• Scientific Goals

  • General
  • For Kids
  • For Scientists

• Why 30 m?

• Partnership

• Design

  General Software Instrumentation Telescope Enclosure

• Adaptive Optics

  • General
  • Why AO?
  • How AO Helps
  • The Diffraction Limit

• Timeline

• Proposed Sites

• Related Projects

• Media

• Business Policies

• Contact Us

• Employment

Telescope Design

Telescope Design:

Once it is built, the TMT will be the largest telescope in the world and will have a light collecting area approximately nine times the size of the largest telescopes that exist today!  The following images give a sense of the grandeur of this telescope.

  Picture of parliament in Ottawa with telescope and enclosure superimposed.
Image credit: Sam Javanrouh - Photographer of parliament.
In order to accomplish this, the telescope incorporates unique design features such as a segmented mirror and platforms at the Nasmyth foci for instruments.  These, among other new design features are crucial to the success of the TMT without which would render it extremely large in mass, size and cause a decrease in its performance.  To illustrate this, if the CFHT (3.58 m primary mirror) were scaled up to a size of 30 m then it would weigh close to 61,000 tonnes as opposed to the TMT's estimated 1,400 tonnes.
This page will discuss some of the novel ideas in the design of the telescope that allow it to be so efficient.

---

Telescope design features in brief:

• 30 m mirror, 574 mirror segments
• Ritchey-Chrétien optical design
• F/1 primary
• F/15 final focus
• Field of view: 20 arcminutes
• Instruments at Nasmyth foci
• Alt-azimuth mount

---

The Segmented Mirror:

The TMT's primary mirror isn't a single piece of glass as in the case of smaller telescopes.  In fact, the TMT primary mirror will be made up of more than 500 hexagonal mirror segments as there are limitations to the size of monolithic mirrors - the largest manufactured are on the order of 8 m such as the primary mirror of the Subaru telescope (8.3 m) on Mauna Kea.  It is possible, however, to use many smaller mirrors so they operate just as one larger one would.  It can be thought of similarly to tiling a floor; it still gets covered either with one large tile or many small ones.

 
Illustrated here the primary mirror hexagonal segments act equivalently to a monolithic mirror.
Image Credit: CIT

The key is getting all the segments to work together. Each one is controlled by actuators and a complex computer program keeps the mirrors positioned so that they form the equivalent of one continuous piece of glass. When the telescope moves, the mirrors must move together easily and accurately. Each mirror is equipped with sensors so a computer can determine how it is oriented relative to the mirrors around it and correct so that the mirror segments together always form a parabola. This positioning is done with such precision that the mirrors can be controlled to distances within fractions of a human hair to where they need to be.

Monolithic mirrors are usually cast in glass and then coated on the front side with a reflective material.  The glass is thick, and a mirror 30 m in diameter would lose its shape as it would deform under its own weight.  In addition to using a segmented mirror, materials other than glass are chosen for the segment blanks such as Zerodur, a glass ceramic. 

Segmented mirror technology, like adaptive optic technology is not new. In fact a man named Guido Horn d'Arturo built a 1 m primary mirror out of 61 segments in the 1930s.  There are three operational telescopes and two under construction that already make use of segmented mirror technology today; the two 10 m Keck telescopes on Mauna Kea and the Hobby-Eberly 11 m (with 9.2 m effective aperture) on Mt. Fowlkes, Texas.  The GTC will be a 10.4 m telescope at La Palma and SALT in South Africa will be 11 m across. Both of these giants are designed and being built using segmented mirror technology. GTC's first light will be sometime in 2005, and SALT is due to be completed in November 2005.

Mirror Blanks and Optical Coatings:

The first step in developing each individual segment of the telescope is to make the mirror blanks.  Each blank acts as a substrate to which a reflective coating is applied and is usually some variation of a glass material.  In the case of the TMT, these blanks are made of a glass-ceramic material which is first formed into the desired optical shape, coated with a reflective material and polished to be very smooth.  In fact, it is so smooth that the largest variation on its surface is on the order of a nanometer!  With such time and effort put towards the optical shape of the mirror, the stability of these blanks is very important as any variation in shape can result in poor imaging.  For this reason they are engineered in such a way to minimize their sensitivity to temperature changes - in more technical terms they are described to have a low coefficient of thermal expansion.  In addition, the blanks must be internally stable over time, meaning that they should not quickly become de-shaped due to internal strain.  As can be seen in the following image, the blanks are also made very thin in order to decrease the overall weight and hence cost of the telescope.  After a segment is cast and figured it is then coated in a reflective material and polished.

Illustration of the mirror blank milling process.
Image Credit: OHARA

  The TMT has three separate mirrors which require reflective coatings; the primary, secondary and tertiary mirrors.  While aluminum is the standard coating material used for many telescopes, the TMT's scientific goals and instruments require an alternate material.  Specifically, this demands that the material have the highest possible reflectance in the optical range of 0.3-30 microns (a section of the visible and infrared spectrum).  As aluminum does not have exceptional performance in this range of wavelengths, a group of TMT scientists are working hard to develop a new coating.

In addition to high reflectance in the infrared, the coating must reflect optical wavelengths well and also keep the emissivity low.  Low emissivity means that the coating does not give off a lot of its own infrared radiation.  As a simple (one substance) coating cannot accomplish all of this, a multi-layer coating is being developed in an attempt to optimize the abilities of many materials.

