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---
title: "L09: Professional Observatories"
subtitle: locations and technology
layout: 'layouts/lecture.html'
sections: [
{"id":'locs', "label":"Locations"},
{"id":'guiding', "label": "Autoguiding"},
{"id":'activeoptics', "label": "Active Optics"}
]
---
<!-- magellan content -->
<div class="sections">
<!-- NEW SECTION: Locations -->
<section id="locs" data-magellan-target="locs">
<div class="grid-container">
<div class="grid-x align-center">
<div class="cell">
<p>There no world-class optical telescopes in the UK. The reason for this is cost. The world's
largest telescopes cost hundreds of millions of pounds. Construction has started on the 32-m
diameter E-ELT, with an estimated cost of €1 billion. It makes sense to build these telescopes
in the best locations on Earth. None of these are in continental Europe. So what makes a site
suitable for a billion Euro telescope?</p>
<h6><strong>Cloud Cover</strong></h6>
</div>
<!-- begin two column for figure -->
<div class="grid-x grid-margin-x">
<div class="cell small-12 medium-6">
{% set fig = figures['cloud_cover_vid'] %}
<div class="responsive-embed widescreen">
<video controls loop>
<source src="{{ fig.url }}" type="video/mp4">
<p>Your browser does not support the video tag</p>
</video>
</div>
<figure id="figure{{fig.fignum}}">
<figcaption><strong>Figure {{fig.fignum}}:</strong> {{fig.caption}}</figcaption>
</figure>
</div>
<div class="cell small-12 medium-6">
<p>Radio telescopes can see through cloud. Optical telescopes can't! This is one of the primary
reasons there are no major research telescopes in the UK.
{%figref "cloud_cover_vid", true, true %} shows monthly average cloud cover. Desert regions
including Antarctica, Australia, parts of Africa and the west coast of the Americas provide
some
of the best cloud-free skies.</p>
</div>
</div>
<div class="cell">
<h6><strong>Light Pollution</strong></h6>
<p>Light from street lights and industry can greatly increase the <a
href="../../principles/l01/#transparency">sky background</a>. The increased background
light causes increased noise in astronomical images, making it harder to detect faint
sources. Therefore, the largest telescopes are placed in regions of low light pollution.
{%figref "light_pollution", true %} shows the artificial night sky brightness around the
world. It can be seen that some of the regions which show low cloud cover in {%figref
"cloud_cover_vid" %}, such as the sparsely populated western coasts of South America and
Africa, are also largely free of light pollution. {%figref "light_pollution", true %}
also clearly shows why no major telescopes are now built in mainland Europe.</p>
</div>
<div class="cell small-10">
{%showfigure "light_pollution" %}
</div>
<div class="cell">
<h6><strong>Seeing</strong></h6>
<p>Selecting a site with good <a href="../../principles/l01/#seeing">seeing</a>
is one of the most important criteria. Lower seeing improves the spatial resolution of
images. Since the light from the stars is also spread out over fewer pixels, good seeing
also improves the <a href="../../stats/l18/">signal-to-noise</a> ratio. Seeing is caused by
turbulence in the atmosphere and this has two causes: heat from the ground rising through
the atmosphere and strong winds higher in the atmosphere (e.g the jet stream).</p>
<p>The atmosphere over the sea tends to be much less turbulent than the atmosphere over land, as
the sea exhibits an essentially smooth, constant-temperature surface compared to the land.
Some of the best astronomical sites are therefore located on small islands in the middle of
oceans, such as Hawaii and the Canaries, as these small land masses cause little additional
turbulence. For the same reason, coastal regions that receive winds predominantly from the
direction of the ocean, such as the western coasts of the Americas and Africa, also exhibit
excellent seeing.
