aggiungo qualche ref.
Radiation events in astronomical CCD images∗
A. R. Smith, R. J. McDonald, D. L. Hurley, S. E. Holland, and D. E. Groom
Lawrence Berkeley National Laboratory, Berkeley, CA 94720
W. E. Brown, D. K. Gilmore, R. J. Stover, and M. Wei
The remarkable sensitivity of depleted silicon to ionizing radiation is a nuisance to astronomers. “Cosmic rays” degrade images because of struck pixels, leading to modified observing strategies and the development of
algorithms to remove the unwanted artifacts. In the new-generation CCD’s with thick sensitive regions, cosmicray muons make recognizable straight tracks and there is enhanced sensitivity to ambient gamma radiation via
Compton-scattered electrons (“worms”). Beta emitters inside the dewar, for example high-potassium glasses such as BK7, also produce worm-like tracks. The cosmic-ray muon rate is irreducible and increases with altitude.
The gamma rays are mostly by-products of the U and Th decay chains; these elements always appear as traces in concrete and other materials. The Compton recoil event rate can be reduced significantly by the choice of
materials in the environment and dewar and by careful shielding. Telescope domes appear to have significantly lower rates than basement laboratories and Coud´e spectrograph rooms. Radiation sources inside the dewar
can be eliminated by judicious choice of materials. Cosmogenic activation during high-altitude flights does not appear to be a problem. Our conclusions are supported by tests at the Lawrence Berkeley National Laboratory low-level counting facilities in Berkeley and at Oroville, California (180 m underground).
Keywords: CCD, cosmic rays, high resistivity, fully depleted, astronomical, Lick Observatory, Lawrence Berkeley National Laboratory
Cosmic Rays and Other Nonsense in Astronom-
ical CCD Imagers
Lawrence Berkeley National Laboratory
Abstract: Cosmic-ray muons make recognizable straight tracks in the new-generation CCD's with thick sensitive regions. Wandering tracks (\worms"), which we identify with multiply-scattered low-energy electrons, are readily recognized as di erent from the muon tracks. These appear to be mostly recoils from Compton-scattered gamma rays, although worms are also
produced directly by beta emitters in dewar windows and eld lenses.
The gamma rays are mostly byproducts of 40K decay and the U and Th
decay chains. Trace amounts of these elements are nearly always present
in concrete and other materials. The direct betas can be eliminated and
the Compton recoils can be reduced signi cantly by the judicious choice
of materials and shielding. The cosmic-ray muon rate is irreducible. Our
conclusions are supported by tests at the Lawrence Berkeley National
Laboratory low-level counting facilities in Berkeley and 180 m under-
ground at Oroville, California.
Keywords: CCD, cosmic rays, high resistivity, fully depleted, back illuminated, Compton scattering, gamma ray
1997 HST Calibration Workshop
Space Telescope Science Institute, 1997
S. Casertano, et al., eds.
Cosmic Ray and Hot Pixel Removal from STIS CCD Images
Robert S. Hill and Wayne B. Landsman
Abstract. The problem of cosmic ray (CR) removal is a general one plaguing spaceborne CCDs, as is the gradual accumulation of single high-dark-rate pixels between CCD annealings. The STIS team at Goddard has developed IDL implementations of standard techniques for dealing with these problems as part of the STIS GTO software. This report summarizes the methods and discusses the pitfalls.
NOISE, NOISE, NOISE ROBERTO BARTALI
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Charge Coupled Devices (CCD) have many advantages, as image devices, over photographic plates, but, like all other electronic devices, temperature, electric noise and exposure to cosmic radiation, reduce their effectiveness. The aim of this project is the description of problems raised due to noise and how they can be fixed or reduced in order to obtain the best from them. Professional and scientific grade CCD are very different from commercial and amateur ones, but they have in common the same type of problems generated by noise, the difference consist in the amount of the figure of that noise and I will compare both type. Specifically I will describe: read noise, dark current, spectral sensitivity response, flat fields, charge transfer efficiency, saturation and blooming, cosmetic defects and cosmic ray effect. For each topic, whenever possible, I will present pictures, curves or graphic representation, this way the reader can understand better the topic; I will also, for each one, compare professional and amateur CCD. Some kind of problems will be reduced or, until some degree, fixed electronically or thermally, but other ones need software specific routines to do that, so I will conclude that noise reduction is a very complex thing and time consuming. Processing a picture, from raw data until obtain from it, scientifically valuable data, can take much more time than that spent for the exposition.