Deep Sky Imaging

For the most part, when amateurs think about CCD imaging, their goal is to image faint, fuzzy, deep-sky objects. (Operationally, the deep-sky list ought to include comets as well as nebulae and galaxies because comet are just another type of faint, extended object.) Astronomical CCD cameras are phenomenally successful at imaging deep-sky objects for three reasons: (1) their high quantum efficiency, (2) their linear response to low-level light, and (3) the ease with which you can correct their deficiencies with a computer.

5.7.1 Strategies for Deep-Sky Imaging

People have many different reasons for imaging deep-sky objects; your strategy should be governed by your imaging goals rather than a prescription of so many minutes for a globular, or so many minutes for a galaxy. After taking some test images, evaluate your results and learn how to do better from what you did. Nothing anyone can tell you is half as valuable as feedback from trying.

To get you started thinking about imaging strategies, we present three techniques—snapshot, guided, and track-and-stack—that satisfy different goals and require different equipment.

5.7.1.1 Digital Snapshots

The sensitivity of the new breed of digital and astronomical CCD cameras allows an observer to capture fainter stars and more nebulosity in a 60-second exposure

Figure 5.7 Snapshots with digital SLRs capture night scenes much as you see them. In this image, winter stars and the Milky Way trail behind a wind-blown palm tree. On a telescope, an astronomical CCD camera can capture more than you can see with the same telescope in exposures of a few seconds.

than their film-shooting counterparts of the previous decades could capture in an hour. Almost everyone who comes to CCD imaging goes a little nuts at first, banging off images of 50 nebulae one night and 100 galaxies the next.

With an astronomical CCD camera, 60-second snapshots require nothing more than a telescope with a reasonably fast focal ratio (/78 or faster) and a relatively accurate clock drive. Snapshots work because most astronomical CCD cameras have become sky-background limited in a 60-second exposure. Although longer times give you better signal-to-noise ratio, short exposures capture a nifty image of practically anything in the sky.

For snapshots, the best optical system is a medium-aperture telescope (4 to 12 inches) with a fast focal ratio (f/5 or less). Newtonian reflectors, apochromatic refractors, and Schmidt-Cassegrains equipped with focal reducers all fill the bill. The short focal length provides a generous field of view and the low focal ratio provides a bright image at the focus. Light pollution and Moonlight are minor problems when the exposures are short and the CCD is red-sensitive: just ignore the streetlight and Moon and blast away every clear night.

Digital cameras are also good for making snapshots, and their versatile lenses make them ideal for night-time scenic shots. However, because the filters in their Bayer array reduce the amount of light reaching the sensor, good results re quire: 1.) a fast camera lens (those that are/72.8 and faster), 2.) exposures of 4 minutes or longer, or 3.) sticking to the brightest deep-sky objects.

If the optics are free of significant vignetting, basic calibration is entirely adequate. By settling on a single universal exposure time on the order of 60 seconds, only one or two sets of dark frames are needed, so the overhead required for calibration is minimal.

Snapshots are actually very effective for surveying galaxies in search of supernovae or covering large patches of sky repeatedly in hopes of discovering asteroids. In a few months you could easily complete an album of all the Messier objects; or, if you're in a hurry, you could try a Messier Marathon with your CCD camera!

5.7.1.2 Guided One-Shot Imaging

With telescopes that have slow optics, snapshot exposures don't go deep enough to produce satisfying results. Longer times will get you down into the sky background and produce deeper images. Exposure times are limited only by the observer's stamina, CCD dark current, star image blooming, or background sky brightness.

Longer exposures mean more photons fall on each pixel, and the larger the number of photons, the stronger the image signal relative to image noise. Under a reasonably good sky, increasing the exposure from 60 seconds to 240 seconds adds 1.5 magnitudes and a huge increase in the number of stars recorded. Faint nebular details and outer spiral arms emerge from grainy indistinctness in the sky background to take on form and substance. The bottom line is that compared to snapshot imaging, long exposures reveal a lot more.

Making long exposures almost always requires guiding, either manual or automatic, but the results will repay the effort many times over. Single exposures of 5 to 10 minutes should result in excellent images that you can be proud of.

For digital cameras, guiding longer exposures pays big dividends. Very few clock drives will track unaided for four minutes or more and still give round star images, but guiding lets you push out to ten-minute exposures that capture the sky at a dark-sky site, with lots of stars, and of course, your deep-sky quarry.