</p>
<h6><strong>Altitude</strong></h6>
<p>The higher the site of an astronomical observatory, the thinner the atmosphere above the
telescope. This reduces <a href="../../principles/l01/#extinction">atmospheric
extinction</a>,
and can also help reduce the seeing if the observatory lies above a turbulent layer in the
atmosphere. High-altitude telescopes are also often above the local inversion layer in the
atmosphere, meaning that local cloud formation occurs below the telescope, significantly
increasing the number of usable nights at the observatory compared to a telescope sited
below the inversion level.</p>
<h6><strong>Other Factors</strong></h6>
<p>Additional factors, such as the difficulty of supplying utilities like water and power,
accessibility, humidity and wind speed all play a lesser role in locating professional
observatories. For a detailed discussion of many of these factors, see Vik Dhillon's <a
href="http://www.vikdhillon.staff.shef.ac.uk/teaching/phy217/telescopes/phy217_tel_sites.html">old
notes</a>. </p>
<p>Political stability is also a factor in the location of large telescopes. This has been a
particular issue in recent years, with protests from Native Hawaiians causing <a
href="https://en.wikipedia.org/wiki/Thirty_Meter_Telescope">severe issues</a> for the
Thirty Meter Telescope project's plans to build their telescope on Mauna Kea.</p>
<h5>The world's largest telescopes</h5>
<p>{%figref "bigscopes", true %} shows the sites of the worlds largest telescopes. For the
reasons
discussed above these are clustered around a few major sites, namely Chile, Hawaii and the
Canary Islands. The gallery below shows photos of these observatories. These sites have low
cloud cover, low light pollution, good seeing and low humidity. They are all at high
altitude
and are politically stable.</p>
</div>
<div class="cell small-10">
{%showfigure "bigscopes" %}
</div>
<div class="grid-x grid-margin-x align-center">
{% set fig = figures['big_three_obs'] %}
<div class="cell small-12 medium-3">
<div class="thumbnail" data-toggle="hawaii">
<img src="{{ fig.thumbnails[0] }}">
</div>
</div>
<div class="cell small-12 medium-3">
<div class="thumbnail" data-toggle="lapalma">
<img src="{{ fig.thumbnails[1] }}">
</div>
</div>
<div class="cell small-12 medium-3">
<div class="thumbnail" data-toggle="paranal">
<img src="{{ fig.thumbnails[2] }}">
</div>
</div>
<div class="cell small-10">
<figure id="figure{{fig.fignum}}">
<figcaption><strong>Figure {{fig.fignum}}:</strong> {{fig.caption}}</figcaption>
</figure>
</div>
</div>
<div class="full reveal" id="hawaii" data-reveal>
<img src="{{fig.filenames[0]}}">
<p>The Keck and Subaru telescopes at the summit of Mauna Kea, Hawaii. </p>
<button class="close-button" data-close aria-label="Close reveal" type="button">
<span aria-hidden="true">×</span>
</button>
</div>
<div class="full reveal" id="lapalma" data-reveal>
<img src="{{fig.filenames[1]}}">
<p>The Roque de los Muchachos Observatory (ORM) on La Palma in the Canary Islands. </p>
<button class="close-button" data-close aria-label="Close reveal" type="button">
<span aria-hidden="true">×</span>
</button>
</div>
<div class="full reveal" id="paranal" data-reveal>
<img src="{{fig.filenames[2]}}">
<p>The Very Large Telescopes (VLT) at the Paranal Observatory in the Atacama Desert, Chile. </p>
<button class="close-button" data-close aria-label="Close reveal" type="button">
<span aria-hidden="true">×</span>
</button>
</div>
<div class="cell">
<h5>Space Based Observatories</h5>
<p>The factors which degrade astronomical images, such as seeing, sky background, transparency
variations and extinction are all atmospheric-induced phenomena. These effects can all be
removed at a stroke by siting optical telescopes in space. The best known example of this is the
2.4 m Hubble Space Telescope (HST), which has helped to revolutionise astronomy since 1990 with
its diffraction-limited imaging capability. Because space-based telescopes don't suffer from
atmospheric extinction, HST also allows astronomical observations in the UV - impossible from
the ground! The main drawback with siting telescopes in space is the cost: the HST cost many
billions of dollars to build and operate, approximately ten times the sum required for the
largest ground-based telescopes.
</p>
<p>Another drawback with space is the risk involved in the launch, and the great difficulty of
fixing problems, servicing the telescope and upgrading the instrumentation once the telescope
has been deployed. For the HST, servicing missions, which cost nearly $1 billion each, were of
limited number and were very risky for the astronauts involved. More recent telescopes, such as
the JWST, have been located much further from Earth and the Sun to keep them cool for as long as
possible; servicingmissions for these telescopes are impossible, so they had better work!</p>
</div>
</div>
</div>
</section>
<!-- NEW SECTION: autoguiding -->
<section id="guiding" data-magellan-target="guiding">
<div class="grid-container">
<div class="grid-x">
<div class="cell">
<hr />
<h5>Autoguiding</h5>
<p>We have already seen that telescopes use mounts to track the motion of stars in the sky. In
practice, it isn't possible to do this accurately enough for professional work, or even for
amateur astrophotography. This is due to mechanical imperfections in the drive systems of the
mount, flexure of the mount, and misalignment of equatorial mounts with regard to the celestial
pole. </p>
<p>The result is that stars drift slowly across the field of view, which would cause smearing in
long exposures. To get round this, it is necessary to guide telescopes. This is done using an
autoguider, a system which monitors the position of a star somewhere in the field of view, and
nudges the telescope to keep the guide star locked onto the same pixel on the autoguider's
detector.</p>
<p>There are two main types of autoguiders in use. <strong>Guidescopes</strong> use a separate,
smaller telescope to image a wider field of view. The wide field of view means that guide scopes
are likely to find a bright star to guide on, but because they are mechanically separate from
the main telescope, they can flex relative to the main scope. This means that, even though the
guide star is held fixed, the main telescope's view of the sky can drift slowly.
