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.

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

CubeSat

10 cm

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.