There's a lot to consider when choosing a CCD camera, and it can be a confusing process. As described below, your choice of camera will partly depend upon the quality of your imaging site, as well as your telescope. This is not meant to be a comprehensive review of the subject, but if you read through it carefully, I think that you will have a good understanding of the more important issues.
Note-
Although this section was written several years ago for CCD cameras,
many of the concepts below apply to CMOS cameras as well.
However, information that is CMOS-specific (like adjusting gain to
achieve a desired read noise and dynamic range) is not included, but I
hope to add this information shortly.
Dynamic
Range
Dynamic range refers to the difference between
the brightest
and the faintest regions of an image that can be simultaneously
recorded by the
CCD camera. The larger the difference, the better you will be
able to
capture faint signal in your image without blowing out (clipping) the
highlights. For a CCD camera, dynamic range can be estimated by
dividing
Full Well Capacity (in electrons) by the Read Noise (RMS electrons).
Take the
U32 camera as an example. The full well capacity of my camera is 55,000
electrons, and my read noise is about 8 RMS electrons (I measured both
parameters for my specific camera), yielding a dynamic range of 6,875.
The read
noise is the denominator since it represents the smallest possible
signal that
can be captured by the camera- you can't capture less than that,
because it
would be buried in the noise (see below for more information about read
noise).
Another valid way of looking at this dynamic range metric (6,875 in my
example)
is that it represents how many steps above the read noise floor the CCD
chip is
capable of recording (since the read noise represents the smallest
increment
that can be resolved by the camera). If you calculate the dynamic range
of
other cameras, many will be lower than 6,875 steps, and some will be
higher. If
it's lower, say around 3000 steps, this could be a potential problem,
especially if you image in a relatively light polluted site (like most
of
us). For instance, if your light pollution takes up 50% of your
well
depth, you will have more room "left over" for actual signal if you
are starting with 6875 steps, as opposed to 3000 steps. So
a higher
dynamic range is generally always better to have than a low dynamic
range,
meaning that you should choose a camera with a high full well capacity,
low
read noise, or both.
Note that dynamic range of a CCD camera is a
measure of
two related properties- 1) the difference between the highest and
lowest signal
intensities, and 2) the number of steps captured between the highest
and lowest
intensities. However, it does NOT guarantee that all of those captured
steps
will be faithfully rendered in the process of converting the electron
signal of
each pixel into a digital read out (i.e., the analog to digital
conversion). Once captured, the ability to faithfully render
those steps
is related to another feature called "bit depth," which is a
property of the camera's analog to digital converter. The analog
to
digital converter of most good quality CCD cameras already operates at
a bit
depth of 16 bits, meaning that it can convert the analog signal into
2^16, or
65,536 digital steps. Since my camera's dynamic range can capture 6,875
steps
(i.e., 55,000 divided by 8), the 16 bit depth of my AD converter (which
is able
to render 65,536 steps) is more than enough! Conversely, a bit depth of
12
would not be adequate for my camera, since it would only be able to
render 2^12
or 4,096 steps, whereas my camera's dynamic range has captured 6,875
steps.
From this description, it should be clear that dynamic range and bit
depth are
two different characteristics of the camera. You need a high
dynamic
range to capture faint signal without blowing out the highlights, and
to
capture fine gradations of intensity ("steps") in the image.
However, you need a high bit depth to actually render all of those
captured
steps into a useable digital output.
A camera can have a high dynamic range but low bit depth, in which case you will not be taking full advantage of all of those fine gradations of intensity that the camera has captured. This wastes the dynamic range and is not optimal. Likewise, a camera can have a low dynamic range but high bit depth, in which case you will certainly take advantage of the dynamic range, but there just won't be many steps available for the AD converter to render. I have gone into this in greater detail than necessary, but I find that there is continued confusion about the difference between dynamic range and bit depth and hope that this clarifies the issue.
Dark
Signal
Every CCD camera
generates a dark signal that varies with exposure time
and
chip temperature. A CCD chip works by converting incoming photons of
visible
light into electrons, which are stored in the pixels and later
converted to a
digital signal. However, it turns out that electrons are not only
produced by photons of visible light that strike the CCD chip.
