A survey of wide-field-of-view optical telescopes
Paul W. Kervin
Air Force Research Laboratory
Abstract
As we attempt to find and catalog smaller, man-made Earth-orbiting objects using
optical telescopes, we encounter challenges in telescope design. The challenge for
telescope design is that to detect smaller, fainter objects, we generally need larger
apertures. However, the usual effect of larger apertures is smaller fields of view, which
has a negative effect on the search for objects. The author will discuss aspects of this
challenging problem, as well as to discuss several telescope systems being developed to
address this problem.
Discussion of Problem
As the angular size of satellites becomes smaller, as observed from the ground, the
optical resolution of those satellites decreases. The angular size will decrease either
because the satellite is small, the satellite is at a large distance, or both. Resolution also
decreases as the telescope aperture decreases. One can attempt to make up for the small
angular size of a satellite by increasing the telescope aperture, but this causes problems
in design, as will be discussed later.
The problem is becoming more pronounced as many operational satellites are
becoming much smaller, while still maintaining many of the capabilities of their larger
counterparts. A good example of this is the large number of CubeSats being launched,
and the corresponding growth in the industries that support the technologies required for
the operation of these satellites.
10 cm
CubeSat
The typical CubeSat is a 10 centimeter cube, weighing approximately 1 kilogram.
The capabilities of these satellites includes attitude determination, attitude control,
communication, propulsion, and Earth imaging. The challenge associated with these
small yet capable satellites is to find them and track them.
Design Implications
There are a number of design implications associated with finding and tracking small
objects such as this, or distant objects that are correspondingly faint.
1) Being able to detect small objects requires a large telescope aperture, since small
objects are likely faint objects.
2) Searching for these objects requires a large field of view so that the search is
performed in a reasonable time.
3) If we want to keep these objects in a catalog, we need to have accurate positional
information, which means we need a fairly small plate scale. That is, one desires the
number of arcseconds per pixel to be relatively small.
4) These requirements are valid whether we are talking about astronomical objects or
Earth-orbiting satellites. But in the case of satellites, particularly for satellites in LEO
orbits, we also need to have short integration times and rapid readout.
Aperture and field of view
The simultaneous requirements for large aperture and large field of view put
significant constraints on the optical system. The design of a system with both of these is
a challenge. There have been many designs for such systems, with many variations, and
new designs are still being developed. This is because there is no right or perfect
solution. The problem is that aberrations become more dominant as you move away
from the optical axis of the system. At the edges of the field of view of large optical
systems, the point spread function becomes increasingly large and asymmetric.
Not only must there be a good optical design, there must also be a good mechanical
design to hold the optical elements in alignment.
Field of view and plate scale
The combination of large field of view and small plate scale puts constraints on the
focal plane arrays that are used. The focal planes must be large, both in the sense of
physical dimensions, and also in the total number of pixels.
Integration time and readout
The requirement for short integration times and fairly rapid readout puts constraints on
the focal plane array, and the associated electronics.
In combination
The combination of these requirements puts constraints on the optical design,
mechanical design, electronics, and software. As a result, these requirements and the
associated constraints result in a telescope system that is usually quite expensive
compared to a system without these constraints.
Solution
There are actually a number of solutions to these design goals, and I’ll talk about
several of them, as well as give examples, both for telescopes designed for astronomical
purposes as well as those designed for surveillance of Earth-orbiting satellites.
Modify an existing telescope
One solution is to modify an existing telescope, which is a fairly popular way to
improve the performance of a telescope, at much less expense than building a telescope
from scratch. I’ll give two examples, the Samuel Oschin telescope, and the Baker-Nunn
telescopes.
Samuel Oschin telescope
The Samuel Oschin telescope is an astronomical telescope, a 1.2-meter Schmidt
telescope located at Palomar Observatory in California. The focal plane is curved, and
was originally populated with a mosaic of glass photographic plates, held in a special
fixture to match the flat plates to the curved focal plane.
The optics were modified in the last few years for two CCD cameras. In 2001 the
NEAT camera was added for detecting asteroids and comets, and more recently, in 2003,
the Quest camera was added for observing quasars. The QUEST camera consists of an
array of 112 flat CCDs, arranged to conform to the curved focal plane, just as the original
glass plates approximated the curvature of the focal plane.
The result is a large telescope, with large focal plane, that has been adapted to the
modern technology of CCDs instead of film.
