Visual Film or Electronic Observing

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The greatest change that has come upon amateur astronomy in recent times has been the shift from visual observing and recording with pen, pencil, charcoal, etc., to the recording of observations electronically, for various purposes, including imaging, positional measurement (astrometry), and brightness measurement (photometry). The use of film photography is still common in some types of observing work, notably the recording of meteors, but it is undoubtedly "on the way out". The CCD (charge-coupled device) and CMOS (complementary metal oxide semiconductor) chips and their associated electronics and digital processing techniques can now do all that film and visual recording could do in the past, and much more. This revolution was affecting professional astronomy as early as the 1970s, and by the 1990s it had hit the amateur world with full force as well.

The planner of an observatory would do well to consider at an early stage what imaging methods and equipment he is likely to be using, as this will influence the design of the project, and interact with the choice of telescope-type and general areas of astronomy in which he is most interested. If he only anticipates ever being a "casual observer", then the observatory need only accommodate the telescope and the observer, along with a few shelves for eyepieces, and maybe a chair. The equipment associated with electronic imaging however, does expand the space that is required. Again, if the telescope, mount and observatory is to be made fully automatic, suitable for remote operation, then a very small observatory could be used, with the electronics transferred to another building, possibly the dwelling-house. But this is a specialised kind of project that few will attempt. Most people will want to observe from their observatory, sometimes visually, sometimes using cameras of various sorts, and sometimes using computers, and most will want sufficient space to accommodate guest observers from time to time.

The ubiquity of the laptop or desktop computer in amateur astronomy is now well-established. Even if not used for collecting observational data, it is likely to be used to display the sky on a planetarium program, to guide or aim the telescope, to keep an observing diary, or perform other functions. Most people will want to think about computer equipment in the planning stages of an observatory. A

laptop can be used on a small shelf or table, which maybe can be folded away when the observatory is required to accommodate more people. A permanent desktop setup with a monitor will be cheaper, more expandable and probably more powerful flexible and ergonomic, but bulkier, and less easy to get out of the way. The advent of cheap LCD displays has alleviated the bulk issue with desktop computers, however. One can be hung on a wall and take up almost no observing space.

If using a desktop computer, there will need to be some means of taking the data away, probably to another computer indoors, for processing - in other words, either some kind of removable disk of sufficient capacity, or a wired or wireless local network. Few people will want to spend hours processing their observations in a cold observatory. The files generated by modern imaging methods are very large, often measured in gigabytes. A laptop gets around this, as it can just be taken inside with all the original data on its hard disk. Also, a laptop is likely to be electrically safer and more reliable in the damp conditions often encountered in an observatory.

One method of using a telescope that has become popular in recent years involves the use of a video camera to take and amplify the image, which is then fed to a TV, and possibly to a video or DVD recorder. Using this method, fainter objects and finer detail can potentially be seen than with the eye at the eyepiece. This is a boon particularly to those contending with light-polluted skies. The way is also open for the observer to operate from a heated room instead of having to sit with the telescope at night air temperature. The cameras are sometimes standard video and "night vision" surveillance cameras, slightly modified for the purpose, or they may be purpose-made, such as the Astrovid and Watec cameras. Standard camcorders have also been used to good effect, particularly on planets. All these cameras can be made to work best if the lens on the front can be removed to allow direct coupling to the telescope. There is a wealth of useful information on the internet (see Appendix 2 for links) placed by amateurs who have developed these methods, and indeed web-based groups, such as the famous QCUIAG (QuickCam and Unconventional Imaging Astronomy Group), have been the principal means of communication between amateurs involved in developing these techniques, linking those who have an interest in writing software, those who understand the hardware, and those who principally observe and record.

The video camera methods are particularly suited to recording precisely timed phenomena such as occultations of stars by minor planets, and a significant area of amateur-professional collaborative research at present is the study of the shapes and sizes of these objects by this method, which yields far higher accuracy than traditional methods using the eye and a stopwatch.