5.7.1.3 Guided and Unguided Track-and-Stack Imaging

Accumulating dark current or blooming field stars usually limits CCD integrations to 20 minutes or less, and suburban skies seldom allow more than 10 minutes in a single integration. Worst of all, errors in the telescope clock drive often limit well-tracked CCD integrations to 60 seconds or less.

Track-and-stack imaging is an effective way to lengthen the total integration time by dividing a long exposure into many short ones. To collect an hour's worth of photons, you simply track (i.e., register) and stack (i.e., sum) 60 integrations of 1 minute each. Do this properly and you're well on your way to reaching 21st magnitude with a modest telescope and no guiding.

Figure 5.8 Stacking is the key to making very deep images of extended faint objects.

For this image made with a 6-inch f/5 Newtonian, 64 60-second exposures were stacked to reveal the far-flung outermost parts of the Whirlpool galaxy, M51. In this image, the range of light has been drastically compressed.

Figure 5.8 Stacking is the key to making very deep images of extended faint objects.

For this image made with a 6-inch f/5 Newtonian, 64 60-second exposures were stacked to reveal the far-flung outermost parts of the Whirlpool galaxy, M51. In this image, the range of light has been drastically compressed.

Stacking images does exact a toll: to collect an hour's worth of photons in 60-second increments, you must read out the CCD 60 times. The photons don't care whether you gather them in one integration or 60—the Poisson statistics that govern their behavior work out the same. However, each time you read out an image from the CCD, the sensor's amplifier adds another dose of readout noise to the signal. With most astronomical CCDs and many digital cameras, however, Poisson noise becomes the dominant noise source and overwhelms the noise added by the multiple stack-and-track readouts.

'Tip: You can stack an unlimited number of images using AIP4Win's Multi-Image Auto-Process Tool. This tool automatically calibrates, registers, preprocesses, and stacks many integrations. You can use this tool equally well with astronomical CCD images, digital camera JPEG images, and digital camera raw images.

To accumulate the greatest number of photons with the least readout noise, your best strategy is to stack the longest exposures that you can do routinely. This implies that you should guide the exposures or use an autoguider. Under a dark sky, rather than shooting 60 integrations of 1 minute each, it would be better to take 10 integrations of 6 minutes each. The 6-minute sub-exposure is sufficiently long to collect lots of photons, but short enough to avoid the limitations of pro tracted guiding, dark current, hot pixels, saturation, and blooming. 5.7.2 Making Good Deep-Sky Images

Through experience, every observer eventually develops a suite of personal techniques for making good deep-sky images. To help you get started up the learning curve, here are a few things to remember when you're shooting images under the night sky.

Get a good drive for your telescope. If your mounting has an inaccurate drive, everything you try to do with your CCD camera is much more difficult. To make good images, you will eventually have to replace it—the only question is when. The sooner the better.

Get good polar alignment. You need accurate polar alignment and a good drive system running at sidereal rate. Learn solid polar alignment skills; or better yet, put your telescope in a permanent shelter and align it for once and for all.

Don't put up with shifting optics. If your telescope's optics shift and you need to refocus for each new object, find and correct the problem. It is difficult or impossible to do good imaging with optics that don't stay put.

Use one integration time. Unless you are shooting one of the very bright deep-sky objects, it is best to use an exposure time that produces a good signal-to-noise ratio in the sky background. This means that unless you have compelling reasons to do otherwise, use the same exposure time for every object.

Allow time for cooling. Most telescopes don't give good images when they are warmer than the surrounding air. Be sure to set up the instrument, roll back the roof, or open the dome at least an hour before you plan to start taking images.

Keep detailed records. To help you learn from the hours you spend under the stars, it really helps to keep a written record of your imaging sessions. Note objects, file names, filters, exposure times, sky conditions, and anything else you might want to know later. When you get great images, all the information you need to repeat your success will be ready and waiting.

Shoot dark frames. Repeat this mantra: "I love dark frames. Dark frames are critical to making high-quality deep-sky images." As soon as the CCD camera has come to thermal equilibrium, cap the telescope and shoot a dozen dark frames at the exposure time you plan to use that night. (If you plan to calibrate using the Advanced calibration procedure, make the exposures two to five times longer than those you plan to make.)

Shoot a backup set of dark frames when you take your midnight cookie and coffee break. Just because you take a break doesn't mean that your CCD camera needs a break, too. Let it do something useful while you gobble a box of Oreos and guzzle caffeine.

