Edit (Sept. 2014): This tutorial was written in 2003 when the imaging of the planets with webcams emerged. At that time the Philips ToUCam Pro was the camera of choice for the amateur astronomer. Since 2003 quite some time has passed, technology has changed dramatically and people don't use ToUCams anymore. Nevertheless, I keep this tutorial online just as a reference because many of the topics are still important for successful planetary imaging.
Back in 2003 I started taking planetary images with a ToUCam Pro webcam on my Celestron 8 telescope. In this little essay I want to share my experiences on how to achieve the best possible results. This is a living document and will be updated in the future as my experiences grow. You are also invited to participate, please give me your feedback.
That's easy to answer. This camera has a
real CCD-Sensor (SONY
ICX098BQ), which allows a high resolution (640×480 Pixels
non interpolated), high color reproductively and sensitivity of
darkness. It is therefore ideally suitable for imaging of bright
objects such as planets, the moon or our sun. It is also
possible to use this camera for deep sky-imaging (integration
time > 1/25 sec.). In order to do that, you have to modify the
cam (which is rather complicated in my opinion) simply search
the web, there are many sites with instructions to modify your
cam. For planetary imaging this is not necessary.
The full resolution is 640×480 or 355×288 pixels. If you observe planets and need high magnification use one of this modes. 640×480 covers every single pixel of the chip whereas the 355×288 mode only covers the central part. I would recommend the 355×288, because you get less dataflow and this reduces the number of artifacts that occur, when the images of an AVI video are compressed during recording.
The 320×240 mode also covers the whole chip size, but 2×2 pixels are averaged to one pixel. This is called 2×2 binning. The result is a better signal to noise (S/N) ratio. The 240×176 and 176×144 modes cover central sections of the 320×240 image. The 160×120 mode is 3×3 binning.
For planetary imaging it is important to know, what part of the sky is captured by the CCD. To calculate the angular field of view of one pixel, use this equation:
α = Angular field of view per pixel [arc sec / px]
206 = Factor to convert radians into arc sec [arc sec]
(1 arc sec = 1/3600th of one degree and 1 radian = 180/π degrees, so one radian = 3600×180/π arc sec = 206265 arc sec; divided by 1000 to convert mm into µm = 206)
d(Pixel) = Edge length of one pixel [μm].
F = Focal length [mm]
Example: We want to observe Jupiter with ToUCam Pro (edge length of one pixel = 5.6 μm) and 2030 mm focal length (C8 telescope):
α = (206" × 5.6 µm) / 2030 mm
α = 0.57"
The number of effective pixels of a Philips ToUCam Pro web cam is 640×480 px, representing 0.10°×0.08° of the sky at this particular focal length.
We can also apply this formula to calculate the focal length required to achieve a favored angular field of view:
The theoretical angular resolution of a telescope is depending on the aperture of the telescope and the wavelength of light we are observing at:
α = Theoretical angular resolution in [arc sec]
206265 = Factor to convert radians into arc sec [arc sec]
(1 arc sec = 1/3600th of one degree and 1 radian = 180/π degrees, so one radian = 3600×180/π arc sec = 206265 arc sec)
λ = The wavelength of light [mm] (1 nm = 10-6 mm)
D = The diameter of the lens or mirror of your telescope [mm]
For a typical calculation we choose a wavelength of 550 nm. This is a good choice for visual observations because our eyes are most sensitive at this wavelength (green light).
If we choose 550 nm (0.00055 mm), this brings us to the famous formula:
In order to achieve the best possible result for high resolution CCD
imaging, matching focal ratio in connection to the pixel size is
of essential importance, this is called
For planetary imaging it is recommended to sample the image at a rate close
to the ideal Nyquist criterion. The Nyquist criterion describes
that the sampling frequency must be greater than twice the band
width of the input signal in order to be able to reconstruct the
original signal perfectly. For our aim this means that the
wavelength of a signal should be covered by two pixels.