<strong>Off-axis guiders</strong> use a mirror on a moveable stage to pick off some light from
the main telescope, and focus it on a separate autoguiding camera. This exhibits less flexure
than the guidescope method, and is the one most often used for large telescopes. Both designs
are shown below in {%figref "autoguiding" %}.
</p>
</div>
</div>
<div class="grid-x align-center">
<div class="cell small-10">
{%showfigstack "autoguiding" %}
</div>
</div>
</section>
<!-- NEW SECTION: active optics -->
<section id="activeoptics" data-magellan-target="activeoptics">
<div class="grid-container">
<div class="grid-x align-center">
<div class="cell">
<hr />
<h5>Active Optics</h5>
<p>The huge mirrors in the world's largest telescopes pose a difficult engineering problem - they
can be too heavy. For example the 4.2 m mirror of the William Herschel Telescope (WHT) on La
Palma, for example, has a thickness of 56 cm and a weight of 16.5 tonnes (see {%figref
"thin_thick_mirrors" %}). Larger mirrors of the same thickness would be too heavy to move. But a
thinner, lighter mirror will bend and change shape as the telescope moves, due to the effects of
gravity. This distortion of the primary mirror would ruin the focusing properties of the
telescope, producing poor quality images. Despite this, the 8-m mirror of the Very Large
Telescope (VLT) in Chile is only 18cm thick and weighs only 24 tonnes. If it were the same
thickness as the WHT mirror it would weigh around 100 tonnes!</p>
</div>
<div class="cell small-10">
{%showfigstack "thin_thick_mirrors" %}
</div>
<div class="cell">
<p>The distortion of modern, thin mirrors is overcome by active optics, where the thin (and hence
flexible) mirror is mounted on a set of actuators, which are devices that transform an
electrical signal into linear motion. The system works as follows (see {%figref "activeoptics"
%}): whilst the telescope is observing a science target, the light from a bright reference star
somewhere in the field of view around the science target is picked off and the quality of its
image is measured. This measurement is usually performed by a <a href="../l10/">wavefront
sensor</a>, which we shall discuss in more detail in the next lecture. The actuators are
used to push and pull different parts of the primary mirror so that its shape changes in such a
way as to improve the quality of the reference star image. Only about one measurement and
correction cycle per minute is required, as the orientation of the telescope whilst tracking a
target does not change significantly on timescales shorter than this. Note that, as well as
correcting the shape of the primary mirror, many active optics systems also correct for the
change in the position of the secondary mirror (see {%figref "activeoptics" %}). This corrects
for changes in the shape of the telescope itself, which can occur due to flexure under gravity
or expansion and contraction caused by changes in temperature.</p>
</div>
<div class="cell small-10">
{%set fig=figures['activeoptics'] %}
<div class="grid-x grid-margin-x">
<div class="cell small-12 medium-7">
<div class="thumbnail"><img src="{{ fig.filename }}"></div>
</div>
<div class="cell small-12 medium-5">
<p><strong>Figure {{fig.fignum}}: </strong> {{fig.caption | safe }}</p>
</div>
</div>
</div>
<div class="cell">
<p>An increasingly common alternative to the single-piece mirrors shown in {%figref
"thin_thick_mirrors" %} are segmented mirrors. This approach has been adopted, for example, on
the twin 10 m <a
href="https://keckobservatory.org/wp-content/gallery/keck-i-keck-ii-telescopes/33R0061-180-2246417083-O.jpg?x32463">Keck
telescopes</a>. The segments are typically hexagonal in shape and usually have an asymmetric
profile so that when all of the segments are combined they form, for example, a hyperbolic
shape. Active optics systems are essential for segmented-mirror telescopes, as the position of
each segment needs to be carefully controlled so that the overall shape of the primary mirror is
retained as it is tilted to different sky positions. Each segment is therefore mounted on its
own set of actuators.</p>
<p>Segmented mirrors are the only feasible way of constructing telescopes with apertures
significantly in excess of 8 m, as single-piece mirrors would become extremely expensive and
ultimately impossible to manufacture, transport, install and maintain. The <a
href="http://www.eso.org/public/teles-instr/e-elt/">European Extremely Large Telescope</a>
(E-ELT) will have a 39 m mirror, composed of 798 elements, each 1.4 m wide and 5 cm thick. </p>
</div>
</div>
</div>
</section>
</div>