Another, related area of technical development on the amateur scene in recent years has been the use of webcams to record detail on the bright solar system objects: the Sun, Moon, Venus, Mars, Jupiter and Saturn. These tiny, mass-produced cameras were designed for purposes such as video-conferencing over computer links, and when the first amateurs started trying to record astronomical images with them, most were highly sceptical that anything would be achieved. However, the technique has been developed spectacularly, achieving resolution undreamed of from amateur equipment only 10 years ago, even surpassing the best results from professional telescopes of that era, and approaching the resolution achieved by some space-probes. The names of the leading practitioners of this art and science have become well-known to readers of the journals and magazines, through their spectacular published images: Damian Peach and Dave Tyler in the UK, Don Parker in the USA, Christopher Go and Isao Miyazaki in the Far East, to name but a few.

A webcam is just a camera containing a small CCD chip and associated electronics, which interfaces directly with a computer, usually through USB 1, USB 2, or FireWire (IEE 1394) connections, and takes a large number of frames in quick succession. Webcams are not all that light-sensitive, hence their use mostly on bright objects, but their strength lies in their high rate of data capture. Used on telescopes with very large effective focal lengths (thus projecting the image of a tiny planet onto the detector chip at a large scale), they take images at a rate of maybe 5-40 frames per second, which are saved to the computer's hard disk in real time. Because the resolution of moderate-size telescopes (above 10 cm or 4 in. aperture) is largely determined by the constantly fluctuating state of the atmosphere, known as the astronomical seeing, webcams will normally capture a mix of good, sharp images, and poor, blurred ones, when run over a few seconds or minutes. Computing techniques are then used to filter out the poor images, and construct a resultant "stacked" image (in other words, an average image) from the good ones.

The more frames that are stacked, in general, the better the results, under even mediocre seeing. The signal-to-noise ratio increases proportionately to the square root of the number of images averaged. The best-known piece of software for performing this image stacking operation is a piece of freeware called Registax, developed by Cor Berrevoets, but there are others. Further image manipulation, in various programs, is then normally carried out in order to achieve the finished result.

Traditional photography never achieved good result on planets, because of the tendency for seeing fluctuations to blur out individual exposures, and in fact visual observation and drawing remained the best method of recording planetary detail as late as the mid 1990s (short of sending a probe up there). The trained human eye and brain is remarkably good at detecting and fixing detail in a badly fluctuating image - performing, in fact, a similar computational averaging procedure to that performed by software such as Registax. However, the webcamcomputer combination can now obtain better results under any conditions than the eye and hand, and these results are more objective and reliable - though they are not automatic, and as much skill and dedication as ever is still required on the part of the observer, to exploit conditions and opportunities to the maximum, and extract the most real data out of the signal recorded. The skill involved in observing is never reduced by technology, but it does change its form.

An advantage of webcams is that they are quite cheap, and little extra equipment other than a telescope and computer is really required. Many people buy the computer for other purposes, anyway. Webcams cannot, in their normal form, be used for recording faint objects, such as nebulae, clusters and galaxies, because they will not permit long exposures. However, ingenious amateurs have discovered that they can be modified so that they can expose for as long as desired, and can be used for recording deep-sky objects. (The limitation on the exposure length is the thermal noise that is intrinsic to the CCD detector, and eventually fogs out the image.) Software has been developed to control these modified webcams, of which the best-known item is K3CCDTools, by Peter Katreniak. The modified cameras can be bought from certain suppliers. They are not so good as true astronomical CCD cameras, but are much cheaper. The main difference between the latter and the former is that the true astronomical CCD cameras are cooled by some method to well below the ambient temperature, to greatly increase sensitivity by decreasing thermal noise. However, fair results may be obtained on the brighter and more compact deep-sky objects using air-cooled modified webcams, such as the Atik Instruments ATK 1HS camera, in colder locations - there are some advantages to living in a cold climate! (See Fig. 1.8).

