CCD Detectors in Astronomy

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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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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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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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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.



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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.



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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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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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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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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.


Dome flats


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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.


Sky flats


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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.



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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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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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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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Preliminary Steps

Bias Substraction


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Preliminary Steps

Dark Current


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Preliminary Steps

Flat Field


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Preliminary Steps

Flat Field


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Preliminary Steps

Flat Field


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Preliminary Steps

Flat Field


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Preliminary Steps

Flat Field


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Preliminary Steps

Flat Field


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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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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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Cleaning

Bad columns and traps


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Cleaning



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Cleaning



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Cleaning

Hair, dust...


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Cleaning

Hair, dust...


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Cleaning

Hair, dust...

3 R d i CCD D t


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Cleaning

Cosmic Rays

3 R d i CCD D t


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Cleaning

Cosmic Rays

3 R d i CCD D t


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Cleaning

Saturation Bleed


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3 Reducing CCD Data


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Cleaning

Oversaturation Dot

3 Reducing CCD Data


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Cleaning

Satellites

3 Reducing CCD Data


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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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CCD Detectors in Astronomy