Capturing Images with a Webcam
Taking pictures with a webcam can be one of the easiest ways to get into astronomical imaging, and it can yield incredible results. Since webcams are used to image the moon and planets, factors such as light pollution and tracking accuracy, which normally affect deep-sky imaging, become unimportant. This makes webcam imaging a possibility for almost any astronomer with almost any telescope. Be sure to see theWebcam page of the Equipment Basics section for more details on how webcams work, and see the Capturing Images with a Webcam andRegistax pages of the Software Instructions section for step-by-step tutorials on taking and processing images with a webcam.
Image scale is an important factor in webcam imaging. Since you will taking pictures of small targets (Mars, Jupiter, Saturn, etc.) you will need to magnify these objects in order to capture sufficient detail. For imaging, magnification is a function of focal length. Very few telescopes have enough inherent focal length to capture images of the planets at a suitable size. Focal lengths of 4000-6000mm are often used for planetary imaging, and even large-aperture Schmidt-Cassegrain telescopes (SCTs) rarely have focal lengths this long.
The easiest way to increase focal length is with a Barlow lens. If an SCT is being used, a 2x Barlow is likely enough to achieve the desired focal length. For example, an 11" f/10 SCT has a 2800mm focal length. When used with a 2x Barlow, the focal length becomes 5600mm, which is appropriate for planetary imaging.
With short-focal-length telescopes, such as smaller refractors and fast Newtonians, an amplification factor greater than 2 will be necessary. An 8" f/5 Newtonian, for example, has a focal length of 1000mm. A 2x Barlow only brings it to 2000mm, still very short for imaging planets. A 5x Barlow, such as TeleVue's Powermate, would bring the scope to a focal length of 5000mm, much better for small targets. Barlows or Barlow-like lenses are commonly available in a variety of amplification factors from 1.5x to 5x.
Focal Length for Webcam Imaging
Focal length determines image scale, and image scale is critical for planetary imaging (being that the planets appear so small). For CCD imaging, it is often recommended that the pixel resolution should be 0.25 arcseconds per pixel. For the popular Philips ToUCam Pro webcam, this requires a focal length of 4600mm. However, longer or shorter focal lengths can be used with excellent results depending primarily on the atmospheric conditions.
Also recommended is a fine focuser for more critical focus adjustments. See the section below for more details.
Besides a Barlow lens, little else is needed beyond the webcam itself and the telescope. A tracking mount is necessary, but it need not be an equatorial mount. This means webcam imaging is possible with popular alt-azimuth mounted SCTs without the need for an equatorial wedge(which would be necessary for deep-sky imaging). In fact, it is possible to take webcam images with a non-tracking scope, such as a Dobsonian reflector, but the novelty of it wears off pretty quickly. Re-centering the image every few seconds can become tedious, so a tracking mount is highly recommended if not absolutely necessary.
Focusing & Framing
Webcams work by capturing hundreds (or thousands) of images in rapid succession. The advantage of this method is that these images can then be sorted by software such as Registax which rejects the blurry images (distorted by the effects of Earth's atmosphere), keeps the sharp ones, and stacks the good images together to reduce noise and allow enhancements to be made. Capturing so many images so fast is impossible with a digital camera or CCD camera, making webcams the ideal solution to the challenges of planetary images.
Focusing with a webcam is a straightforward procedure, especially compared to the techniques needed for deep-sky CCD imaging. Since webcams display a real time image, focusing is achieved in the same manner as for visual observing. Focus is critical for planetary imaging, so some care is needed, but the procedure goes quickly.
Tips for Focusing a Webcam
Go slowly. Take your time in looking for the best focus, and wait for a few seconds at each focus position to allow a moment of sharp seeing so you don't mistake poor seeing for bad focus.
Look for a distinct feature. Keep an eye out for a fine feature that may only appear when the telescope is at its best focus. For example, look for the Cassini division in Saturn's rings, which in a raw webcam video may only be visible intermittently and only at best focus. Small lunar craters or peaks are suitable features to use when imaging the moon. Jupiter's moons are easy focusing targets for that planet.
If possible, use a fine focuser. The standard focuser on most telescopes is fairly coarse, making it hard to produce very small adjustments to focus. Fine focusers such as Starlight Instruments various lines of Feathertouch Focusers can be extremely helpful. Motorized focusers can also be beneficial.
Framing the planet involves not only the obvious idea of putting the target into the image as desired, but also a less obvious consideration. Since tracking and polar alignment are less critical for planetary imaging than for deep-sky imaging, highly accurate alignments are not normally done to the telescope mount beforehand. Nor are they necessary. However, less-than-perfect polar alignment does have a noticeable effect which has an easy work-around. A telescope that is not perfectly polar aligned will result in image drift. (See the section on Polar Alignment for details.) This means that over a couple minutes, the target will drift away from the center of the image (specifically, it will drift north or south).
When framing the target, watch the live video image for a minute to see if the target drifts and, if so, which way it moves. Then, offset the target as necessary to keep it in the field of view for the duration of the imaging sequence (usually 60-90 seconds). This doesn't seem like much time for drift to occur, but at the magnifications employed for planetary imaging, the field of view is tiny and drift can appear very quickly.
Another aspect of framing involves the rotation of the camera. If you prefer Saturn's rings to appear horizontal, now is the best time to rotate the camera to achieve that orientation. Rotating later using software is possible, but results in a slight loss of detail. Framing certain lunar features and moons of Jupiter would also be done in this way.
Control programs differ somewhat from camera to camera, but the basic functions are the same. The basic routine involves setting a directory to save the image file, setting an appropriate frame rate, then simply taking a short video clip of suitable duration.
Setting the Exposure
To set the proper exposure you need to disable the software's automatic exposure settings. Using the capture software's manual exposure feature, select an appropriate frame rate and gain setting. The key is not overexposing the target. Look for the highlights, the brightest parts of the frame, and make sure they are not too bright and washed out. If so, detail will be lost in that area. Keeping the frame rate closer to maximum value (shortest exposure) is preferable as it will further reduce the effects of atmospheric turbulence.
Capturing a Video Clip
Once the frame rate is set and a file name and directory have been assigned, begin taking the video clip. A running total of the number of frames will be displayed in most software. Once the desired number of frames has been reached (say, 500 or 1000), stop the video capture. The video file is saved to the preset directory and can now be processed using Registax or similar software.