Download Optical Astronomy Imaging Chain: Telescopes & CCDs

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
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