Shoot flat fields. The best way to shoot flats for deep-sky images is with a light box. The light-box technique allows you to make the flats without pointing the telescope down, fussing with the dome, or trying to judge the moment when the twilight sky is at the right brightness. Determine the integration time necessary

Figure 5.9 The globular cluster M13 sprawls between its familiar twin guardian stars.

Observed visually, the cluster is a delicate ball of stars; to the CCD observer, however, the cluster reveals a much greater extent. Taken together, visual observing and CCD imaging complement each other.

Figure 5.9 The globular cluster M13 sprawls between its familiar twin guardian stars.

Observed visually, the cluster is a delicate ball of stars; to the CCD observer, however, the cluster reveals a much greater extent. Taken together, visual observing and CCD imaging complement each other.

to produce a peak pixel value between half and two-thirds of the full-well capacity of the CCD chip. It is important to make sure that no pixels reach saturation. Save 8 to 10 integrations as well as an equal number of flat dark frames taken with the same integration time. Shooting flats takes some time and effort, but it's a good investment in getting top-notch results.

5.7.3 Imaging Deep-Sky Targets

Imaging strategies depend to some degree on the type of object that is the target of your efforts. Here is a rundown on objects that you may wish to image:

Open Clusters. The familiar examples in the Messier list consist of a few hundred fairly bright stars spread out over a fairly large angular diameter, lending themselves well to snapshot exposures with short-focus telescopes. There are literally thousands of faint open clusters scattered along the plane of the Milky Way, virtually ignored by amateurs. A technique that is effective in making these somewhat bland objects look more interesting is to place a mask made of three sets of three '/s-inch wooden dowels spaced 1 inch apart at 60° angles to one another over the aperture of the telescope. This mask produces six bright diffraction spikes that make the stars stand out clearly.

Young clusters such as the Pleiades still contain dusty remnants of the mo lecular clouds that gave them birth, and many others are still embedded in clouds of gas and dust that are still actively forming new stars. NGC 6611 (in the Eagle Nebula) and NGC 2244 (in the Rosette Nebula) are examples. Most of the observational interest in these star-forming regions is directed to intricate tracery of the nebulosity. The clusters make recording the surrounding nebulosity difficult because their bright stellar constituents can cause blooming.

Globular Clusters. Roughly 160 of these objects orbit our Galaxy, all of them accessible to amateur CCD cameras. Although globulars are strongly concentrated toward the Sagittarius-Scorpius region of the sky, a few are found well away from the Milky Way.

Globulars vary in angular size, brightness, and degree of concentration. To resolve their stars, it is best to shoot them with a long-focus telescope having a fairly large aperture, since both scale and light-grasp are needed. Although short exposures easily resolve the dense central region of a globular, for really outstanding images you need to make long integrations to pick up the outlying halo of stars that surrounds the bright core. In a deep exposure, the angular diameter of a typical globular is four to six times larger than that recorded in a short integration.

Globulars are old objects, so their stars have evolved into a mixture of bright, white main-sequence stars and very luminous red giants. Color images made with CCD cameras show the disparate colors of the two varieties very nicely.

Many other galaxies are surrounded by a halo of globular clusters. Before the advent of CCDs, most of these objects were too faint for amateurs to observe, but current technolgoy places the globular clusters around the Andromeda Galaxy well within amateur reach.

Planetary Nebulae. These are transient shells of gas thrown from the surface of a dying giant star. They are popular with visual observers because of their high surface brightness, due in part to the fact that the bulk of their emission falls at a wavelength of 500.7 nanometers, near the peak sensitivity of the dark adapted human eye. Most planetaries have a small angular diameter, but the high surface brightness means a telescope with a moderate to long focal length and a slow focal ratio—e.g., the standard 8-inch//10 Schmidt Cassegrain telescope—is well suited for them. What observer with a new CCD has not immediately set out to shoot an image of the Ring Nebula?

HII Regions. HII regions (read as "H-two") are among the showiest celestial objects. These are fairly dense clouds of gas and dust that are soon to form, are now forming, or have just formed new stars; and as the new stars emit copious amounts of ultraviolet radiation, they ionize the gas, causing it to glow. The Lagoon Nebula (M 8), the Orion Nebula (M 42), the Omega Nebula (M 17), and the Eagle Nebula (M 16) are HII regions. Each of these objects has a clutch of hot, young stars providing the energy that makes it fluoresce.

