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/fD

• 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 1000x1000 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

• Response is large in visible region, falls off for ultraviolet (UV) and infrared (IR)

300 400 500 600 700 800 900 1000

Incident Wavelength [nm]

RelativeResponse

Visible Light IRUV

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