To calculate the desired focal length for a given wavelength and equipment we can apply the following formula:
F = Desired focal length [mm]
d (Pixel) = Size of one pixel, incl. the gap between two pixels [μm].
D = Aperture of the telescope [mm]
λ = Wavelength [nm] (I choose 550 nm in accordance to Rayleigh)
F = (2000 × 5.6 × 203) / 550
F = 4133.8 mm (Focal ratio of f/20.36)
If we strong oversample an image, we need longer
integration times without a gain of detail, we
waste integration time. If we
undersample an image, we waste resolution which also is of an disadvantage. So for
planetary imaging we want to sample close to the Nyquist criteria or slightly oversample the raw images.
To better understand the term
oversampled, please review the following explanation:
The white point in the first image represents the smallest visible detail (diffraction disc) that your telescope can resolve. If the angular resolution of the CCD sensor is better than the theoretical angular resolution of the telescope, the smallest visible detail is only represented by one pixel (Image 2). In this case we experience a maximum utilization of the sensitivity of the CCD but we suffer a loss of information, the image is called undersampled:
But on the other hand we don't want too many pixels representing the smallest visible detail. Images sampled with more than 2 pixels across the diffraction disc (Image 3) are called oversampled. In this case the angular resolution of the telescope is far better than the angular resolution of the CCD and the sensitivity of the CCD is not optimal utilized (longer integration times needed). As an example, the star in image 4 is heavily over-sampled:
In image 5 the angular resolution of the telescope matches the angular resolution of the CCD, the diffraction disc is two pixels in diameter. As a result (Image 6), we have a good or critical sampling:
In chapter 6. we have calculated a focal length required to fulfill the Nyquist criterion for a C8 telescope. Now, one could
say that we choose a 2.5x barlow lens to achieve this. Well, that
could be true, but before you go and buy
you must know that some barlow-lenses have varying
extension-factors. On some barlows, the extension factor is
depending on the distance between the lens top surface to
the surface of the CCD. With increasing distance, the
extension factor is also increasing and maybe your 2x barlow
becomes a 4x barlow without your knowledge.
As an example I found a chart on the TeleVue web site. On this chart we can see, that the 2x and 2.5x Powermates don't change their extension factor significantly, but the 5x Powermate can easily turn into a 7.5x barlow.
So the true extension factor of a barlow lens must be calculated by yourself. In order to calculate the true focal length, it is necessary to image an object which apparent diameter is known from the ephemerides (e.g. Saturn's planetary disc at the equator) and then carefully measure the diameter of the planets disc on the CCD sensor. If you know, how many pixels the object is in diameter, then you can re-calculate the true focal length of your imaging system.
We can re-calculate the effective focal length with the following procedure:
I imaged Saturn on March 13, 2009 with a Celestron C11 (focal
length = 2800 mm) and a TeleVue 2.5x barlow lens, using a
The Imaging Source. The amplification factor of
the TeleVue barlow is 2.5x, but if you have checked the link
above (TeleVue web site), you know that the amplification factor
is slightly decreasing with growing distance between the surface
of the CCD sensor and the top lens surface of the barlow lens.
So what is the
effective focal length of my setup?
At first we must check the ephemerides for the apparent angular size of Saturn's disc (at the equator) for this particular date. Result: On March 13, 2009 Saturn's disc was 19.7 arc sec in diameter.
Now I want to measure the diameter of Saturn's disc on my CCD image, see the following picture:
With Photoshop, I measured the coordinates (in pixels) of the mark 1 and mark 2. Photoshop measured for mark 1: x1 = 81, y1 = 112 and for mark 2: x2 = 176 and y2 = 61. Now we must calculate the length of a and b using the following formula:
When I apply these formulas I get the result for length a = 95 px and length b = 51 px.
In order to calculate the length of
c, we can see this as a
right angled triangle and calculate the length of
When I apply this formula, I get the result that the length of c = 109 px.
Now we know that the planets disc has an apparent size of 19.7 arc sec and this is represented by 109 px on the CCD sensor.
