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CCD Detectors
inAstronomy
Alejandro Lavrador
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1. CCD Detectors
Photographic plates:
Limited dynamic range and response to the
brightness of the illuminating light is non-linear.
unlike the eye they were an integrating detector:
fainter objects could be detected by making
longer exposures to accumulate more light,
the images were objective and reproducible
(unlike a sketch),
the photographic image constituted a quantitative
measure of the light distribution across the
luminous object (at least in principle).
Photoelectric photometers:
Wider dynamic range
More sensitive, accurate and
brightness.
Dont produce an image(outp
brightness in sky in the point
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1. CCD Detectors
CCD: Charge-Couple devices
High sensitivity, linear response.
Large dynamic range.
Produces image of sky being viewed.
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1. CCD Detectors
Semiconductor chip
Face sensitive to light
Grid of rectangular areas(Picture
elements or Pixels)
Photon generates small electric
charge that is stored.
That charge is cumulative, higher
charge = brighter signal.
Resolution or arrays of 64, 256,
512, 1024, 2048, etc...
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1. CCD Detectors
Analog-to-digital Converter (ADC) with
16-bit accuracy at least.
Reads charge and gives back a value
(number) depending on the amount of
charge.
This number, Analogue Data Units, is not
in physical Units. We calibrate with the ADC factor,
proportionality between ADUs and
charge.
Computer gets image directly.
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1. CCD Detectors
Quantum Efficiency of 80%.
Broader range of wavelengths. More sensibility on Red lights.
Bad Response in Blue and UV light.
Large Dynamic Range(Ratio from
brightest to faintest signal):
It can get a range of 14.5 magnitudescompared to Photograph which gets
7.5.
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1. CCD Detectors
Plate scale: p=/mm ( Units of seconds arc/mm)
If you know the plate scale and the size of the grid or a pixel, yo
Angle on the sky or field of view given by that pixel.
The plate scale is usually given by the manual of the instrument
calculated from the effective focal length:
p=1/f ,
where p is in radians or any unit f is in. If f is in meters and we a
conversion from radians to seconds of arc:
p=206.26/f
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2. Instrumental Effects in CCD Detectors
Bad Pixels
Read-out signal:bias
Non-linearity
Thermal noise, dark current
Pixel sensitivity,flat fielding
Cosmic-ray events
Photon noise
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2. Instrumental Effects in CCD Detectors
Faulty pixels that return inaccurate signals.
These are called hot, cold or bad
Because of the readout method to get the values from pixels, t
contaminate the whole column or row, leading to bad rows or c
To fix them we replace them with values computed from neighpixels.
Bad Pixels
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2. Instrumental Effects in CCD Detectors
Caused by the amplifier boosting the signal before sending it to
to-digital converter, thus biasing the signal by some amount.
There is 2 methods to estimate and correct them:
Bias Strips: Bias Frames
Read-out signal, bias
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2. Instrumental Effects in CCD Detectors
CCD generates 2
regions, that contain
only readouts of the
CCD without sampling
its charge.
These regions contain
only values of noise and
bias, so these can be
substracted from all
pixels in the row.
Read-out signal, bias
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2. Instrumental Effects in CCD Detectors
In the Bias Frames, all CCD is read-out without sampling charg
and this frame is used to substract noise from the final image
frames.
Read-out signal, bias
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2. Instrumental Effects in CCD Detectors
If the incoming light is bright enough, then it may become non-
linear and saturate(it doesnt produce change in the recorded
signal anymore)
It may be prevented by taking a larger number of short exposu
shots.
This technique offers also advantage to remove cosmic-ray ev
Non-linearity
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2. Instrumental Effects in CCD Detectors
Offset from zero that is generated thermally within the CCD, ev
without light.
Its called dark current.
Varies with time and from pixel to pixel slowly, and its minimize
keeping the CCD cool with liquid nitrogen.
It can be measured by taking long exposure with shutter close
removing bias and cosmic-ray events and dividing by the expo
time.
Its usually insignificant for visible light, but important for infrare
light.
Thermal noise, dark current
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2. Instrumental Effects in CCD Detectors
Pixel sensitivity vary slightly between them across the grid.
Can be calibrated by taking an image of an evenly iluminated
source, such as twilight sky.
This is known as flat fielding, and pixel-to-pixel sensitivity varia
change with wavelength, so flat field must be acquired using sa
filter as the observation of the target objects.
It also corrects for other effects: small sharp dark features, produced by dust particles
ring or torus features, produced by dust on filters that are out of focus.
vignetting, dimming of objects in the edge of the field of view, produced by
obstruction of light, eg secondary mirror support
Pixel sensitivity, flat fielding
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2. Instrumental Effects in CCD Detectors
Dome flats are images of the inside of the telescope dome, ev
iluminated.
The interior surface is usually a smooth diffuse reflector and its
focus for the telescope optics so the image obtained is feature
This is convenient because dome flats can be taken during day
night or twilight.