The cooled CCD cameras are expensive - anticipate spending as much on one as on a high-quality telescope. They are capable of amazing results, as any trawl through an astronomy magazine, or the websites of imagers, will show. The major manufacturers are Starlight Xpress (UK) and SBIG (Santa Barbara Imaging Group) (USA). At the top-price end they merge into the equipment used by professional astronomers. For work such as surveys of distant galaxies for new supernovae explosions, or imaging faint comets, or showing the spectacular details of nebulae imaged in narrow optical wavebands using filters, they have no competition. The cooling allows for long exposures, of many minutes, but, of course, long exposures require very accurate tracking. There is no point embarking on long-exposure imaging if your mount cannot be made to track accurately, with either a very low periodic error (PE), or a mechanism that satisfactorily compensates for PE and other causes of image-drift.

Figure 1.8. Messier 1, the Crab Nebula, imaged by the author with an ATK 1HS modified webcam and a 20 cm (8 in.) driven (but unguided) SCT.

The two possible compensatory mechanisms are as PE training, by which the computer control is taught by the observer what the mechanical errors are over a period, and then automatically compensates for them, and auto-guiding, where a separate camera, or guide chip, detects drift due to PE and other causes and, operating through a computer and an interface to the mount, adjusts the drives in real time to compensate. Traditional manual guiding using a subsidiary telescope of long focal length (a guidescope) and crosswires is also possible. It should be noted that even given a mechanically perfect mount, with negligible PE (and some mounts do approach this), guiding is still necessary for very long exposures at typical telescopic focal lengths because of changing atmospheric refraction with changing altitude, telescope flexure, and because, in the case of comets and other solar system bodies, the target does not move at sidereal rate, it moves additionally with respect to the stars.

The requirements of PE correction and guiding are eased if image scale is reduced, as then the drift is slower and less noticeable in a given length of exposure. Additionally, for diffuse objects, such as nebulae and galaxies, the exposure required goes down as the light in the image becomes more concentrated, at smaller effective focal lengths. Hence the recent popularity of smaller, shorter focal length telescopes, particularly small apochromatic (triplet objective) refractors, on high-quality oversized computerised mounts, for use with CCD imagers. These produce spectacular results on large, diffuse nebulae, such as the North America Nebula, which are hardly obtainable any other way. Larger telescopes, unless they are specially constructed astrographs, devised specifically for widefield imaging, do not give a wide enough field to do such objects justice. A complicated imaging setup based on a small telescope probably requires an observatory almost as much as a simpler large telescope does, as the setup time can be impractical if all the equipment, cameras, computers, etc. have to be brought outside every time and connected up. The wonder of an observatory is that everything can be left connected, ready and waiting, in situ.

Image-scale is also reduced by using a larger CCD chip combined with high-sensitivity "binning mode", so that individual elements in the detector have their signals combined in groups, thus reducing resolution, but increasing signal. This is analogous to using a high ISO film in photography. CCD cameras with larger chips and binning capabilities become rapidly more expensive. The chips in "entry-level" CCD cameras are less an 8 mm (0.3 in.) across, and it is important to realise this when trying to predict what result will be achieved with such a camera, compared to that from a film camera using film 35 mm across. There is not space here to go in detail into the techniques of CCD imaging, but there are excellent published accounts.3

A cheaper option than the cooled CCD, particularly viable for wider-field work, remains the film SLR camera, connected to a wide-field telescope by T couplings, or used with a telephoto lens (effectively a small-aperture wide-field telescope), a standard or a wide-angle lens, with the camera mounted "piggyback" on the main telescope, using its drives and guiding. Fine shots of the Milky Way, the

3See, for example, Digital Astrophotography: The State of the Art by David Ratledge, in this series.

Magellanic Clouds, constellations and open clusters, such as the Pleiades, can be taken in this way, but a dark, unpolluted sky is really required. The orange glow of sodium lights, so ubiquitous in towns now, quickly fogs film, though it can be partially filtered out with light-pollution filters. Of course, exploitation of film photography to its potential requires the user to have their own darkroom and processing rig. Sensible results cannot be expected from commercial processing labs, whose staff will have no idea of what the astrophotographer is trying to show. Because of this, and the rapidly developing digital competition, film astrophotography is being practiced less and less.

