Pixel sampling and matching to the plate scale

There are two issues to be considered when matching the spatial or spectral resolution element to the physical size of the detector pixels (1) maximizing observing efficiency, meaning more light onto a pixel and therefore keeping the required integration time to a minimum and (2) accomplishing this task without compromising the ability of the camera system to obtain very accurate brightness measurements (photometry). The spatial resolution element may be determined by seeing conditions or by...

Case Study Design Of A Twomirror Telescope

This discussion follows the excellent exposition given by Dan Schroeder (2000) in Astronomical Optics and is intended to illustrate the basic rationale behind the design of modern large two-mirror telescopes. Spherical aberration is eliminated by the use of conic surfaces, and for a parabolic primary on-axis imaging is perfect. However, for a parabolic mirror receiving parallel light at an off-axis angle 9, with a vertex radius of curvature R, the angular aberrations to third order (AA3) (i.e.,...

Diffraction gratings

Grating Anamorphosis Image

The usual dispersing element is a diffraction grating and the general grating equation is m d(sin i + sin 9) cos 7 (5-11) where d is the spacing of adjacent grooves or slits i is the angle of incidence of the collimated beam 9 is the angle of the emergent diffracted beam 7 is the angle out of the normal plane of incidence (usually 0 , hence cos 1) and m is an integer called the order of interference. For zero order (m 0), sin 9 sin i or 9 i. The negative sign comes from the fact that we have...

Multipinned phase CCDs

Several years ago it was realized that the dominant contributor to the dark current in CCDs was thermal generation due to surface states at the Si-SiO2 interface. In fact, surface dark current is 102-103 greater than dark current generated by the bulk of the CCD. Two factors control the dark current at the silicon-silicon dioxide interface the density of interface states and the density of free carriers (holes or electrons) that populate the interface. Electrons that thermally hop from the...

Cryogenics And Vacuum Methods

Most state-of-the-art detectors require cooling for optimum performance. There are several categories of cooling systems which might be required in cameras and spectrographs for ground-based astronomy depending on the application. 1. Thermoelectric coolers and liquid circulation coolers these normally operate over the range 20 C to 50 C and are suitable for photomultiplier tubes, certain CCDs which have low dark currents, and high-speed applications such as telescope-guiding cameras. This class...

The astronomical push

Many astronomy-related groups familiar with imaging technology, usually with vidicon-type systems, were alert to the potential of CCDs in the early 1970s. Gerald M. Smith, Frederick P. Landauer, and James R. Janesick of the Advanced Imaging Development Group at the NASA Jet Propulsion Laboratory operated by the California Institute of Technology (Caltech) in Pasadena, and Caltech scientist James Westphal (1930-2004) were among the first to recognize the potential advantages of such an imaging...

Historical Development

By the end of the 13th century in Europe, artisans in glass-making centers like Venice and Florence had already found practical techniques for grinding and polishing glass relatively cheaply and easily. Moreover, people of that time were aware that the condition known today as presbyopia, in which the aging eye can no longer focus on something held at a comfortable distance, could be helped with a simple magnifying glass. But two smaller disks of glass, convex on both sides and supported in a...

Basic optical properties

In this section we summarize and apply useful relationships that are derived in any good book on optics. It is well-known that when a ray of light strikes the boundary between two different transparent materials, such as air and glass, it is divided into a reflected ray (which is why you can see a faint reflection of yourself in a transparent window) and a transmitted ray whose direction in the glass is different from that in the air for oblique incidence this ray is said to be refracted. A ray...

Clock Voltages And Basic Electrical Factors

CCD manufacturers provide a data sheet which gives the electrical pin connection diagram for the device (Figure 8.1), the names and symbols for each pin, the voltages or range of voltages to be applied to each pin, and the timing diagram (i.e., a diagram showing the time sequence of the CCD drive signals and the relationship between them). Terminology varies, but certain basic functions are common to all. Voltages applied to CCDs are of two types fixed voltages, referred to as dc bias'' levels,...

Multiobject spectroscopic surveys 3D maps of the Universe

Spectrograph Scanner Wood

Spectroscopic measurements can be painfully slow, even with very sensitive CCD detectors. Typically, the signal at any point in the spectrum of an astronomical source will be at least several hundred times fainter than the signal in a direct image. Thus, the possibility of recording spectra from several objects at once is very attractive. Slit-less spectroscopy and objective prism spectroscopy are two similar ways to observe the spectra of many objects at the same time. Removing the slit is...

Very Large Telescope Design 341 Segmented primaries

In the vanguard of the wave of new large telescopes was the pair of 10 m collectors at the W.M. Keck Observatory (WMKO). The WMKO is managed by CARA, the California Association for Research in Astronomy, and comprises the California Institute of Technology (Caltech) and the University of California (UC), in collaboration with the University of Hawaii and NASA. The optical layout of each telescope is summarized in Figure 3.10. Each primary mirror is composed of 36 hexagonal segments of Zerodur,...

Surveys and data archives the virtual observatory

The last decade of the 20th century was remarkable in astronomy for many reasons. Not only did many large telescopes with apertures of 8 m-10 m appear, but also many smaller telescopes were built and designed to capitalize on the power of electronic imaging devices by performing deep astronomical surveys. Perhaps the best known of these is the Sloan Digital Sky Survey (SDSS). This remarkable project, spearheaded by Jim Gunn of Princeton University, uses a dedicated 2.5 m telescope located on...