Dark
signal refers to electrons that are generated in the absence of light,
as a
result of heat produced by the CCD camera chip itself. These
"thermal electrons" create hot pixels which increase in intensity
over the exposure duration, and which can be minimized by cooling the
CCD
chip. Some chips (like the Sony "Exview" chip used in the
SXV-H9 camera) have very low dark signal. Most other chips like
the Kodak
series have enough dark signal to warrant dark frame calibration
(meaning that
the dark frame is subtracted from the light frame, in order to remove
the hot
pixels that represent the effects of thermal electrons). Although
it's
nice to not have to worry about dark frame calibration, it's not a big
deal
either. Almost all CCD cameras that use Kodak series chips will
be
temperature regulated, meaning that you can specify the desired chip
temperature during imaging. This allows you to generate a series
of dark
frames at the same temperature (and duration), to be used as a dark
frame master
for future images taken at the same temperature. Creating a dark
frame
master library makes the process of dark frame calibration a relatively
painless process.
Choice
of Blooming (NABG) versus
Anti-Blooming (ABG) Cameras
Blooming is a
phenomenon that occurs when electrons fill the well of a
given
pixel and spill over into adjacent pixels, causing a bright, vertical
streak
that destroys the data contained within those adjacent pixels.
CCD chips
that bloom are called "non anti-blooming gate" chips (NABG) and are
typical of many Kodak KAF series chips (although note that some KAF
chips do
have anti-blooming gates). Anti-blooming chips do not have this
problem. They contain an "anti-blooming gate" (ABG) that bleeds
off electrons before they can spill over into adjacent pixels.
ABG chips
are typical of the Kodak KAI series and the Sony Exview series.
Sounds
like we should all be using ABG chips to avoid blooming, right?
In order
to appreciate why the choice isn't always so simple, take a look at the
Quantum
Efficiency (QE) curves of a NABG versus an ABG camera, and you will see
the
problem (QE curves are usually available on CCD vendor websites).
QE is a
measure of how efficiently a chip converts photons to
electrons.
Because the ABG technology takes up space in the pixel, less surface
area is
available for detecting photons. Thus, the QE of ABG chips is
comparatively quite low when compared to a NABG camera. So
how do
we choose between an NABG camera that blooms but has greater
sensitivity, versus
an ABG camera without blooming but with lower sensitivity? As
explained
below, the choice is largely dependent upon how long your subexposure
times
will be, and this in turn is dependent upon sky noise and read out
noise.
When taking a
subexposure, we want to maximize signal and minimize
noise (i.e.,
maximize the signal to noise ratio). Noise is uncertainty in the
true
value of the pixel, which shows itself as variability in the results of
a given
measurement. For instance, if we expose the chip to a constant
light
source for a fixed period of time, measure the number of photons being
captured
by the pixel, and repeat this 10 times in exactly the same way, we will
not
always get the same result! The degree to which the results
fluctuate is
referred to as noise, and it can be quantified. Noise is related
to 3
main effects:
1)
"Photon Noise" is a
property of the signal from collected light (i.e., the desired signal
plus any
sky background). Photons arrive in packets, at irregular
intervals, and
it's this unpredictability in arrival times that generates their noise,
which
is also referred to as shot noise;
2) "Dark Noise" is a property of the signal generated by
thermal
electrons (mentioned above);
3) "Read Noise" is another layer of variability in the signal that is
introduced by the chip amplifier responsible for converting the analog
signal
(i.e., the electrons in each pixel well) into a digital signal that our
image
processing program can use. In simple terms, if the pixel had 100
electrons
to read out (and remember that this value itself is subject to the
noise
mentioned in points 1 and 2 above), then the analog to digital unit
converter
might read this out as 96 electrons instead of 100 (for example).
This
extra layer of variability is referred to as read noise.
Photon
Noise is unavoidable and is
largely contributed by sky background (light pollution).
The longer
you expose, the more photon noise you will have, but the greater the
chance of
acquiring your desired signal. So think of photon noise as a
necessary
evil. Dark Noise is also unavoidable but can be minimized by
cooling of
the CCD chip. That brings us to Read Noise. As stated above, read noise
is a
fixed amount of noise that is caused by the ADU converter every time an
image
is downloaded from your camera into your computer. Every camera has a
certain
amount of read noise (some less than others- check the
specifications).
