The Imaging Chain for Optical Astronomy

Review/overview The imaging chain typically includes the following elements: –energy source –object –collection –detection –processing –display & analysis

Source/object In astronomy, the source of energy (light) is almost always also the object of the imaging –exceptions: planets, dust reflecting starlight Astronomical sources place specific requirements on astronomical imaging systems –requirements are often conflicting: excellent angular resolution; wide field of view high sensitivity; large dynamic range broad wavelength coverage; spectral lines

More luminous objects can be detected out to larger distances Lines of constant apparent brightness

More distant objects are usually larger in physical size Lines of constant angular size

Angular sizes span a wide range

Atmosphere modifies source For ground-based optical astronomy, Earth’s atmosphere plays a large role in determining the character of the source –scintillation modifies source angular size twinkling of stars = smearing of point sources –extinction cuts down on light intensity atmosphere scatters a small amount of light, especially at short (bluer) wavelengths water vapor blocks out specific wavelengths, esp. in near-IR –scattered light produces interfering “background” astronomical images are never limited to light from source alone; always include “source” + “background sky” light pollution worsens sky background

Collection: Telescopes Refractor telescopes –exclusively use lenses to collect light –have big disadvantages: aberrations & sheer weight of lenses Reflector telescopes –use mirrors to collect light –relatively free of aberrations –mirror fabrication techniques steadily improving

Optical Reflecting Telescopes Use parabolic, concave primary mirror to collect light from source –modern mirrors for large telescopes are lightweight & deformable, to optimize image quality 3.5 meter WIYN telescope mirror, Kitt Peak, Arizona

Optical Reflecting Telescopes Basic optical designs: –Prime focus: light is brought to focus by primary mirror, without further deflection –Newtonian: use flat, diagonal secondary mirror to deflect light out side of tube –Cassegrain: use convex secondary mirror to reflect light back through hole in primary –Nasmyth focus: use tertiary mirror to redirect light to external instruments

Optical Reflecting Telescopes Schematic of 10-meter Keck telescope

Big Optical Telescopes Largest telescopes in use or under construction : –10 meter Keck (Mauna Kea, Hawaii) –8 meter Subaru (Mauna Kea) –8 meter Gemini (Mauna Kea & Cerro Pachon, Chile) –6.5 meter Mt. Hopkins (Arizona) –5 meter Mt. Palomar (California) –4 meter NOAO (Kitt Peak, AZ & Cerro Tololo, Chile) Summit of Mauna Kea, with Maui in background Keck telescope mirror (note person)

Why build big telescopes? Larger aperture means more light gathering power –sensitivity goes like D 2, where D is diameter of main light collecting element (e.g., primary mirror) Larger aperture means better angular resolution –resolution goes like lambda/D, where lambda is wavelength and D is diameter of mirror

Why build small telescopes? Smaller aperture means less chance of saturation (“overexposure”) on bright sources Smaller aperture generally means larger field of view –recall F ratio, F=f/D, where f is focal length of collecting element and D is diameter of aperture –for two reflecting telescopes with same F ratio and the same size detector, the telescope with smaller D produces images that cover a wider angle

Detection: Cameras for Astronomy Camera usually includes: –filters most experiments require specific wavelength range(s) broad-band vs. narrow-band –reimaging optics enlarge or reduce image formed by primary collecting element –detector Most common detectors: –The eye –Photographic emulsion film plates –CCDs

The eye as astronomical detector Must reimage the image formed by the primary (or objective) such that the light rays are parallel as they enter the eye (i.e. rays appear to come from infinity) –reimaging is accomplished by the eyepiece Point sources (stars) appear brighter to the eye through a telescope by a factor D 2 /P 2, where D is telescope diameter and P is the diameter of the eye’s pupil –for maximum effect, magnification has to be sufficient for light to fill pupil Extended sources (for example, nebulae) do not appear brighter through a telescope –Gain in light gathering power is exactly compensated by magnification of image, which spreads light out

Photographic techniques: silver halide film –large amount of work is still done by amateurs using highly sensitive B&W and color film plates –from the earliest development of AgX techniques until advent of CCDs in late 70’s, most images were captured on photographic plates panes of glass overlaid with silver halide emulsion

CCDs charge coupled devices (CCDs) are now standard light detection medium for professional and amateur astronomical imaging systems alike numerous advantages over film: –high quantum efficiency (QE), meaning most photons incident on a CCD are detected –linear response, meaning signal builds up in direct proportion to number of photons collected –fast processing turnaround (CCD readout speeds ~1 sec) –regular grid of pixels (as opposed to random distribution of AgX grains) –image delivered in computer-ready form

Image processing Once images are collected, they need to be corrected for: –Atmosphere (to the extent possible) e.g., sequence of images obtained at a variety of telescope elevations usually can be corrected for atmospheric extinction –CCD defects and artifacts dark current –CCD pixel reports a signal even when not exposed to light bad pixels –some pixels will be dead, hot, or even “flickering” variations in pixel-to-pixel sensitivity –every pixel has its own QE –can be characterized by “flat field”

Image display & analysis Often, this step in the imaging chain is where the astronomy really begins. Type and extent of display and analysis depends on purpose of imaging experiment Common examples: –evaluating whether an object has been detected or not –determining total CCD signal (counts) for an object, such as a star –determining relative intensities of an object from images at two different wavelengths –determining relative sizes of an extended object from images at two different wavelengths