Imaging the Moon


Although they are designed mainly for use in the home to enable communication between individuals over the Internet, webcams can be used to capture high resolution images of the Moon and planets. Priced at just a fraction of the cost of dedicated astronomical CCD cameras, webcams are extremely lightweight and versatile. Any commercial webcam hooked up to a computer and a telescope can be used to image the Moon and the brighter planets. Electrical signal noise greatly hampers the use of webcams to capture faint deep-sky objects like nebulae and galaxies, but the electronic circuitry within some makes of webcam have been modified by some amateurs to take longer exposures with reasonable success. The Moon is such a bright object that webcams can produce brilliant lunar images.

Although webcams may not have as sensitive a CCD chip as a more expensive astronomical CCD camera, their ability to record video clips made up of hundreds, or thousands of individual images gives them a distinct advantage over a single-shot astronomical CCD. By taking a video sequence, the effects of poor seeing can be overcome by selecting (either manually or automatically) the clearest images in the clip. These images can then be combined using stacking software to produce a highly detailed image – this may show as much detail as a visual view through the eyepiece using the same instrument.

It is possible to photograph the Moon afocally with webcams, but they are usually used at the telescope’s prime focus, minus the webcam’s original lens. A number of webcams have easily removable lenses, and commercially available adapters can be screwed in their place, permitting easy attachment to a telescope. Some webcams however require disassembly to remove the lens, and the adapter needs to be home made. CCDs are sensitive to infrared light, and the lens assembly may contain an infrared blocking filter – without the filter, a really clean focus through a refractor is not possible, since infrared is focused differently to visible light. IR blocking filters are however available to fit between the telescope and webcam, allowing only visible light wavelengths to pass through to a sharp focus.

Using basic equipment alone, focusing a webcam can prove to be time consuming. To achieve a rough focus, it is best to set up during the daytime and focus on a distant terrestrial object using the telescope and webcam, viewing the computer monitor and adjusting the focus manually. If your telescope is some distance from the monitor, this may require a number of trips to and from the telescope and computer! A laptop in the field, near the telescope, would save a great deal of time, both during this initial process and during lunar imaging itself. Once the terrestrial object has been focused, lock the focus or mark the focusing barrel with a chinagraph pencil.

During the imaging session, the Moon is been centred in the field of view using the telescope’s finderscope (which must be accurately aligned), and the Moon will appear on the computer screen, probably still requiring further focusing. It is best to focus on the lunar terminator, where features are most sharply defined. When the telescope’s focus is adjusted manually, care must be taken not to nudge the instrument too hard, as the Moon may disappear altogether out of the small field of view. Patient trial and error will eventually produce a reasonably sharp focus – once achieved, lock the focuser and mark the focusing tube’s position so that an approximately sharp focus can be found quickly during subsequent imaging sessions.

Achieving a good focus makes the difference between a good lunar image and a great one, and a fraction of a millimetre can make the difference between a good focus and a tack-sharp one. Focusing by hand is exceedingly time consuming, and a perfect focus is more likely to be found by chance than trial and error. Electric focusers enable the focus to be adjusted remotely from the telescope, and they are considered an essential accessory to the lunar imager rather than a luxury. Electric focusers save a lot of time, making a great difference to your enjoyment of imaging, but more importantly they offer infinitely more control over fine focusing. A webcam attached to a computer through a high speed USB port will deliver a rapid refresh rate of the image, enabling fine focusing in real time.

Video sequences of the Moon can be captured using the software supplied with the webcam. It is necessary to override most of the software’s automatic controls – contrast, gain and exposure controls require adjusting to deliver an acceptable image. Many imagers prefer to use black and white recording mode, which cuts down on signal noise, takes up less hard drive space and eliminates any false colour that may be produced electronically or optically.

Most webcams are able to record image sequences using frame rates of between 5 and 60 fps (frames per second). A ten second video clip made at 5 fps will be composed of 50 individual exposures and may take up around 35 Mb of computer memory. At 60 fps there will be 600 exposures, and the amount of space taken up on the hard drive will be proportionately greater. The sheer number of images provided by webcams is their greatest strength. A single-shot dedicated astronomical CCD camera costing perhaps ten times as much as a webcam only takes one image at a time. An image produced by an astronomical CCD may have far less signal noise and a higher number of pixels than one taken with a webcam, but in mediocre seeing conditions, the chances that the image was taken at the precise moment of very good seeing are small. Webcams can be used even in poor seeing conditions, as a number of clearly resolved frames will be available to use in an extended video sequence. Video sequences are usually captured as AVI (Audio Video Interleave) files.

Astronomical image editing software is used to analyse the video sequence, and there are a number of very good freeware imaging programs available. Some programs are able to work directly from the AVI, and much of the process can be set up to be automatic – the software itself selects which frames are the sharpest, and these are then automatically aligned, stacked and sharpened to produce the final image. If more control is required, it is possible to individually select which images out of the sequence ought to be used – since this may require up to a thousand images to be visually examined, one after another, this can be a laborious process, but it can produce sharper images than those derived automatically. Images can be further processed in image manipulation software to remove unwanted artefacts, to sharpen the image, enhance its tonal range and contrast and to bring out detail. Unsharp masking is one of the most widely used tools in astronomical imaging – almost magically, a blurred image can be brought into a sharper focus. Too much image processing and unsharp masking may produce spurious artefacts in the image’s texture and a progressive loss of tonal detail.