In contrast, photon noise is mainly due to sky background and is
dependent upon
your imaging site. At a given imaging site, sky background is
proportional to the subexposure duration. If your subexposure
time is too
short, the sky background noise will be minimal (and so will your
desired
signal), and your image will be dominated by read noise (your exposure
is
"read noise limited," which is not ideal). Conversely, if your
subexposure time is long enough, sky background noise is very large
compared to
your read noise, and you essentially drown out the effects of read
noise.
Your image is said to be "photon noise limited," which is good.
In other words, by exposing your subs long enough so that the sky
background
noise overwhelms the read noise, you effectively minimize the influence
of read
noise in your image. Once you reach a subexposure duration where
the read
noise contribution becomes less than 5-10% of the total noise in the
image,
there appears to be no major advantage to prolonging the duration of
the
subexposure further. If you are interested in learning more about
this,
please check out my subexposure
duration
page for additional details.
It follows that subexposure duration is largely
dependent on two
factors, sky
background noise and read out noise. We want to aim for a
subexposure
duration that will reduce the read noise contribution to about
5-10%. John
Smith has done some nice work in this area, and his subexposure
calculator is very
instructive to use. I have also analyzed subexposure
duration and
provide an alternative
subexposure
calculator for this purpose. So what does this have to
do with
the choice between NABG and ABG cameras? Here are some "bottom
line" observations that are useful to consider:
1.
At a dark site, sky background noise
is very low. In order to generate enough sky noise in an individual
subexposure
to drown out the read noise, the subexposure duration will therefore
have to be
quite long. Subexposure times in the range of 30-60 minutes
(unbinned)
may be necessary at a dark site in order to get the read out noise
contribution
down to 5-10% for most CCD cameras. At f ratios in the f4-f8
range, most
NABG cameras will bloom like crazy during a 30-60 minute exposure,
making this
type of camera impractical for a dark site. So a compromise has
to be
made for dark sites- because of the need for long subexposures (it's a
need,
not really a choice), an ABG camera is ideal in order to avoid
blooming.
The lower QE of ABG cameras is accepted as a necessary evil. Most
imagers would
want a higher QE, but they accept the lower QE of an ABG camera in
order to
avoid the hassle of blooming. The fact that they are
imaging at a
dark site makes up for the lower QE in most cases, since there is
greater
chance for detecting faint signals that are not drowned out by sky
noise.
2. For the rest of us who image in relatively light polluted sites, sky
noise
is much higher. Therefore, subexposure times in the range of only 5-15
minutes
are usually sufficient to drown out the read noise contribution to less
than
10% of total noise (it's really true- crunch some numbers using John
Smith's
calculator to convince yourselves). At my imaging site, where
I've
measured sky flux with the U32 on several different occasions, my
typical
subexposure times are in the range of 5 minutes for luminance, 8
minutes for
RGB, and 10 minutes for Ha (6 nm bandpass), in order to achieve a read
out
noise contribution of around 10% or less. These exposure
durations were
not chosen by accident- they are chosen to achieve a low read out noise
contribution
at my imaging site. With relatively short subexposure times, the
amount
of blooming seen in a typical star field with a NABG camera is
generally easy
to manage with currently available software. And given the need
for
shorter subs based upon sky noise (it's a need, not really a choice),
one could
make the argument that it's better to have a chip with a high QE, such
as a
NABG camera, in order to maximize signal during a relatively short
subexposure. It's ironic that the presence of light pollution
makes a
more sensitive NABG camera a viable option, whereas those under dark
skies
often must use a camera that is intrinsically less sensitive (ABG).
Despite all
of these considerations, it is not necessary to use a NABG camera just
because
your subs will be relatively short. You could certainly choose an
ABG for
light polluted skies, realizing that you will need a longer cumulative
exposure
to compensate for the lower QE of such a camera. Still, an ABG
camera
will permit you to take photos of objects such as the Pleiades and M42
without
worrying about blooming (which will occur with a NABG, even at short
exposures,
for these types of bright targets).
Putting
it all together
1.
