Download Characterization of a CCD

Characterization of a CCD
The physical properties and function of a CCD camera are described by the terms read
noise, dark current, signal-to-noise, bias, quantum efficiency, gain, dynamic range and a
few others. The following will attempt to demystify these terms and give a better
understanding of what they mean.
Readout Noise (or Read Noise)
Read noise comes from the electronics used to “read” the CCD. There are two
components to read noise: 1) the conversion from voltage (an analog signal that is
actually what is read from each pixel) to a digital number, and 2) passage of the signal
through an amplifier and analog to digital converter. As an example of the conversion
from voltage to digital numbers, lets say you are reading the same pixel twice, each time
with an identical charge. You will get a slightly different digital number each time. This
is because you can never measure anything exactly. There is a statistical distribution of
possible answers centered on a mean value. Second, the electronics themselves will
introduce spurious electrons into the entire process, generating unwanted fluctuations in
the output. These two effects combine to produce an uncertainty in the final output value
for each pixel. The average of this uncertainty is called the read noise. So if the read
noise of your CCD is 20 electrons, each pixel will, on average, contain 20 extra electrons
each time the CCD is read.
Gain
The output voltage from a given pixel is converted to a digital number during readout.
The amount of voltage needed (which translates into the number of collected electrons or
received photons) to produce 1 count (also called an analog-to-digital unit, ADU) is
called the gain of the CCD. A typical gain might be 10 electrons/count, which means that
for every 10 electrons collected within a photosite, that pixel will produce, on average, 1
count.
Take two bias frames and two flat field frames with either the Ap7p or the ST-2000XM
CCD.
1) Determine the mean pixel value within each bias and each flat field.
2) Now crop the two flat fields to remove the dark corners. Calculate the mean pixel
values for the two cropped flat fields.
Image
Flat 1
Flat 2
Bias 1
Bias 2
Flat 1 cropped
Flat 2 cropped
Mean Pixel Value
a. What is the difference between the mean pixel values for each uncropped
and cropped flat image?
b. How do you explain this difference?
3) Now create three difference images: (Flat 1 – Flat 2, Bias 1 – Bias 2, Flat 1cFlat2c) . Measure the standard deviation in each of these image differences.
Image
Flat 1 – Flat2
Bias 1 – Bias 2
Flat 1c – Flat 2c
Standard Deviation
4) Determine the gain of the CCD using:
Gain = (F1+F2)-(B1+B2)
(⍺2 (f1-f12) - ⍺2(B1-B2))
5) Recalculate the gain using the cropped flat images
Images
Gain
Normal flats
Cropped flats
6) How does using the cropped flats affect the gain? Which do you think is
appropriate to use given the definition of gain?
7) Now calculate the read-noise of the CCD using
Read Noise = Gain * ⍺(B1-B2)
Sqrt(2)
8) Imagine you are observing the planet Jupiter with a CCD with the read noise and
gain you determined above. Jupiter has an average signal of 100000 electrons per
second per pixel.
a. What, on average, would be the counts coming from each pixel exposed to
Jupiter?
b. You want to do a long exposure of Jupiter to bring out details in its
atmosphere. Given what you know about read noise, do you think it is
wise to do a single long exposure or co-add several shorter exposures?
Why?
9) Now you are observing a faint spiral galaxy in Virgo with an average signal of
100 electrons per second per pixel.
a. What, on average, would be the counts coming from each pixel exposed to
the galaxy?
b. Do you think it is wise to do a single long exposure or co-add several
shorter exposures?
Linearity
One major advantage of a CCD is that it is linear in its response over a large range of data
values. Linearity means that there is a simple linear relation between the input value
(charge collected within each pixel) and the output value.
The largest output number that a CCD can produce is set by the number of bits in the A/D
converter. A bit can have the value of 0 or 1. The output is based on powers of two.
With a 14 bit A/D converter, numbers 0-16383 (214) can be represented. With a 16 bit
A/D converter (both of our CCDs are 16 bit), numbers 0-65,535 can be represented (216).
At high count rates, the CCD can start to become non-linear. It is important, when using
your observations for scientific analysis, to know when and how the CCD becomes nonlinear.
There are several factors that can limit the largest usable output pixel value in a CCD
image: two types of saturation (A/D saturation and pixel saturation), and nonlinearity.
The CCD is said to be “saturated” when the A/D converter can not output higher
numbers, and also when each pixel cannot hold any more electrons without leaking into
surrounding pixels.
1) The Ap7p has a full well capacity of 300,000 electrons. The ST-2000XM has a
full well capacity of 45,000 electrons. Using the gain you calculated above for the
CCDs, calculate how many photons each CCD can hold before the A/D converter
saturates.
A/D max * gain = input photons
2) Is this more or less than the number of electrons a pixel can hold?
Both types of saturation lead to noticeable effects, such as “bleeding” of signal into
adjacent pixels. Far more dangerous is “nonlinearity”, which can occur without the
observer being aware. This it is important to know the response of your CCD to higher
and higher counts.
1) Set up the Ap7p on the 24 inch and/or the ST-2000XM on the Meade 10 inch (if
not already done).
2) Find a field of stars with a range of brightnesses (Double cluster in Perseus might
be good for the Meade, check on the ECU computer for a cluster for the 24 inch).
Don’t include any stars that are really bright.
3) Obtain exposures starting at 0.5 second and doubling the exposure time until one
or more of the stars begins to saturate (or bleed). For example, you might end up
with exposures of 0.5, 1, 2, 4, 8 , 16, 32, and 64 seconds. You generally can’t go
longer than 120 seconds without the stars trailing a bit.
4) Take your images downstairs and view with AIP.
a. Pick 5 stars with a range of brightness.
b. Plot the output ADU values of the star, the sky, and star+sky versus
exposure time.
c. This will produce a linearity curve for the CCD. Where does the CCD
start to become nonlinear?