Home > Webcam Imaging
-Frank Brandl-
A couple of months ago 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! Please give me your feedback on this (
).Why Philips TouCam Pro?
That's easy to answer. This camera has a "real" CCD-Sensor (SONY ICX098BQ), which allows a high resolution (320x240 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.
Equipment
1) You need a PC with USB, the parallel port is not supported. Minimum system requirements are 32 MB RAM, Win 98, ME or 2000. With XP you need an additional program to run the included software (could be downloaded on the Philips homepage), the driver is integrated in the OS. To achieve a fluent dataflow I would recommend a minimum of 128 MB RAM and a processor with a minimum of 600 MHz. (You are lucky if you have a notebook).
2) A Telescope with motor drive in right ascension and (optional) in declination. You can try without motor, but you’ll loose patience soon because the object passes through the viewing field really fast.
3) An adapter to connect your cam to the scope. I use two adapters: The first is 1 ¼" in diameter and fits on the eyepiece holder, so I can use it with a barlow lens. The second suits to my T-Adapter, so I can connect it to the tele-extender and use it in eyepiece projection. It is important that the CCD sensor of the cam is in the center of the optical axis of the system, when it is mounted to the telescope. If it is shifted (even slightly), you'll later get problems to find the object and center it on the sensor. I would recommend to purchase a drilled adapter, search the web there are sites that offer good drilled adapters.
4) Adequate disc space! (I would recommend 2 Gigabyte minimum!).
The Correct Focal Length
In order to achieve the best possible result, you have to consider the correct focal length in combination with the CCD array. As a basic rule, the theoretical resolution of your telescope (smallest visible details) should be detected of two or more pixels.
The theoretical resolution is easily calculated with the equation: Alpha = 120 / D
Where "Alpha" is the theoretical resolution in arc sec. and "D" is the diameter of the objective (lens or mirror of your scope).
For example:
My 8" SCT is 203mm in diameter, the theoretical resolution is: 120/203 = 0.59 arc sec. In this configuration, one pixel of the sensor should at least cover 0.30 arc seconds. This is achieved with the correct focal length. Use this equation:
F = 206.265 * (d pixel / alpha* pixel)
F = Minimum required focal length [mm];
d pixel = size of one pixel [micron] (TouCam = 5.6 microns square);
alpha* pixel = what one pixel must cover at least [arc sec.].
In this example: F = 206.265 * (5.6/0.3) = 3850.28 mm
Illustration:
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| Image 1 | Image 2 |
The white point in the first image represents the maximum visible detail of the telescope. If the resolution of the CCD-Array is better then the theoretical resolution of the telescope, the smallest visible detail is only represented by one pixel (image 2), the image is "under sampled".
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| Image 3 | Image 4 |
In image 3 the resolution of the telescope is better then the res. of the CCD. As the result (Image 4) you see more pixels "involved" in the smallest visible detail!
The angular field of view
For planetary imaging is important to know, what part of the sky is captured by the CCD. If we work with too long focal ratio, the planet could be too large for the CCD. To compute the angular size of one pixel, use this equation:
Beta = (L/F) * (180/Pi)
Beta = part of the sky that is represented by one pixel [°]
L = size of one pixel [mm]
F = Focal length [mm]
Example: We want to observe Jupiter with TouCam Pro and 3.850 mm focal length:
Beta = (0.0056/3850) * (180/Pi) = 0.0000833° = 0.30" Not surprisingly :-)
The number of effective pixels (TouCam Pro) is 640 x 480 Pixels, representing 0.053° x 0.04° of the sky at this particular focal length. Jupiter is max. 46" in diameter so no problem, it fits completely on the CCD. Now enlarge the focal length and see, how the field of view is shrinking! Remark: All these equations above are for 640 x 480 mode. Before computing please refer to the next chapter.
The Resolution
The full resolution is 640 x 480 or 355x288 pixels. If you observe planets and need high magnification use one of this modes. 640 x 480 covers every single pixel of the chip whereas the 355 x 288 mode only covers the central part. I would recommend the 355 x 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 x 240 mode also covers the whole chip size, but 2 x 2 pixels are averaged to one pixel. This is called 2 x 2 binning. The result is a better signal to noise (S/N) ratio. The 240 x 176 and 176 x 144 modes cover central sections of the 320 x 240 image. The 160 x 120 mode is 3 x 3 binning.
Please keep in mind: When using binning-modes, the magnification is changing!
The Planets Rotation
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 rotating and maybe the surface details are getting blurred after stacking due to the rotation. Following there is an example on Jupiter how to calculate this effect:
Given information:
1) Date: 16. Feb. 2003;
2) Diameter of Jupiter at this time is 45.1''
3) Rotation period of System I (area between NEB and SEB): 9h 50' 30'' or 9.84167h
4) Sequence: 160s or 0.0444h
5) Telescopes resolution: 0,59''.
A surface detail will pass the whole planet disc of 45.1'' in 9.84167 / 2 = 4.920835 h.
And which angular size will it pass in 0.444 h?
Use this equation:
X = (Planet size ["] * Length of sequence [h]) / (Rotation Period System I / 2)
X = (45.1'' * 0.0444 h) / (9.84167 / 2) = 0.407"
The result is below my telescopes resolution, so its o.k., surface details are not getting blurred.
Now it's time to get started
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. You won't get high resolution, if the optic is misaligned. (Check out Thierry Legaults homepage for more information).
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 x 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 = 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, but do not adjust gain too high. 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 x 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.
Some Remarks in Image Processing
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, centering and adding them to one final image.
I use the excellent IRIS-Program (http://www.astrosurf.com/buil/us/iris/iris.htm) for this operations, all these functions are carried out automatically by this software.
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| Single raw image. | Processed image:
200 frames stacked, unsharp mask |
© 2003 Frank Brandl - All rights reserved.