The direct alternative to the traditional high-quality single lens reflex (SLR) film camera is the digital SLR (DSLR) camera. These cameras are based on CMOS detectors, which have higher thermal noise and hence lower sensitivity than CCD chips, but they can be manufactured in large sizes much more cheaply. The size of the detector chip in many fairly inexpensive digital cameras now approaches the old 35 mm film standard.

Most consumer-level digital cameras, though excellent in many other respects, have one major disadvantage for astronomy - the lens cannot be removed. It is possible to use these cameras with a telescope, utilising some method of attaching the camera so that it "looks into" an eyepiece, but such arrangements, even involving specially-manufactured accessories, tend to be slightly rickety, unstable and unsatisfactory. Fairly good images of lunar and solar features are the best that can be expected from this so-called "afocal" method of photography. A level of manual control on the camera is desirable, so the automatic settings can be overridden, and also a timed or cable-operated shutter release.

The DSLR camera is far more suitable than these consumer-level cameras for all astronomical purposes. The lens can be unscrewed, allowing direct coupling to the telescope using suitable adaptors (usually two adaptor rings, one having the universal T-thread, and the other adapting this to the particular make of camera.) The camera can be used at prime focus, in other words, using no optics other than the primary (and secondary in a reflector or catadioptric scope) for a low image scale, or it can be used with Barlow lenses or eyepieces in the light path (Barlow projection and eyepiece projection) using suitable accessories, increasing the effective focal length (EFL), or it can be combined with a focal reducer, a lens that is essentially the opposite of a Barlow, in that it decreases the EFL, to reduce image scale further beyond the prime focus scale. Focal reducers can be used with modified webcams and CCD cameras as well, but these generally will still give smaller fields than the DSLR due to the smaller detector size. Focal reducers are popular with SCTs, generally reducing them from f10 to f6.3 or f3.3, but can be used with other telescopes as well with suitable couplings.

DSLRs can be used with telescopes to record fine wider-field views of open clusters, extensive nebulae, star clouds, the Andromeda Galaxy, etc., but they are rapidly fogged-out by light pollution (and moonlight), limiting exposure in badly-polluted areas to only a minute or so. Various narrowband and broadband filters can mitigate the pollution problem, as they can with CCD cameras. Postprocessing in an image manipulation program can also be used to reduce the effects. (Later in this book, I will discuss the observatory and methods of Bob Garner, who achieves amazing deep sky imaging results from one of the most unsuitable, light-polluted sites imaginable.) One strategy is to take several shorter exposures and stack them, using similar software to that used for processing webcam images. This reduces the build-up of background signal, and also alleviates the requirement for precise tracking. Again, this is a method that can also be used with modified webcams and CCD cameras, but not, so easily, with film.

The DSLRs are rapidly getting better and cheaper, with larger detectors and greater sensitivity. The most popular makes with astronomers are Canon and Nikon. The most popular models are undoubtedly the Canon EOS 300D and EOS 350D at present, but no doubt improved successors will more prevalent before this book has been long in print. DSLRs allow rapid downloading of the images to a computer for perfecting in a graphics program, such as Photoshop, PHOTO-PAINT or Astroart, or the images can be recorded direct to a computer. The exposures can also be controlled by computer with suitable connections and software.

DSLRs, like SLRs, can be used alternatively with standard, telephoto, or wide-angle lenses in telescope-piggyback mode to record wider views of the sky (as indeed can most CCD units). Taken together, the CCD and CMOS cameras have ushered in a new world of amateur astronomical imaging (arguably the word photography should no longer be used), with easy processing, free from the vagaries and mess of chemicals and darkrooms, and independent of commercial photo processors who have no idea of astronomical photography. Some astronomers will continue just to watch the sky and wish for no more, and some will continue to draw and paint, but most will wish to use the new technologies to record both things they can and cannot see. The exciting possibilities opened up by the new cameras will no doubt continue to advance into the foreseeable future.

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