Now we can use the following equation:
F = Effective focal length [mm]
206.265 = factor to convert radians into arc sec, divided by 1000 [arc sec]
d(Pixel) = Edge length of one pixel [μm]
c = Size of the object on the sensor [px]
D = Diameter of the object [arc sec.]
F = (206.265 arc sec × 5.6 µm × 109 px) / 19.7 arc sec
F = 6391 mm
So the true extension factor of the 2.5x TeleVue Powermate in fact was 2.28x for my setup, resulting in a focal ratio of f/22.8!
This is very important, when you are observing planets with fast
rotation period such as Jupiter, Saturn or Mars. If you are
recording a sequence, you have to consider that the planet is
still in rotation while you are recording and there is a risk,
that fine details can get blurred after stacking if the duration
of the recording was exceeding a critical time span. In this
chapter we want to calculate the maximum permitted time span of
a video record without noticeable motion of the planet.
When we observe a planet through the eyepiece, we see a round disc, but we know that in reality it is a globe with a linear rotation. When we observe details on the planets surface or in the atmosphere, we see that the motion of these details appears to us relatively slowly at the limb and it is of the fastest motion when it passes the central meridian. In particular, a surface-detail is seen in its fastest motion when it is passing the area, where the equator and the central meridian interfere.
The following calculations are considering this
scenario for a detail that is exactly in the area where the
equator and central meridian interfere.
We can express the linear rotation period of a planet with the formula for the angular speed:
ω = Angular speed
φ = Angle of 360° for one complete revolution
T = Time to complete a rotation of 360°
For further calculations, we must convert the angle φ = 360° into radians (RAD), that's easy because one revolution (360°) of the unit circle = 2π radians.
So the formula for the angular speed of a planets rotation period looks like this:
Now we have evaluated a formula for the angular speed. With this formula we can express how long it takes for the planet to rotate one complete revolution. For example we know that the planet Jupiter's rotation period is 9h 50' 30" (System I), meaning the planet will turn 360° or 2π (RAD) in this time period.
For a linear rotation period, the angular rotation is constant. For our aim this doesn't help us much because a planet always appears to us at a different angular size and so the apparent velocity of a surface detail always appears to us differently (if a planet is close to us, we see more detail and so it seems to us to rotate faster). What we need to know is the tangential velocity.
Imagine you are right above the north pole of a planet and you see an object on the equator which is constantly moving with the rotation of the planet. The tangential velocity describes the path that has been covered by this object in a certain time frame. This is depending on the radius that this object has to the center of the circle. The bigger the radius, the longer is the path that has been covered (whereas the rotation period is constant).
In order to calculate the tangential velocity of
an object which is passing the
critical area (where the
equator and the central meridian interfere), we must add the radius
to that formula.
The formula for the tangential velocity is:
R = Radius of the apparent diameter of the planet [arc sec]
Now we must take into account the angular resolution of our our equipment. The angular resolution per pixel can be calculated with this formula:
If we want to know, how long it takes for the smallest visible detail to move about the value of one pixel on the CCD sensor (otherwise we will not detect a motion with our equipment), we just have to divide α by v:
or in full detail:
t max = Maximum permitted time span of a video recording [s].
d(Pixel) = Edge length of one pixel [mm]
T = Rotation period of the planet [s]
F = Focal length [mm]
R = Radius of the apparent diameter of the planet [arc sec]
206265 = Factor to convert radians into arc sec [arc sec]
(1 arc sec = 1/3600th of one degree and 1 radian = 180/π degrees, so one radian = 3600*180/π arc sec = 206265 arc sec)
Following there is an example on how to calculate this effect on Jupiter:
t max = (206265 arc sec × 0.0056 mm × 35430 s) / (2π × 6391 mm × 22.55 arc sec)
t max = 45.2 s
As I mentioned earlier, this is a
worst case calculation. So I
think that in real live you can exceed this calculated
max. duration time
by the factor 2 and still have a good final image.
