ASTROPHOTOGRAPHY WITH SIMPLE DIGITAL CAMERAS ------------------------------------------ John Pazmino NYSkies Astronomy Inc www.nyskies.org firstname.lastname@example.org 2010 January 10 initial 2011 August 3 current
Introduction ---------- I routinely take chemopictures of the sky with a range-finder camera on tripod. I tried from time to time pictures by prime focus and projection at the Brooklyn College Observatory. However, some 99- 44/100 percent of my astrophotos are skyscapes taken with the camera itself supported for a time exposure. In the last couple years I drifted, like virtually all of my photography colleagues, to electronic cameras, shooting on 'silicon film'. After sending out the last of my slide films for processing, I just about stopped shooting chemopicutres. For me, at least, the transition from film to silicon was easy. Many of the techniques from film carried over with little effort to silicon photography.
My cameras -------- In chemophotography I used both single-lens reflex (SLR) and range-finder (RF) cameras. The former is built around the Miranda system, a major brand of the 1960s-1970s. In fact, my cameraS for astrophotography thru telescopes or large lenses is the Miranda G and Laborec bodies, once a supreme choice for film astrophotography. The main SLR is the Miranda Senorex with a full complement of lenses and accessories. Unlike for many other camera brands, all Miranda devices are interchangeable on all of its bodies. What's more, Miranda accepted the lenses of many other brands with adaptor rings, allowing the choice from a much wider diversity of optics. A few lenses could be attached only with focusing and metering limitations. The range-finder is the Canon QL7. This is the camera I take on trips for general picture-taking, it being light in weight and sturdy in build. It's F1.8 lens captures images in dim light, where other RF cameras fail by freezing the shutter or capturing dark pictures. In 2003 I acquired as a charity performance award a point-&-shoot Kodak digital camera. It worked for two years and I gained fluency in silicon imaging with it. It suddenly died in 2005. I took it to a repair shop to learn that it just was infeasible to fix. The store had other reconditioned Kodak cameras which I could choose from. For $100 I took a Kodak DX6340 model for its general feel in the hands and bit of heft. This camera I use all the time, carrying it with me for being so compact and lightweight. The obvious question I had since then was: Can I get nice astrophotographs with this camera? SInce daytime or terrestrial pictures came out so well, by applying much of the smarts from chemopictures, why not try for star pictures?
Preparation --------- Most of the litterature on digital astrophotography quickly urges the reader to the SLR models coupled to solidly mounted telescopes. Yes, like for film work, the SLR is the best overall machine for astrophotography thru a telescope or long lenses. It also, in the silicon world, has the most features and flexibility to control the exposure and do some initial in-camera processing of the image. Many of these functions are nonexistent for chemophotography. I wasn't ready for a digital SLR yet, altho prices declined, partly due to the Depression, to very attractive levels. I really don't need one yet. I wanted to see what a RF digital camera can do. Can't I take star pictures with my RF the way I do with my Canon QL7? Can't I just prop it up, set focus to infinity, open the lens, and trip the shutter with a cable release? It seemed not so simple, yet so plausible. I collected assorted litterature on digital astrophotography and skipped all the stuff requiring heavy telescope rigs, coupling tubes, exotic focusing devices, and stayed with the sections on point-&-shoot cameras.
Peculiar SLR feature ------------------ One feature of silicon SLRs I quickly noticed during my reading is the fixed pentaprism and focusing screen. Even brands whose film SLRs have exchangeable prisms and screen now seem to make their digital SLRs with fixed screens and prisms. Substantial text is devoted to the obstacles of the unitized digital SLR, its loss of light in the prism and screen, fanatical tricks to aim and focus the camera, contortions to look thru the prism, and more. Having a removable prism and screen allows for right-angle viewing thru the lens, use of screen with a brighter focal image, fitting magnifying eyepieces. In some models the metering is in the prism, so removing it makes the camera smaller and lighter. In chemoastrophotography, a camera with interchangeable prisms and screen was one of the most prized possessions.
Encouragement ----------- The reading was encouraging. The authors showed really pleasing pictures of constellations, comets, planet alignments, just the things I like to capture. And they characterize the simple cameras as 'point- &-shoot' cameras. I got the same encouragement from photo galleries in astronomy magazines. The skyscape pictures were for the most part taken with simple silicon cameras. In chemophotography, a point-&-shoot camera is essentially a modern box camera with a button to snap the picture and a knob or lever to wind the film. The shutter speed, lens opening, and focus are fixed to yield acceptable (to whom?) images under the usual range of well-lighted scenes. A builtin flash kicks in automaticly for dim light, even if it'll be useless for distant night scenes. In as much as almost all pictures are taken on print film, where the final product throws away a major percent of the original detail of the in-camera film, point-&-shoot cameras didn't really need high quality. Their flaws showed up with slide film where the film in the camera is the final rendition of the image. Such cameras can not take good sky pictures. They are just too limited in the range of application to astronomy. Yes, they can -- and do! -- capture the landscape during solar eclipses and so-so views of lunar eclipse partial phases. They could capture solar halos and something of twilight planets. But that was about it. On the other hand a range-finder camera can be used like a box camera with everything automaticly adjusted. Press a button and a picture is captured. The focus can be set on the lens to a mid range to accommodate the anticipated scenes. The camera selects the shutter and f-stop according to a builtin scale. For the great bulk of pictures, this use of the range-finder works amazingly well! But the auto features can be disabled, leave every thing manually adjustable. That's how I take starfields with the Canon QL7. I turn a dial off of 'A' for 'auto', turn the focus ring to the 'infinity' mark, set the shutter dial to 'B' ('bulb', time exposure). A connected cable release holds the shutter open while I count orangutans. On my Kodak DX6340 there has some manual adjustments. They are buried in the menus displayed on the camera's screen. The instruction book gave many hints, but few explicit details. The menus change according as the operating mode the camera is in at the instant. It was at first frustratng to find the feature I wanted. Because this camera, and likely others of its class, do have manual override, to at least some degree, they belong more to the range-finder category than in the point-&-shoot one. Here I call these cameras, potentially good for star pictures, the range-finders. Of course there is no actual range finder, the split image in the finder that you have to superimpose for focus. Focus is still either automatic or set manually thru a menu.