Disadvantages are:
Light reflected from dome is incident at different angle to light from sky, it a
and shape of images formed by dust particles
The colour is not the same as of the night sky.
Pixel sensitivity, flat fielding
Dome flats
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2. Instrumental Effects in CCD Detectors
Sky flats are images from the sky taken during twilight when it
The sky must be much brighter than any stars but not enough
The time to acquire the flat field depends on the filter.
A narrow filter ( wavelength for which the chip is insensitive, or
range where sun emits little light), can be taken nearer to sunr
than a broadband filter.
Pixel sensitivity, flat fielding
Sky flats
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2. Instrumental Effects in CCD Detectors
Possible to combine different sorts of flat fields. To obtain
advantages of each.
Example:
Use Dome flats for pixel-to-pixel sensitivity and twilight flats for
large-scale effects, such as vignetting.
Pixel sensitivity, flat fielding
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2. Instrumental Effects in CCD Detectors
Signals in CCD frames caused by ionising radiation.
When a cosmic-ray particle hits a pixel it increases its charge.
They appear as a set of pixels with intense values scattered ov
CCD frame.
In an exposure of minutes we might have a hundred of cosmic
hits.
To correct it, if we take several frames of the same object, the
cosmic-ray hits will be random in different positions, so it will b
possible to detect and remove them by comparing the pixels in
different images.
Cosmic-ray events
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2. Instrumental Effects in CCD Detectors
The last source of noise is photon noise, due to the way of cou
photons.
This noise is irreducible and proportional to the square root of
signal.
Photon noise
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3. Reducing CCD Data
There are many steps between the data coming out of a came
the final image that we see on internet.
It may be split intro 3 sections:
Preliminary: from the telescope to Photoshop
Cleaning: Remove the artifacts remaining in the image.
Final touch: Remove large scale effects, color correct, balance, p
Introduction
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3. Reducing CCD Data
The CCD can only take shots in black and white, for more flexibility a
sensitivity, so filters must be used to rebuild the colour information.
The procedure is to take shots with the different filters and then recom
them to form the colour image.
Basic filters are Red, Green and Blue, but there are many others that c
used to expand even more the colour range, such as UV, IR or a comm
one, red H_alpha, which highlights Hydrogen gas.
This frames must first be cleaned from all instrumental and electronic e
that affect them.
Preliminary Steps
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3. Reducing CCD Data
Preliminary Steps
Bias Substraction
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3. Reducing CCD Data
Preliminary Steps
Dark Current
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3. Reducing CCD Data
Preliminary Steps
Flat Field
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3. Reducing CCD Data
Preliminary Steps
Flat Field
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3. Reducing CCD Data
Preliminary Steps
Flat Field
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3. Reducing CCD Data
Preliminary Steps
Flat Field
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3. Reducing CCD Data
Preliminary Steps
Flat Field
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3. Reducing CCD Data
Preliminary Steps
Flat Field
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3. Reducing CCD Data
At this stage, the images are still in FITS format, the astronomical form
And we have one FITS file per filter.
FITS files are 16 bits , this means 65536 levels of gray.
Even if we can work in photoshop with 16 and 32 bits images, the stan
on internet, called sRGB, is 8 bit images with a colour space determine
So we must do this in photoshop, before publishing them.
We import the different filters in photoshop and align them, and we get
colour image, but the result is still horrible because we need to clean m
artifacts on them.
Preliminary Steps
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3. Reducing CCD Data
Even if we removed the bias and vignetting and some other artifacts, tstill more artifacts to be cleaned now in our photoshop file.
We can clean them manually or using filters in Photoshop.
Cleaning
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3. Reducing CCD Data
Cleaning
Bad columns and traps
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3. Reducing CCD Data
Cleaning
Bad columns and traps
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3. Reducing CCD Data
Cleaning
Bad columns and traps
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3. Reducing CCD Data
Cleaning
Hair, dust...
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3. Reducing CCD Data
Cleaning
Hair, dust...
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3. Reducing CCD Data
Cleaning
Hair, dust...
3 R d i CCD D t
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3. Reducing CCD Data
Cleaning
Cosmic Rays
3 R d i CCD D t
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3. Reducing CCD Data
Cleaning
Cosmic Rays
3 R d i CCD D t
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3. Reducing CCD Data
Cleaning
Saturation Bleed
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3 Reducing CCD Data
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3. Reducing CCD Data
Cleaning
Oversaturation Dot
3 Reducing CCD Data
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3. Reducing CCD Data
Cleaning
Satellites
3 Reducing CCD Data
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3. Reducing CCD Data
Cleaning
Diffraction pattern
3 Reducing CCD Data
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3. Reducing CCD DataCleaning
Background Steps
3 Reducing CCD Data
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3. Reducing CCD DataCleaning
Ghosts
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