Baker-Nunn telescopes
Another set of telescopes that have been modified have been the Baker-Nunn
telescopes. 12 of them were built by the Smithsonian Astrophysical Observatory in the
1950s. They had 50 cm apertures, a highly curved focal plane, with photographic film
being the medium, with a 5 degree by 30 degree field of view.
The first satellite that they observed was Sputnik, launched almost exactly 50 years
ago.
The locations for these telescopes is listed here, including locations in both Northern
and Southern hemispheres.
Woomera, Australia
Jupiter, Florida, US
Organ Pass, New Mexico, US
Olifansfontein, South Africa
Cadiz, Spain
Mitaka, Japan
Naini Tal, India
Arequipa, Peru
Shiraz, Iran
Curaçao, Netherlands West Indies
Villa Dolores, Argentina
Haleakala, Maui, Hawaii
These telescopes were retired in 1976. However, some of these telescopes have been
modified to accept a CCD, using field-flattening optics. Here are the new specifications
of four of the telescopes that have been modified, along with the field of view:
Siding Springs, Australia
• University of New South Wales
• 3 x 2 degrees
Calgary, Canada
• University of Calgary
• 4 x 4 degrees
Catalan Pyrenees, Spain
• San Fernando / Fabra collaboration
• 5 x 5 degrees
Maui, Hawaii, US
• US Air Force Research Laboratory
• 7 x 7 degrees
New Design
Another solution is to start from scratch and design something new. I’ll briefly
mention the design of a new telescope, and the design of a new type of sensor.
Telescope Design
The Large Synoptic Survey Telescope (LSST) is a telescope designed for
astronomical purposes, with an 8.4-meter primary and a field of view of 10 square
degrees, addressing the problem mentioned earlier of combining a large aperture with a
large field of view. The camera will be a 3 gigapixel camera, and will generate an
astounding 30 terabytes of data per night. Data will be disseminated via an arrangement
with Google.
This telescope will be located in at the 8800-foot peak of Cerro Pachon, in northern
Chile, and should be operational in 2013.
Sensor design
One can also design a new camera system, which is what the University of Hawaii has
done for their Pan-STARRS telescope, which will be described in more detail later in the
paper.
One of the most interesting aspects of the Pan-STARRS telescope is its camera. It is a
mosaic of 60 orthogonal transfer arrays.
What is unique about those OTAs is that charge can be shifted in both X and Y axes in
real time, and the charge on each OTA can be shifted independently of all of the other
OTAs. One can then track a bright star in each OTA with a resultant point spread
function that is much smaller, increasing the signal to noise ratio.
These sensors are built by MIT Lincoln Laboratory. The Pan-STARRS camera itself is
built by the University of Hawaii.
Multiple telescopes
The last solution I’ll talk about employs a number of telescopes ganged together. I’ll
talk about Pan-STARRS and the RAPTOR telescopes.
Pan-STARRS
The Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) is
designed for astronomical purposes, although the Air Force is looking into using similar
technology for surveillance of Earth-orbiting satellites.
Rather than have one large telescope, Pan-STARRS will use 4 telescopes, each with a
field of view of 7 square degrees and an aperture of 1.8 meters, and a 1.4 gigapixel
camera, each of which will generate 2 to 3 terabytes of information per night, which was
described earlier.
The prototype system is located on Haleakala on Maui, and achieved first light with
the gigapixel camera in 2007. The 4-telescope system is planned for the 13,800-foot
summit of Mauna Kea on Hawaii island.
Rapid Telescope for Optical Response (RAPTOR)
Another group that is using smaller telescopes in an array is Los Alamos National
Laboratory. Originally designed for astronomical purposes, they are looking at
applications for surveillance of Earth-orbiting satellites.
There are currently 5 systems on line, with another 3 under construction. The system
shown in the first photo is currently on line, while the other will be fielded shortly.
One of their systems consists of 4 40-cm telescopes, each of which can use a different
filter for simultaneous observations of the same object.
These telescopes are currently located in New Mexico. They are controlled by
software that autonomously hands objects off from one telescope to another.
Conclusions
In conclusion, to search for small Earth-orbiting objects puts challenging requirements
on the optical system and the sensor system.
This can be accomplished in a number of ways, and I’ve given examples of
astronomical and space surveillance telescopes for each of these approaches.
Each of the systems designed for space surveillance, or which can be used for space
surveillance, differ dramatically in basic design, as well as concept of operations. Each is
likely to fill a niche that should be complementary to the other systems.