Optical Astronomy Imaging Chain Telescopes CCDs Reflector telescopes

Optical Astronomy Imaging Chain: Telescopes & CCDs

Reflector telescopes: basic principles • reflection: angle in = angle out – as a result, spherical mirrors would suffer from spherical aberration • the virtues of parabolas – parallel incident rays are brought to common focus – => primary mirrors are ground to paraboloid shape

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

Telescope f-ratio • f = F/D where F is focal length and D is diameter – must consider focal length of primary & secondary mirrors combined • Determines “plate scale” – plate scale is measured in e. g. arcsec per mm at the focal plane – can be estimated from our friend, the small-angle relation theta=S/F • plate scale = theta/S = 1/f. D • for an f/16 10” telescope, plate scale = 50 arcsec per mm

CCDs: pixel scale and field of view • Example: CCD pixel scale – take a plate scale of 50 arcsec per mm – CCD pixels are about 25 microns – => pixel scale would be 1. 25 arcsec per pixel • Example: CCD field of view – For a 1000 x 1000 CCD with 1. 25 arcsec pixels, field of view is 1250” or about 21 arcmin (could image most of Moon’s surface)

CCDs: pixel scale and field of view • Want to match CCD pixel scale to image “smear” = point spread function – remember main sources of image smear • telescope angular resolution • atmosphere – ideally, arrange pixel scale such that 2 CCD pixels cover width of PSF • image field of view then limited by format (number of pixels) of CCD – the bigger the better, but bigger means more expensive

CCDs: noise sources • dark current – can be “removed” by subtracting image obtained without exposing CCD • leave CCD covered: dark frame • read noise – detector electronics subject to uncertainty in reading out the number of electrons in each pixel • photon counting – Poisson statistics: if I detect N photons, the uncertainty in my photon count is root(N)

CCDs: artifacts and defects • bad pixels – dead, hot, flickering… – methods for correcting: • replace bad pixel with average value of the pixel’s neighbors • dithering telescope: take a series of images, move telescope slightly to ensure image falls on good pixels • pixel-to-pixel differences in QE – can construct and divide images by the flat field • flat field is what CCD would detect if uniformly illuminated • saturation – each pixel can only hold so much charge (limited well depth) – at saturation, pixel stops detecting new photons (like overexposure) • charge loss occurs during pixel charge transfer & readout

Spectral Response (sensitivity) of a typical CCD UV Visible Light IR Relative Response 300 400 500 600 700 800 900 1000 Incident Wavelength [nm] • Response is large in visible region, falls off for ultraviolet (UV) and infrared (IR)

Filters • Because CCDs have broad spectral response, need to use filters to determine e. g. star colors in visible • broad-band: filter width is about 10% of filter’s central wavelength – example: V filter at 550 nm will allow light from 500 to 600 nm to pass through – astronomers use BVRI: blue, ‘visible’, red, IR • narrow-band: filter width is <1% – example: “H-alpha” covers 650 to 660 nm