It takes a lot of money and time to recoat mirrors, and with more than 500 of them, it can turn into a very costly endeavour.  The TMT team is trying to minimize recoating costs and downtime by requiring that whatever coating is developed has a long lifetime - on the order of five or more years. Coatings are made at the Pacific Northwest National Laboratory and its ability to reflect light from 0.3-30 microns is measured at Lick Observatory with a spectrophotometer.  To test their effectiveness, accelerating aging tests are carried out.  Promising coatings will be placed in telescope dome environments for 6 months at a time to better study long term degradation.

Optical Design Specifics:

It has already been mentioned that the TMT is a reflecting telescope, but more specifically it has a Ritchey-Chrétien optical design.  It is a modified Cassegrain design, typical of large telescopes, that uses specifically shaped concave mirrors to form an image; the primary (M1) and secondary (M2) mirrors are hyperbolic while the tertiary (M3) mirror is a flat reflector.  In a Cassegrain telescope the secondary mirror reflects light back toward a hole in the centre of the primary mirror where instruments are placed for imaging.  However, the TMT has a tertiary mirror in between the primary and secondary, reflecting light out the side of the telescope's tube and into the instrumentation located on the so-called nasmyth platform.  This design is illustrated in the following 'ray diagram' which tracks the path that incident (incoming) light rays follow as they enter the instrumentation of the telescope.

Ray diagram showing how light travels from the sky and into the TMTs instruments.
Image Credit: CIT - Telescope Conceptual Design

This type of telescope is beneficial because it corrects very well for the effects known as coma and spherical aberration which are typically a source of poor imaging. 

Instrument Location and Mounting:

As mentioned above the tertiary mirror or 'Nasmyth flat' mirror reflects light toward instruments located on the Nasmyth platforms.  The TMT will have two of these platforms for very large, delicate instruments to reside upon and will rotate in the horizontal plane (the azimuth plane) to accommodate the various observing positions of the telescope.  Having this platform rotate in one plane alleviates major power requirements which would be needed if these heavy instruments were to rotate in two planes (horizontal and vertical).    

The Thirty Metre Telescope:
Note the two Nasmyth platforms and
the size of the people compared to the instruments.
Image Credit: CIT - Telescope Conceptual Design

Having these bulky instruments on the nasmyth platform also simplifies their design; instruments that are attached at the Cassegrain focus (just behind the primary mirror) move in altitude as well as azimuth just like the telescope.  Depending on how they are oriented, the instruments will then experience different kinds of stresses which will need to be compensated for in a more complex design.  However, if the instrument is kept on a platform that only rotates horizontally then the stresses on the instruments stay constant as its orientation does not change.  In other words, it is a lot simpler to design the instruments to sit one way, as opposed to designing them to be flipped upside down or held out sideways!  This has proven to be an effective way to mount instrumentation; for example, the Keck telescopes on Mauna Kea and the William Hershel Telescope on La Palma all use Nasmyth platforms to mount their instruments.

Telescope Mounting and Structural Design:

The telescope will be mounted using an alt-az (altitude-azimuth) system which has the telescope turn about two axes: a vertical one (altitude) and a horizontal one (azimuth).  The telescope then rotates about both axes simultaneously in order to follow objects as they move across the sky.  Alt-azimuth mounts are good in that they are simple to manufacture and in conjunction with a computerized control system, track the sky very well. 
    As primarily a truss structure, the alt-az configuration of the TMT is structurally more efficient than a scaled up version of the CFHT in that its geometry allows for a direct load path to the ground.  Alternatively, the CFHT's equatorial configuration requires a massive yoke structure to support the telescope due to its indirect load path.  More specifically, the structural members of the TMT are described to work in tension and compression, making it very efficient, while many of the CFHT's structural members work in bending which is less efficient.

The top-end mount is the structural component of the telescope that supports the secondary mirror above the primary and must be as steady as possible.  The design of the top-end mount is very important as this section of the telescope will be subject to the greatest amount of wind buffeting.  Thinking of this region as a sort of sail, a larger cross sectional area would catch more wind causing more movement of the secondary mirror.  The objective then, is to minimize the cross sectional area while still supporting the secondary mirror very steadily.  Three designs that were considered are presented here:

 
Three designs considered for the top-end mount -
Conventional (left), Radio Telescope (center) and an Innovative design (right).
Image Credit: CIT - Telescope Conceptual Design
Notice that in the case of the conventional design (far left) the top end has too much cross sectional area allowing for a lot of wind buffeting.  The radio telescope design (center) has tripod struts that are too long with respect to buckling, although it does have a small cross sectional area.  The design that was finally arrived upon (far right) is somewhat of an amalgamation of the first two: it has a partial truss tube as well as a short tripod which reaches the best compromise between cross sectional area and strong structural support for the secondary mirror. 

Field of View:

The TMT's field of view is 20 arcminutes and can be described as the angular quantity of sky that you see when looking through the telescope.  But how much sky is 20 arcminutes?  Stick your pinky finger out and hold it out at arm's length. The distance from one end of it to the other is 1 degree of arc and is equivalent to 60 arcminutes.  So, 20 arcminutes would be one third of the distance across your pinky finger!  Comparing this to the rest of the sky, it's a pretty small patch!

Now let's discuss focal ratio. It is the length that light travels inside the telescope divided by the telescope's aperture (diameter).  The notation used to identify a focal ratio is "F/(focal ratio)" Focal ratios that are small numbers (such as F/3 or F/4) are considered to be fast.  These give wide fields of view and are used in conjunction with low magnification eyepieces.  On the other hand, a slower focal ratio such as the TMT's total focal ratio of F/15 yields a small field of view that can use high magnification.  The F/15 final focal ratio optimizes the telescope for the type of observing that the scientists plan to do with it.

©