How to Choose a CCD Camera
There
are lots of things 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.
Pixel Size:
Image resolution is largely related to a characteristic of your system
known as "image
scale," expressed as arcseconds per pixel. For simplicity's sake,
we are ignoring the influence of your optics and tracking on image
resolution, although these factors are obviously important as
well. The image scale of
your system is dependent upon only two factors- your CCD camera's pixel
size, and your telescope's effective focal length. A low number
for image scale (like 0.5 arcsec/pixel) means high resolution, and a
high number for image scale (like 10 arcsec/pixel) means lower
resolution. This makes sense- if 10 arcsec worth of data are
represented by only one pixel, you have essentially crammed all of that
detail into one point! Calculation of image scale (in units of
arcsec/pixel) is easy: 206 x pixel size (in microns) / effective
focal
length (in mm). Ron Wodaski has a nice calculator available on his
website for this purpose. Stan Moore has an excellent
discussion of CCD pixel size on his website.
Regardless of your image scale, however, the best resolution that you
can achieve is still limited by your seeing conditions (as well as
optics and mount). For
instance, in my imaging location, seeing is typically around 3.5
arcseconds FWHM (full width at half maximum, which refers to the width
of the star profile measured at 1/2 the maximum pixel intensity of that
star). This means that the
best that I could resolve is only around 3.5"/pixel. So
what is the best image scale to aim for (in other words, what pixel
size should we want in our CCD camera, for a given focal length
telescope)? Surprisingly, it's not 3.5"/pixel at my site.
The Nyquist theorem suggests that in order to efficiently record this
information and convert it into digital format, our system should be
sampling the image more aggressively, by operating at an image scale of
about 1/3 times the seeing, or in this example 1/3 x 3.5", or
1.17"/pixel. (Actually, it should be 1/3.3 times the seeing, but
who's
counting). This means that my CCD camera/scope combination
should ideally have an image scale of 1.17"/pixel, in order to take
full advantage of my (suboptimal) seeing conditions and produce a
final resolution of approximately 3.5"/pixel.
This places a lower limit on the image scale that we should aim for
with our CCD camera/scope combination. Anything lower than this
is wasted effort for our seeing conditions, and in fact makes imaging
more difficult. In our example above, anything lower than about
1.17"/pixel will be associated with less sensitivity (smaller pixels)
and greater potential to reveal guiding errors. Note that for
better seeing conditions, the optimal image scale will be
different. So
if my seeing is 2" FWHM, I should choose my camera/scope combination to
produce an image scale of about 1/3 times 2", or 0.66
arcsec/pixel. For most of us who don't live in areas of great
seeing, an average value for seeing on a decent night will be in the
3-3.5" FWHM range. So for practical purposes, a lower limit for image scale
of about 1"/pixel is a perfectly reasonable value to keep in
mind. But what about an upper limit?
To answer this, remember that the Nyquist theorem tells you what the
ideal image scale should be, but it doesn't mean that a less ideal
image scale is necessarily bad. Take a look at the images
on my website and realize that the image scale for the Sky90 f4.5
photos is about 3.26"/pixel, and that the image scale for many of the
FS102 f6 photos is about 2.29"/pixel, whereas the ideal image scale
according to Nyquist would be around 1.17"/pixel (with seeing of 3.5"
FWHM at my site). In other words, essentially all of the images
on my website are referred to as "undersampled." Needless to say,
I don't lose any sleep over this. Although it's true that the
FS102 images at an image scale of
2.29"/pixel show more detail, the Sky90 images at
an image scale of
3.26"/pixel are pretty nice too. A good rule of thumb for most
imaging sites and objects is
this: an upper limit
image scale of about 3.5"/pixel is a safe bet for producing nice images
with reasonable detail. Higher than this (e.g., 5-10
arcsec/pixels)
will produce "softer" images lacking in finer detail (although this may
be perfectly acceptable for wider field views if the imager is only
interested in showing large scale structure). And as mentioned
above, going below an
image scale of 1-1.5
arcsec/pixels for most sites will not produce better images,
since your resolution will be seeing-limited. Thus, there is some
flexibility in the image scale, with a range of 1-3.5 "/pixel being
safe for most of our average seeing conditions. Obviously, the
rules are different if you are lucky enough to be imaging in New Mexico.
Dynamic Range:
In its simplest terms, 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 CM10 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 operate 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 Noise:
Every
CCD camera has dark noise 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 noise 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 noise. Most other chips
like the
Kodak series have enough dark noise 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 the Kodak KAF series.
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? The choice is largely dependent upon how long your
subexposure times
will be, and this is dependent upon sky noise and read out noise.
Let
me explain...
When taking a subexposure, we want to
maximize signal and minimize noise (i.e., maximize the signal to noise
ratio). Noise is related to 3 main effects: 1) "Photon
Noise" is due to the collected photons themselves (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" (mentioned above); 3) "Read Noise," which is generated
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. 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 CM10 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:
Here are some general rules to follow:
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 noise:
Don't worry too much about dark noise. 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 current.
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 current, 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
very pleased with the depth of my Ha signal with my new 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).
I hope that this information is helpful to you in your decision-making!
Steve
Home