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Imaging

Imaging the Moon

Great lunar results can be obtained using digital cameras. If you haven't tried to photograph the Moon with your digicam or mobile phone held up to the telescope eyepiece, give it a go -- you may be pleasantly surprised at the results.

Digital cameras capture light using the CCD (charge coupled device) a small flat chip – about the diameter of a match head in most commercial digicams - made up of an array of tiny light sensitive elements called pixels. CCDs in very low-end digicams may have an array of 640 x 480 pixels, while a more expensive digicam CCD may boast 12 megapixels.

Light hitting each pixel is converted to an electrical signal, and the intensity of this signal directly corresponds to the brightness of the light that struck it. This information can be stored digitally in the camera’s own memory or transferred to a PC, where it can be processed into an image. Digital images are infinitely easier to enhance and manipulate than a conventional photograph in a photo lab darkroom.

Digicams, digital camcorders, webcams and dedicated astronomical CCD cameras have enabled amateur astronomers with quite modest equipment the opportunity to obtain wonderfully detailed images of the Moon. Indeed, just about anyone can take an acceptable lunar snapshot by pointing a digital camera through a small telescope. It’s one thing to capture an image of the whole Moon showing features along its terminator, but it requires considerable skill and expertise – both in the field, and later at the computer - to produce a high resolution, close-up image of the Moon capable of impressing a seasoned lunar observer. Digital imaging devices are not all identical to use – for example, afocal digicam imaging requires different techniques to imaging the Moon with a webcam and Barlow lens at prime focus.

Digital cameras

Most low-end digicams have a fixed optical system, with non-removable lenses, and some may not even have an LCD (liquid crystal display) screen, so the principles involved in photographing the Moon through these cameras are just the same as with afocal photography using conventional fixed lens film cameras (see above).

As a general rule, the digicam with the highest pixel rating will produce the clearest, highest resolution views of the Moon. Most digicams can take images in a number of resolutions – a low resolution image will take up less space in the camera’s memory, and more images can be stored. Lunar photography requires the maximum resolution possible, so the setting should always be on high. 

Most digicams provide instant results, their stored images being viewable on the camera’s small LCD screen. The photographer can review each image individually to decide whether it’s good enough to keep, or whether to delete it and free the memory for a better image. LCD screens are usually on the small side, and the display is coarser than the captured image itself. To save the amount of time spent focusing the image, first focus the Moon in the eyepiece using your own eyes. When the digicam is fixed to the same eyepiece, the focus ought to be about right. Fine focusing is best achieved by viewing along the lunar terminator, where most relief detail is thrown up by the low elevation of the Sun, and features appear at their sharpest. If the digicam has a zoom facility, the lunar terminator can be magnified, aiding focusing further. By hooking a digicam to a TV monitor or PC screen, the problems of focusing using the camera’s inbuilt LCD screen are bypassed, as a larger image is far easier to gauge a focus.

Digicams are designed for everyday use, and their automatic settings may pose considerable problems when attempting to photograph the Moon, so it’s essential to experiment with the digicam’s various settings to produce the best results. One of the most important settings to get to grips with is the digicam’s exposure settings, as many afocal lunar images tend to be somewhat overexposed, the bright part of the Moon appearing washed out and lacking in any detail. The digicam’s automatic exposure works best if there is a uniformly bright image across the entire field. A digicam may judge exposures perfectly fine when the Moon is centred in the field of view or when taking close-up shots.

Colour images of the Moon taken with digicams may show particularly vivid tones that aren’t visible through the telescope eyepiece. While colour can enhance the aesthetic quality of an image, it can also be undesirable. Computer processing can easily tone down any colours in an image. Capturing the Moon’s colours in an exaggerated or visually realistic fashion may produce a pleasing image, but the same amount of topographic detail can be recorded in black and white. If your camera has a facility to take black and white images, try it out – the results may be noticeably sharper than those taken in colour. Black and white images will take up less space in the camera’s memory, too.

Using the digicam’s zoom facility (if it has one) can eliminate the problems of image vignetting that tends to plague afocal photographs. Zooming adjusts the position of the camera’s internal lenses – the magnified image becomes progressively dimmer, and vibrations in the telescope will show up more. When ‘digital zoom’ comes into play at high magnifications, the quality of the image begins to degrade and the advantages of zooming are completely cancelled out. Optimum zoom is not maximum zoom – the amount of zoom used depends on the seeing conditions, the resolving power of the CCD chip and the telescope, as well as the stability of the system and the accuracy of the telescope drive.

Camcorders

Camcorder footage of the Moon viewed in real time conveys a striking sense of actually viewing through the telescope eyepiece. The viewer is using the same cerebral processing to make sense of the lunar landscape as in a real observation, as atmospheric shimmering distorts the view and the eye fixes upon individual objects to attempt to make out the fine detail. A high magnification camcorder sweep along the Moon’s terminator, taken under good seeing conditions using a well-aligned driven telescope and slow motion controls, enables the viewer to experience the grandeur of the Moon in a way that viewing still photographs cannot convey. It is possible to produce a wonderful tour of the Moon and its terminator, taking time to linger over features of interest and zoom in on them. Such footage makes a fantastic presentation at any astronomical society meeting – but think twice before showing all of your hard-won lunar camcorder footage to visitors and relatives, as they may not appreciate half an hour of wandering around the crater-crowded southern uplands as much as you!

Camcorders have fixed lenses, and lunar footage must be obtained afocally through the telescope eyepiece. The same problems that affect afocal imaging using conventional film cameras and digicams apply to camcorders. Camcorders tend to be somewhat heavier than digicams, and it is essential that the camcorder is coupled to the telescope eyepiece as sturdily as possible. Some of the same equipment designed to hold digicams in place when taking afocal lunar images can be used to secure a lightweight camcorder.

Digital camcorders are the lightest and most versatile camcorders, and their images can be easily transferred to a computer for digital editing using the same techniques as images obtained with a webcam (see below). Once downloaded onto a computer, individual frames from digital video footage can be sampled individually (at low resolution), stacked using special software to produce detailed, high resolution images, or assembled into clips that can be transferred to a CD-ROM, DVD or videotape. The process can be time consuming – the time spent running through the video footage and processing the images can amount to far longer than the time that was actually spent taking the footage. Digital video editing also consumes a great deal of a computer’s resources, in both terms of memory and storage space – the faster a computer’s CPUs and graphics card, the better. For basic editing of short video clips, at least 5 gigabytes require to be free on your computer’s hard drive. 

Webcams

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.