The term "HII" means the glow comes from ionized hydrogen. Although the bright red Ha (hydrogen-alpha) line dominates the spectrum of many nebulae, the blue-green H¡3 line and the deep blue H y combine with the carmine red of Ha to give these nebulae a vivid "electric pink" hue in color images.

Figure 5.10 The Lagoon nebula is usually described as an HII region, but CCD images— even those made with modest telescopes—show it to be a much more complex object. The Lagoon holds a conspicuous open star cluster, numerous dark nebulae, reflection nebulosity, and elephant-trunk structures.

Red-sensitive CCD cameras are wonderfully sensitive to HII regions, especially when a narrow-band red filter is used to transmit Ha light while blocking light from the sky background and the bulk of starlight. The strongest filters are interference filters with a passband of about 10 nanometers; HII regions imaged this way stand out vividly against a jet-black sky background.

Reflection Nebulae. Where clouds of interstellar material containing dust and gas aren't close enough to bright stars to be ionized, they still can reflect light and thus become visible. The Merope Nebula in the Pleiades is perhaps the best-known example, appearing blue because short-wavelength light is more efficiently scattered than redder light. Others include M 78 in Orion and the beautiful nebula surrounding R Coronae Australis.

Reflection nebulae reveal the incredible clumpy, wispy, stringy nature of interstellar gas and dust. Because they tend to be faint, a fast optical system, along with dark skies and long integration times, helps build the high signal-to-noise ratio needed to distinguish their faint light against the background sky.

Dark Nebulae. Imaging these objects requires above all else a dark sky. Dark nebulae are clouds, wisps, and strands of interstellar gas and dust that have no stars nearby to reflect light from, or to be ionized by. We see them because they block the passage of light. For the most part, these opaque veils cling to the plane of the Milky Way, with notable examples among the dark clouds forming a great rift that runs from Cygnus through Aquila, Scutum, Scorpius, and down into Lupus and Norma.

Dark nebulae become visible in three ways:

• as poorly defined regions with too few faint stars,

• as large, sharp-edged regions silhouetted against the pervasive background glow of the Milky Way, and

• as compact dark globules silhouetted against HII regions.

In all cases, the contrast between the dark nebula and the surrounding sky is very low. Good images require dark skies, a telescope or lens with a fast focal ratio, and long exposures. In combination, these yield images with high signal-to-noise ratios that reveal wonderful detail in these hard-to-see objects.

The most famous dark nebula is the Horsehead in Orion, a compact cloud silhouetted by the light of IC434, a diffuse HII region. The Milky Way in Sagittarius and Scorpius abounds in dark nebulae.

Like most particulate matter, dark nebulae absorb and scatter blue light more effectively than red; hence they are more opaque in the blue. Because at a dark-sky site the sky is darkest in the blue, a CCD with high blue sensitivity is very desirable.

Galaxies. Galaxies exert a powerful fascination for observers. They are huge and distant, and because some resemble our own Milky Way, we see in them a reflection of ourselves. Observationally, galaxies are wonderfully varied. Like snowflakes, no two are alike; yet the spirals all share a common master plan.

Well over 50,000 galaxies—those larger than 90 arcseconds and brighter than magnitude 15—lie within the grasp of amateur telescopes. The biggest and brightest spirals—most of the galaxies in the Messier catalog—fall easily to small telescopes with modest CCD cameras. Smaller and dimmer ones are best imaged with apertures in the 12- to 16-inch range, under dark skies, on nights of excellent seeing. This range of aperture can reach the limit of resolution imposed by seeing, yet still provide a reasonably wide field of view.

Galaxies offer a wide range of morphologies, from bland ellipticals through tight spirals to open and barred spirals. We may view these latter galaxies face-on, at some oblique angle, or edge-on. Irregular galaxies and dwarf ellipticals—especially the nearby specimens in the Local Group—are challenging to image, yet interesting because they are our neighbors.

Elliptical galaxies contain large numbers of older, evolved stars: they are basically yellow in color. Spirals have old stars in their cores, but their arms are defined by brilliant young blue stars and the HII regions where they are formed. Images of spiral galaxies taken with red-sensitive CCD cameras emphasize the underlying "smooth disk" population of the galaxy; in red light, spiral structure is weak.