In order to achieve a better contrast, most amateur astronomers
add to the Red, Green, Blue data a fourth channel, the
In the deep-sky business, this is mostly a high resolution 1×1
bin, long exposure clear filter image (sum or mean of multiple
images). This adds to the picture the sharpness
and contrast, whereas the R, G, B images deliver the color information.
When we want to image the planets, we use this technique to add
a high resolution luminance image to the R, G, B data but the
image is a R, or IR image. Since most planets show interesting albedo features in the
R or IR wavelength which we want to enhance.
But which filter is the one to choose? It depends on the conditions:
IR pass filter:
Bpixels are not sensitive for IR, only the
Rpixels are sensitive for this wavelength (the IR-pass filter turns off the G and B pixels). So the picture is getting
darkand needs longer integration times. For the use of an IR-pass filter, the monochrome cameras are more appropriate.
If you have good seeing, choose the red color filter instead of the IR, if the seeing is bad and you are using a monochrome camera, you will gain better results with the IR pass filter.
However, if our interest lies in details of the atmosphere, maybe blue or green filters are appropriate. On Mars, a blue filter greatly intensifies ice-clouds or the polar caps and a green filter can intensify dust storms in the atmosphere. Some amateur astronomers also apply a green filter as a luminance layer in order to reveal the smallest details on the Moon.
It is very important that the temperature of your telescope is equal to the surrounding temperature. So let your scope at least 30 minutes to cool down. Orient the mounting to the true north as close as possible, otherwise the object is permanently drifting out of the field of view. The next step is to check collimation, this is a very important step - you won't gain high resolution, if the telescope is misaligned.
Lets assume we want to observe Jupiter. Remove the star diagonal, pop in an eyepiece of high magnification (10–15 mm), center and focus the planet carefully. Now replace the eyepiece with one of lower magnification (I use 25 mm) and assemble the webcam behind this configuration.
Now start the driver of the webcam and disable the automatic functions. Change the settings to maximal brightness, integration time 1/25 sec. and 10 – 15 frames per second. Change the resolution to 320×240. You see nothing on the screen? Simply repeat the procedure if it’s still not working, remove the eyepiece and repeat the procedure again. If you see a bright blob on the screen, congratulations! Now try to center the blob carefully with your motor drive. Make yourself familiar with the controls for the motor drive, because one slight push and the planet could drift off the field of view dramatically.
When turning the focus knob of a Schmidt-Cassegrain-Telescope the object is shifting. So be patient when turning the focus knob and always re-center the object in short intervals. It is recommended that it is the best to turn the focus knob counter-clockwise for focusing (then the mirror is pushed and is more stable in this position). When you focused too far, simply go back by turning the focus knob clockwise and then again counter-clockwise to focus.
When the planet is in the center and you think it is relatively sharp, change the settings in the control box. For the ToUCam the best adjustment is:
- Brightness: 50 %
- Gamma: 0 (or near to 0)
- Saturation: 80-90 %
- Gain: 50 %
Change the settings for the integration time so that details of the planet become visible. (With a C8 and a 25 mm EP 1/25 and 1/33 sec. works fine). Only use integration time and gain for changes. Now again try to focus the image as sharp as possible. (This procedure sometimes takes me 15 to 20 minutes).
Now if you have a sharp image showing some details, we can start the recording. Change the image size to 355×288 pixels (check focus again!). Select 5 or 10 fps - and start! It is recommended to collect as much data as possible, meaning at least 500 frames per sequence, this means a lot of data.
Take several sequences with different settings, always check focus before starting the recording. Seeing sometimes changes within minutes and so the sharpness of the image often changes very quickly.
This is all half of the work. Back indoors, the sequences must be processed. The main steps consist in dividing up the sequence into individual frames. Then sorting the best images, registering and stacking them to one final image. If you want to save the file, choose a 16-bit TIF format, don't save as 8-bit JPG or BMP, otherwise you will loose the dynamics in the image.
I use Registax for this operations, all these functions are carried out automatically by this software. Nowadays I heard that Avistack should produce better results on the Moon, but have not yet found the time to proof this.
That's all, thanks for reading.