Download
1 / 33
330 likes | 379 Views
Delve into the evolution of light detection in astronomy, from naked eye observations to the advancements in CCD detectors. Learn about historical techniques like daguerreotypes and the significance of photometry. Discover how modern instruments like the Apogee Ap47p CCD are reshaping astronomical research. Understand the importance of calibration processes like bias framing and dark frames for accurate data analysis. Explore the impact of different emulsions on capturing celestial phenomena and the significance of Flat fields. Unravel the mysteries of the night sky through innovative tools like the Spartan Infrared Imager and the Sloan Digital Sky Survey. Navigate through techniques and technologies that have revolutionized our understanding of the cosmos.
E N D
Light Detectors • Eye • Use averted vision to see Faintest objects Only the brightest stars show color with the naked eye
Naked eye limiting magnitude • A difference of a factor of 100 in brightness corresponds to a difference of 5 magnitudes • Limit usually taken as around 6.0, but by taking special precautions some have seen to magnitude 7 or fainter with the naked eye
Photography • 1852 daguerreotype • Daguerreotypes and wet plate photographs were very insensitive to light and were inconvenient to use • Dry plates developed in 1870s
Harvard College Observatory • Collection of some 500,000 photographic plates taken between 1880s and 1980s • Provide a record of changes in the sky
Different photographic emulsions were sensitive to different wavelengths of light • Early emulsions were mostly sensitive to blue and ultraviolet light • Early photographic magnitudes • “pg” blue • “pv” or “pvis” yellow
Kodak emulsions blue to red • Oh, John, George doesn’t eat flannel underwear nor milk zebras • OJGDEFUNMZ • Not made anymore
National Geographic Palomar Sky Survey 1950-57 103aO 103aF
POSS II • 1980s and 1990s • IIIaJ • IIIaF • Finer emulsion than in POSS I
UK Schmidt telescope in Australia • Southern counterpart to POSS • IIIaJ and IIIaF Digitized versions of these surveys are downloadable
Photoelectric Photometry • Advantages • Linear • Higher quantum efficiency than photography
1930s: Photomultiplier tubes • IP21 workhorse photomultiplier tube after WW II • One star at a time photometry
1950s: UBV filter system • U 365nm FWHM = 68nm • B 440nm 98 • V 550nm 89 • B-V = color index (bigger means redder)
1980s: CCD detectors (charge-coupled devices) • Array detectors for visible to about 1000 nm • Combines high quantum efficiency and imaging capability
CCDs were originally much smaller than photographic plates, but they are getting bigger
CCD in use at the 24-inch on campus • Apogee Ap47p 1024x1024 CCD • Thermoelectrically cooled • Designed for observing fainter objects
Infrared Observing • Has also gone to area arrays • Spartan Infrared imager (1-2.5 microns) • 4 2048x2048 HgCdTe detectors (mercury cadmium telleride)
Inexpensive planetary imagers • Celestron NexImage • Meade Autostar
Basic CCD observing • CCD images should be calibrated • Bias • Dark frame • Flat field
Bias Frame • Sets the zero-point of the CCD output • Essentially a 0 second exposure with the shutter closed • Should be subtracted from all images
Dark Frames • Even if the shutter is closed, every CCD image will have some signal • The warmer the CCD, the bigger this “dark signal” is • To minimize this we can cool the CCD, either electrically or cryogenically
Dark Frames • Take one or more exposures with the shutter closed • Ideally about as long as the real exposures and at the same CCD temperature • Subtract these dark frames
Flat fields • Not every pixel on the CCD will be equally sensitive to light • With the same filter you will use for observing, taken an exposure of a uniform light source, such as the twilight sky
Flat fields • Flat field images should be divided into the object image
Stardial TT Mon