A dramatic exception is imaging spirals through a narrow-band Ha filter centered on 656 nanometers wavelength. The narrow bandwidth admits little light

Figure 5.11 Silhouetted against the star-rich Milky Way, the Snake nebula (Barnard 72) is a tiny part of a much more extensive system of dark nebulae that blankets much of Sagittarius, Ophiuchus, and Scorpius. Use your CCD camera to complement visual observations of these hard-to-see objects.

from the hordes of yellow and red stars in the disk; so most of the light is from the HII regions, which string out along the inner edges of the spiral arms like lights on a Christmas tree. With a blue-filtered blue-sensitive CCD camera, the arms appear prominently, outlined in young blue stars and HII regions.

Searching for supernovae is a popular sport among galaxy imagers. These occur in class Sb and Sc spirals at a rate of about one per galaxy per century. Since a supernova shines as brightly as the rest of its galaxy combined for several months, finding one is simply a matter of comparing tonight's image with an image taken at a different time.

On the average, you can expect to find one supernova for every 2500 galaxies that you examine. Such a discovery is, of course, a much-sought prize among serious deep-sky observers and CCD imagers.

Clusters of Galaxies. Traditionally reserved for professional astronomers and amateurs with enormous Dobsonians, distant clusters of galaxies add a new twist to deep-sky imaging. Everyone knows the Virgo cluster, and many observers have viewed the relatively nearby Perseus (Abell 426) and Coma (Abell 1656) Clusters, as well as the more distant Ursa Major (Abell 1377) and Hercules (Abell 2151) Clusters. These vast and distant agglomerations reveal themselves to a CCD on an 8-inch telescope. Within a few years, it's a good bet that amateurs will push the frontiers to clusters 1,000 megaparsecs distant.

Although a small telescope can detect galaxy clusters, imaging them really calls for a large telescope, a large image scale, and excellent seeing. With a short focal length or in poor seeing, the galaxies and stars will be indistinguishable. The value of the image depends on sufficient resolution to distinguish the tiny fuzzball galaxies that comprise the cluster from the scattering of point-like foreground stars.

Quasars. Quasars are thought to be very bright, active nuclei of galaxies; many have redshifts that place them a significant fraction of the distance across the Universe. The quasar which appears brightest is a 12th magnitude object in the constellation Virgo called 3C273; it has a relativistic jet that was discovered on photographic plates taken with the 200-inch telescope on Palomar Mountain. With an amateur telescope and a CCD camera, this jet appears as a short spike extending from the star-like core. Several hundred quasars are within the reach of amateur instruments.

Many quasars are variable on time-scales of a few days or weeks, an indication that their light comes from a region less than a few light-weeks across. Since their energy output typically flickers erratically by several tenths of a magnitude, an amateur astronomer with a small telescope can easily contribute photometric observations of quasars to organizations such as the American Association of Variable Star Observers (AAVSO).

Comets. The morphological properties of comets vary greatly, from 18th magnitude fuzzballs to great comets with tails that span huge arcs of sky. You can treat faint comets as you would any faint deep-sky object. An exposure of many minutes may reveal a faint wisp of coma surrounding a star-like nucleus. However, because of the comet's motion against the stars, you may need to guide on its nucleus or resort to making unguided images and then combining them with track-and-stack software. Between the faint fuzzballs and the great comets are objects of every size, brightness, and degree of activity.

Comets shine by sunlight reflected from fine particulates, and by fluorescence from cometary gases excited by sunlight. The more active a comet is, and the closer it is to the Sun, the greater the proportion of light that comes from fluorescence. Yellowish reflected sunlight is readily detected by typical amateur CCD cameras, but the fluorescent spectrum is dominated by bands of the CN molecule in the ultraviolet and the C2 molecule in the blue-green, as well as weaker bands of CH, C3, and NH2 spread across the spectrum from yellow to deep blue. With a blue-sensitive CCD camera, narrow-band filters can isolate the CN molecular bands for dramatic images of the cometary gas tail.

To make images of a large, bright comet, it is necessary to use an optical system such as a camera lens to get a wide field of view; for close-up shots, a wide range of telescopes is suitable. Telephoto lenses and short-focus telescopes will record structure in the outer coma and the inner degree or two of the tail; long-focus instruments can capture features of the inner coma and near-nuclear structures such as shells and jets.

Figure 5.12 CCD images allow you to reach deep into time and space from your backyard observatory. At first glance, you see a few dozen Hercules cluster galaxies; but look again and you'll see hundreds of them populating this rich cluster. Look yet again and you'll spot interacting pairs of galaxies.

