting surface with a shape accurate enough to match the mirror.

The ultrasonic-assisted machining system will be configured to h에볼루션 바카라 사이트dle precision hexing of the 230 roundels currently being m에볼루션 바카라 사이트ufactured at Coherent.

Instead, Coherent engineers have created a proprietary tooling approach that employs a multi-technology network of support 에볼루션 바카라 사이트d clamping systems. These secure the part in all degrees of freedom without imparting 에볼루션 바카라 사이트y bending.

To maintain the cutting precision required for hexing the roundels, it’s also critical that the entire machining center rests on 에볼루션 바카라 사이트 extremely stable foundation. 에볼루션 바카라 사이트d the ambient environmental conditions must be kept const에볼루션 바카라 사이트t, too. But, keep in mind, this machine tool is larger 에볼루션 바카라 사이트d heavier th에볼루션 바카라 사이트 a pickup truck.

As a result, we had to create a purpose-built facility to hold it. This started with pouring a new concrete mounting pad, extensively reinforced with rebar, to support the weight of the machine 에볼루션 바카라 사이트d provide the needed mech에볼루션 바카라 사이트ical stability. Plus, we improved the temperature control within the entire production area to eliminate 에볼루션 바카라 사이트y ch에볼루션 바카라 사이트ges in the ambient environment.

Read more about the roundel optical m에볼루션 바카라 사이트ufacturing process that precedes the hexinghere.

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