Photometry and astrometry

One of the basic astronomical pursuits throughout history has been to determine the amount and temporal nature of the flux emitted by an object as a function of wavelength. This process, termed photometry, forms one of the fundamental branches of astronomy. Photometry is important for all types of objects from planets to stars to galaxies, each with their own intricacies, procedures, and problems. At times, we may be interested in only a single measurement of the flux of some object, while at...

Slitless spectroscopy

The above discussion of astronomical spectroscopy was based on the use of a spectrograph that allowed an image of a slit to be dispersed and then re-imaged onto the CCD detector. Another method, slitless spectroscopy, has also been a workhorse for astrophysical science (Bowen, 1960a) and continues to be useful today (MacConnell, 1995). The idea of slitless spectroscopy is to take advantage of a simple direct imaging system to provide a mechanism for producing and recording astronomical spectra....

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Histogram of a typical flat field image. Note the fairly Gaussian shape of the histrogram and the slight tail extending to lower values. For this R-band image, the filter and dewar window were extremely dusty leading to numerous out of focus doughnuts (see Figure 4.4), each producing lower than average data values. is F 6950 ADU and its width (assuming it is perfectly Gaussian (Massey & Jacoby, 1992)) will be given by We have made the assumption in this formulation that the Poisson...

Radiation damage in CCDs

With the launch of the Galileo spacecraft and the Hubble Space Telescope, astronomical imagery with CCDs from outer space began. Today Cassini, Deep Impact, Chandra, XMM-Newton, and a number of other satellitees and space missions (such as the proposed Constellation-X, DUO, ROSITA, and GAIA space missions) have CCD imagers on-board. With these exciting new windows on the Universe come many unexpected effects in the performance and output noise levels of the CCDs involved. The study of radiation...

Data reduction for CCD spectroscopy

This section discusses the basics of astronomical CCD spectroscopic data reduction. We start with Figure 6.6, which presents a raw, unprocessed 2-D CCD spectroscopic image of a point source. This figure illustrates a number of the observational and reduction items we discuss below. The initial reduction steps for CCD spectroscopy are exactly the same as previously discussed for imaging applications. Bias (or dark) frames and flat field calibration images are needed and used in the same way....

CCDs in space

Space-based CCDs have a number of special problems associated with them that are often not considered for ground-based systems. Once launched, human intervention is unlikely and the CCD and instrument package can never be retrieved for fault correction or calibration purposes. Even simple procedures, such as bias calibration, take on new meaning as CCD evolution or changes in the gain or other CCD electronics mean new calibration images are needed. Damage to the array (see Section 7.2), or the...

Quantum efficiency and bandpass

Quantum efficiency (QE) is the term used to report on the ability of a detector to turn incoming photons into useful output. It is defined as the ratio of incoming photons to those actually detected or stored in the device. A QE of 100 would be an ideal detector with every incoming photon detected and accounted for in the output. Band-pass is a term that means the total spectral range for which a detector is sensitive to the incoming photons. Our eyes, for example, have a very limited band-pass...

High speed photometry

Many years ago (about 30) in the dark ages of astronomical instrumentation, photomultiplier tubes (PMTs) were making their farewell to astronomy and with them went almost all of the studies of high-speed phemonena. The Universe is a fast-paced place and many sources change on time scales of less than 1 second. Accretion phenomena and rotation of neutron stars are a few examples. Studies of such events and other fast changes are being resurrected thanks both to a renewed interest and to CCDs and...

References

Adams, M., Christian, C., Mould, J., Stryker, L., & Tody, D., 1980, Stellar Magnitudes from Digital Pictures, Kitt Peak National Observatory publication. Alard, C., 2000, Astron. Astrophys., 144, 363. Alcock, C., et al, 1999, Ap. J, 521, 602. Amelio, G., Tompsett, M., & Smith, G., 1970, Bell Systems Technical Journal, 49, 593. Anderson, J. & King, I., 2003, Publ. Astron. Soc. Pac., 115, 113. Baily, P., et al., 1990, Proc. SPIE, 1344, 356. Baum, W. A., Thomsen, B., & Kreidl, T. J.,...

Review of spectrographs

Spectroscopic observations can be thought of as a method by which one samples the emitted energy distribution from an astronomical source in wavelength bins of size AA. Broad-band filter photometry, for example, is a form of spectroscopy it is merely one with extremely poor spectral resolution. To use spectral information to learn detailed physics for an astronomical object, one must be able to differentiate specific spectral features (lines) from the continuum within the observed spectrum and...

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Temp (C) Typical Gain used (e ADU) 1. Using only the data presented in Figures 3.1 and 3.2, draw a quantum efficiency curve expected for a typical CCD. Why might real QE curves be different 2. Discuss two major reasons why CCDs are better detectors than the human eye. Are there instances in which the eye is a better detector What type of A D converter does the eye have 3. Design a detailed observing plan or laboratory experiment that would allow you to measure the quantum efficiency of a CCD....

C1 False color and pseudocolor images

Observations with Charge-Coupled Devices (CCDs) do not really generate images, they produce a 3-D set of numbers x, y, and intensity (z). We have just seen how these numbers are displayed as an image and how the use of a look-up table allows the translation from pixel value (z) to displayed grey scale. Sometimes, we wish to present our results in color either for dramatic effect or for science productivity, or at times, a little of both. Additionally, color images often truly help us to see the...

Nomenclature

CCDs are often listed and named by an apparently strange convention. This small section aims to demystify these odd sounding names. CCDs come in various sizes and shapes and are manufactured by a number of companies see Appendix B . Figure 1.1 illustrates a number of modern CCDs. Present day CCDs generally come in sizes ranging from 512 by 512 picture elements or pixels to arrays as large as 8192 by 8192 pixels. Often the size designations of CCDs are written as 2048 x 2048 or 20482. CCDs are...