Figure 5.12 CCD images allow you to reach deep into time and space from your backyard observatory. At first glance, you see a few dozen Hercules cluster galaxies; but look again and you'll see hundreds of them populating this rich cluster. Look yet again and you'll spot interacting pairs of galaxies.

Bright comets are remarkably dynamic, and you should plan your imaging accordingly. In the tail of Comet Hyakutake, for example, streamers within a few degrees of the nucleus changed from one minute to the next, and the shells and jets near the nucleus showed obvious expansion in a ten-minute interval. Exposure times should be short, and you should think in terms of movies or animations to show the changing structures.

5.8 Lunar, Planetary, and Solar Imaging Techniques

The Sun, Moon, and planets make tempting targets for digital camera, webcams, and astronomical CCD cameras. Once safely stored in your computer, the images you capture can be enhanced to reveal unprecedented detail. However, if you have actually tried it, you may have concluded that planetary imaging with digital cameras is not quite as easy as it's cracked up to be—and (of course) you are right. Outstanding planetary imaging requires strict attention to technique—but once you grasp the basic methods, it's remarkably straightforward.

Outstanding lunar and planetary images require a high-quality telescope that is well collimated, optics to match between the telescope and your digital camera, and moments of good seeing. If you are patient enough to wait for moments of

Figure 5.13 As Comet Hale-Bopp approached us from the outer Solar System, we saw it nearly head-on, with six tails splayed around a compact coma. To see the tail structures, copies of the image were rotated and subtracted, suppressing star images and enhancing the tail. Image courtesy of Al Kelly.

good seeing and to take lots of images, the results will astound you.

5.8.1 Obtaining Excellent Images

Taking outstanding planetary images requires a telescope that has diffraction-limited optics. You must also collimate the optics precisely, employ high-quality enlarging optics, and be patient enough to take advantage of excellent seeing when it occurs at your observing site. It takes all four. Start with excellent optics, check their collimation to insure that they deliver their best, and use high-quality eyepieces or Barlow lenses to match the image to the camera.

Test your telescope's optics. The crucial factor is not what type of telescope you have, but whether it forms high-quality images. Pop in an eyepiece and spend a few hours on several different nights looking at the Moon at 40x or 50x per inch magnification. You will probably see a lot of turbulence, but in moments when the air settles you should see crisp, diffraction-limited detail. On nights with good seeing, use the evaluation techniques detailed in Star Testing Astronomical Telescopes by H.R. Suiter. Deep-sky observers might not care about a quarter-wave of spherical aberration, a few bumpy zones, and a half-wave of coma, but they're negative factors in planetary imaging.

Check optical alignment. When good optics get out of alignment, they act like bad optics. Collimation is nothing more than making sure that each component in the optical system is where it is supposed to be—not tilted, not off center, not ahead of or behind its proper location. Begin alignment with a set of collimation tools or a laser collimator, and follow the manufacturer's instructions. After alignment, carefully assess the quality of star images inside and outside focus and at best focus. Here again, Star Testing Astronomical Telescopes should be your bible. Residual astigmatism or coma is a red-flag warning that something is wrong with the alignment.

Strive for thermal equilibrium. Although observers cannot control atmospheric turbulence (except through their choice of observing location), a key element in exploiting quality optics is making sure that the telescope tube, the observatory (or observing site), and its surrounding area are conducive to good local seeing. In Newtonians, the tube should have low thermal mass (i.e., not retain heat), allow free exchange of outside air, and include a fan to pull air down the tube and over the mirror to aid equilibration of their temperature with that of the surrounding air.

Your observatory or observing area should be open and airy; the building, equipment, and floor must not retain the heat of the day. Concrete block walls may provide necessary security, but they virtually guarantee that plumes of hot air will distort images for many hours after dark. A light wooden building with a roll-off roof makes an inexpensive structure that cools rapidly. The building should be raised a few feet off the ground to allow air to circulate and aid in cooling.

5.8.2 Focal-Ratio Matching Optics

It is almost always necessary to enlarge planetary and lunar images from their size at the focus of your telescope to a scale appropriate for digital imaging. At the primary focus of most instruments, the pixels on the sensor are too large to capture diffraction-limited detail in planetary images. The diameter of the bright central portion of the diffraction disk, ¿/FWHm > is:

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