Pixel size / Image scale:
If your seeing is average (applies to most of us), consider cameras
with pixel
sizes that will yield an image scale in the range of 1.0-3.5
arcsec/pixel. If
you have a good mount that guides well, aim for the lower end of this
image
scale range in order to maximize resolution. Much below
1.0"/pixel
is usually wasted effort for most of us, since conditions are often
seeing-limited. A bit higher than 3.5"/pixel can be fine as well,
as
long as you don't mind a softer look to your images. These are
only
general guidelines- don't get hung up on any of this. You can
produce a
great astroimage even if you are not using the ideal Nyquist value for
image
scale, although staying within this general range (1.0-3.5
arcsec/pixel) is a
good idea for most of us. These rules don't apply for those with great
seeing,
where optimal image scales would be well under 1.0 arcsec/pixel.
2.
Dynamic range:
All things being equal, get a camera with a higher dynamic range (full
well
capacity divided by read noise).
3.
Dark signal:
Don't worry too much about this, given the quality of modern-day,
cooled chips.
If it's there, you will remove it with dark frames. If it's not, you
won't.
Certainly, if you have two cameras that are equivalent in all other
important
aspects (image scale, dynamic range, read noise, QE, etc.), then get
the one
that has the lower dark signal. However, after reading this
primer and
looking at camera specifications, you will see that it's not always
that
simple. A camera may have a higher dark signal, and yet have
features
such as better dynamic range and higher QE that make it a more
attractive
choice, despite the need to dark subtract.
4.
NABG versus ABG:
If you image at a reasonably light polluted site which requires short
subexposures, you have a choice of either NABG or ABG cameras.
With NABG
cameras, the QE will be higher, and the blooming will be manageable for
most
star fields over relatively short subexposure durations. For me,
a NAGB
camera was a logical choice for my second camera, especially since I
was
interested in a chip with high Ha sensitivity, and since my subexposure
times
would be short at my imaging site. I've been pleased with the Ha
sensitivity of
the KAF3200 chip (U32 camera), and it's related in large part to the
NABG
feature. If you are just starting out and plan to take lots of
photos of
bright star clusters like the Pleiades, then an ABG camera is perhaps
the
better choice, since you won't have to deal with blooms (my first
camera was
the SXV-H9, which is ABG). You can always take longer cumulative
exposures to compensate for the lower QE of an ABG camera, if you have
the time
and patience. If you are at a darker site where you must use longer
subexposures, an ABG camera is the way to go.
5.
Chip size: The CCD chip
dimensions and your scope's focal length will dictate the field of view
(FOV)
of your images. Ron Wodaski's calculator mentioned above will
provide the
field of view for a given camera/scope combination. The
calculation is
easy: FOV (in degrees) = 57 x CCD chip dimension (in mm) /
effective
focal length (in mm). However, a bigger chip is not always
better.
Remember that with a larger chip you will be more likely to have
problems with
1) field curvature if your scope does not provide a flat field over the
chip's
entire surface area, 2) vignetting, 3) camera sag/flexure due to
increased
camera weight, resulting in non-orthogonality of the chip to the
optical axis,
which introduces optical aberrations at the edge of the field (do not
underestimate the frustration that can occur due to this last
point). If
you have to crop out a significant amount of the image due to oblong
stars or
severe vignetting at the periphery of your field, you would have been
better
off with a smaller-sized chip in the first place (less money, less
frustration,
smaller files, etc.). Make sure that you talk with the telescope
vendor,
or e-mail astroimagers who are using specific types of telescopes, to
determine
whether a given scope will support the chip size of the camera that you
are
interested in.
6.
Monochrome
versus one-shot color:
I didn't discuss this above, but will mention it briefly now. One-shot
color
CCD cameras have lower sensitivity and resolution compared to
monochrome
cameras. The Bayer matrix present in one-shot color cameras is
responsible
for this problem, and there are many websites that discuss this issue
in great
detail. However, it's possible to take nice images of relatively
bright
objects with one-shot color CCD cameras, and you wouldn't need to
invest in a
separate filter wheel and costly filters. Note that
one-shot CCD
color cameras seem to be very susceptible to the effects of light
pollution,
and most require some type of LPS filter in the imaging train, which
will
further decrease sensitivity. Most CCD imagers use monochrome
cameras,
but the one-shot color CCD camera is a viable alternative, as long as
you
realize the potential downsides (decreased resolution being the most
important
problem in my view).
Steve