In early 2001, after having gathered experience in taking webcam images of the planets and the Moon, I thought of imaging the International Space Station ISS with the internet-camera. It may sound absurd, as the station has a very high angular motion relative to the stars, on an ever-changing orbit to add to the difficulties. With the computer tracking available in telescopes like the Meade LX200, it is possible to track satellites quite accurately, but the motor of my "New Polaris" equatorial mount was not even able to follow the motion of the stars correctly!
Suddenly I had an idea. When imaging the planets, I had aligned my finderscope so precisely with the main telescope, that an object centered on the crosshairs was also right in the middle of the small 320x240 pixel chip. Then it should also be possible to hold the ISS centered in the finder and capture it with the webcam. The only problem, of course, is the motion of the station. Using some basic photographic knowledge, I realized that a very short exposure time would "freeze" the motion of a fast object. The faster this object is moving, the shorter the exposure must be in order to still get a sharp image. To allow for the shorter exposure, the aperture on a camera lens must be opened (the focal ratio increased), i.e. more light must hit the chip/film during a certain unit of time.
Telescopes have a focal ratio that cannot be changed as in a camera lens, but not all is lost, as the exposure of the webcam can be influenced with more than just the exposure time. In addition, brightness and contrast can be changed, so that a good exposure can be made of an object with changing brightness while still using the same exposure time. Luckily, the ISS is very bright (this method of imaging does not work with fainter satellites!). As it is an extended object, the required exposure time depends on the sensitivity of the CCD chip and the focal ratio of the telescope being used. Unfortunately, there is no formula with which it is possible to calculate the correct settings for different webcams, so one must try different exposure values. The best way to begin is by choosing a relatively short exposure time.
The longer the focal length of the telescope, the more the station will trail during the exposure due to its proper motion, so an even shorter focal length must be choosen. (In classical photography, there is a rule saying the maximum exposure time that can be used when shooting a hand-held camera is the reciprocal value of the focal length, meaning 1/1000 sec. for a f=1000mm lens. However, I believe that with steady hands, much longer exposure times can be used to get sharp images, especially as the scope is still attached to a mount holding it somewhat steady.) Brightness and contrast values (and any other values that can be changed with the particular webcam) should be chosen in order to get a picture of maximum brightness despite a short exposure. In this case, it is almost always possible to see the station, and to reduce the exposure if necessary. If the image is too dark from the start, there is no way to know if the station was simply underexposed, or if it was never on the chip.
If you have chosen to try imaging the station, your best source for pass times is Heavens Above. When you have a favorable pass (at least 50 degrees above the horizon at culmination), you can start planning the observation. Set up at least an hour beforehand, especially if your setup takes longer (you don't have a permanently mounted scope or the mount consists of many parts). Manual tracking is greatly simplified if the polar axis of the mount is aimed at the apparent pole of the station's pass. For this, imagine the path of the station as the equator of a sphere and "polar align" the mount, provided you can do this, not on Polaris, but on the pole of this sphere.
Now it's time to start setting up the webcam. It is possible to take a desktop PC onto the balcony or the garden, but a laptop saves a lot of hassles. I only have an old laptop without a USB port, so I can only use it with my QuickCam VC. When I wish to use the new ToUcam Pro (needs a USB port), I turn the monitor of my desktop PC so that I can see it through the window to the balcony. After the webcam has been attached and the computer booted, put the camera into the focuser of the telescope and look for a star of about 2m brightness to focus on. When it is centered, the mount's motor (if present) should be turned on to facilitate focusing. The brightness should be reduced to the point where the pixels at the star's position are not saturated, otherwise focusing will not be precise.
A mask with three holes helps with finding the exact focal point. Placed in front of the telescope, it creates three separate images of the star when it is not focused. The closer you get to perfect focus, the closer the three stars move together, until they finally become one single star. Now, it is time to adjust the finderscope. Keeping the star exactly in the center of the field of view, the alignment of the finder should be changed so that the star is also exactly centered on the crosshairs. If it is already (or still) quite dark, you may notice that the crosshairs are extremely difficult to see. Either you build yourself a permanent finder illumination (difficult) or you quickly attach a red flashlight to the telescope tube so that it shines into the finder at an oblique angle (might not work). For best results, make sure the crosshairs are just slightly illuminated and that the entire field of the finder is not bathed in red (test with a star!).
At this point, the preparations are almost complete. Only the exposure settings remain to be adjusted to the appropriate values, so that the station is neither too bright nor too dark. If you are chasing the station for the first time, I recommend using the method described above to get the brightest image possible. Another option is to take a 2m star and adjust the brightness to the level at which the star is just at the threshold of visibility. Remember to choose a very short exposure time, so that the station appears as sharp as possible. (The star you used for focusing and adjusting the finder is perfect for adjusting the webcam.)
You're ready! If you started your preparations too late, you probably noticed that the station has already passed. Because a lot can go wrong (computer crash!), I always take more time than I need to prepare for a pass. If you still have to wait for the station, just look up at the stars - there's enough beauty to see there even if you had to wait for hours! With current orbital elements (or the newest predictions from Heavens Above) and an accurate clock, you are prepared for the moment the station is first visible. Start recording the video about a minute before this time. If you don't know how to do that, check the webcam's operating manual - every camera is different. On one very good pass, I had tracked nearly perfectly, and I was very excited to see the results when I realized that I had never started the exposure. Therefore, double-check that you have done so.
It makes no sense tracking the station until is has reached about 40-50 degrees altitude, as it is nearly impossible to see any details until this height if you are using a small telescope. In addition, the station is by far not as bright as it gets when it reaches the zenith, being 2000-3000km away when it is at the horizon, and only 400km when it culminates at the zenith. A pass through the zenith takes about 8 minutes from horizon to horizon, and hand-tracking that long might as well be an olympic discipline. Moving the telescope several degrees a second at a very high accuracy, mostly standing in positions reminiscent of yoga, burns quite a few calories, which you will notice afterwards.
As soon as the station has reached 50 degrees altitude, it is almost always possible to resolve details. There is no exact "tracking technique", as everyone has a different telescope and mount, and should find the best method for himself.
When the space station dives towards the horizon again, you can stop the video and watch as it continues towards the horizon. Now it's time for others to observe the station high in the sky. The recorded video should show the station a few times. On your first try, it may be visible on only one or two frames of several thousand, but it's an amazing feeling, seeing your own images of this spaceship. The further processing of the images is also a matter of taste. I first use a program like AVI2BMP to make a bitmap image of every frame of the AVI video. Then, using AstroStack I stack the bitmaps showing the station from the same angle, and do an unsharp mask with a graphics program.
If you have lots of money or someone willing to give it to you, you can go completely overboard with satellite tracking, but at the same time create amazing images. The first step should be computer tracking, which you can control with software like SatTracker (developed for the Meade LX-200 and compatible tracking systems). The more precise tracking allows increasing the focal length for more resolution using a barlow lens. As the station is also visible more often, stacking leads to much better final images. As soon as the tracking works well, you can buy good barlow lenses and better cameras (bigger chip, more sensitive, etc.). And finally, you know that the resolution of a telescope is directly proportional to its aperture. And big scopes require big mounts. You see, the sky's (not) the limit....
[1] Heavens Above: http://www.heavens-above.com
[2] SatCal Download: http://www.3rdplanet.de/freeware.htm
[3] Webcam Images of ISS: http://www.analemma.de/english/ccdsatel.html