More encouragement ---------------- Over the yearend holiday of 2009 I stormed the reading about digital astrophotography and asked around for sample cameras to experiment with. Three friends let me play with cameras, all simple models. Since all their owners used the cameras straight out of the box, they offhand didn't know what manual setting they offered. This actually was a good thing, because I now had a collection of three cameras, plus my own, of the sort most people, home astronomers included, may have to hand. The set of cameras was: ---------------------- Kodak DX6340 Nikon Coolpix 4600 Panaxonic Lumix LZ7 Samsung S760 ------------ I also got from Internet their instruction books,tests, and reviews. All these materials boasted the cameras as good general photography machines, with no serious deficiencies. All accept an SD card and run off of AA batteries. This made life vastly easier. I could swop chips and batteries from a single stock. Please understand that I did not look for cameras suitable for starfield pictures. I obtained cameras of the kind likely in your hands already, as their owners favored me to test them. All have a 'auto everything' mode, the one used by their owners so far. All seemed to have some degree of manual control. The fact that all four are a few years old may mean that today's models have more and better features, perhaps better fitted for astrophotography. To keep peace with my friends I did not attempt any alterations with the camera firmware, even tho, in principle, i could restore the original one after I finish my experiments. This, too, was a good idea because people will not cotton to messing around with the internals of their cameras.
Amplification ----------- Also called 'light gathering power', this is the aperture thru which the photons enter the camera, like the aperture of a telescope. Camera lenses are usually specified by focal length and maximum f- stop, but you can get the aperture by
(aperture) = (focal length) / (maximum f-stop)
For digital cameras, the focal length is the actual one, not the 35mm equivalent. This is somewhere given in the instruction book as a technical specification. One reason SLRs are favored for astrophotography is that they can mount lenses with larger apertures, tens of millimeters, to take in photons from fainter stars and allow shorter exposures. When attached to a telescope, which then becomes the camera lens, the aperture is many centimeters The silicon range-finder camera, with a fixed lens, can have only a small aperture within their tiny barrels. Apertures are in the millimeter range. The small aperture is partly made up for by the huge photon efficacy of the sensing cell and the ability to select high ASA or ISO values. There are constraints to this ability, but at least it's there in most digital RF cameras.
Ideal parameters -------------- In a film-based RF camera the photographic parameters can be independently adjusted by the dials, rings, knobs on the camera body. A digital camera has very few mechanical adjustments. The parameters are altered by picking in the menus displayed on the camera screen. Every camera brand, and model within a brand, has its own scheme of menus, feature organization, and naming convention. It is not possible, like for a film camera, to instruct, 'turn the aperture ring to widest open'. Your peculiar camera may require drilling down several levels of menus to reach the choice of lens f-stop. A careful study of the instruction book and playing with the camera is pretty much a necessary preparation for starfield photography. The one great blessing is that you do not 'waste film'. When trial pictures come out wrong, just delete them. Here are many of the settings that should be examined, altho some may be missing from your camera or forced to a certain value. --------------------------------------------------------- * meter - Set to 'off' or 'manual'. In my sample cameras, the metering is overrided when the shutter is manually set. On the other hand for a zoom view of the Moon, try a 'spot' or 'center' mode to get the exposure off of the lunar disc without the dark surrounding sky. For twilight scenes, try the 'multi' setting, which samples several spots in the field for an best-fit exposure. * focus - Set to 'infinity'. None of the cameras in my trials allowed explicit focus setting. All had some scenery mode that locked the focus on infinity. * shutter - Set to several seconds. In my initialexperiments I found that less than 10 seconds for starfields at night was too short. Later with adjustment of other parmes I could reduce the exposure to just a few seconds. * ASA or ISO - set to a mid, but not very high, level. A digital camera can vary its equivalent film sensitivity for EACH FRAME. At the highest level, some cameras introduce into the picture too much static or noise in the form of lighted pixels that can be confused with stars. These artifacts can be removed by special processing but for starts it's best to avoid getting them in the fist place. * burst or multishot - Set to 'off' or 'single'. It may be locked out when certain other parms are changed manually. A potential use of this feature is for afocal pictures, when you take a picture of the view thru a telescope's eyepiece. It can be hard to hold the camera centered and steady over the eyepiece. Taking a rapid series of pictures may capture one rally good image. * aperture - Set to widest, smallest f-number, opening. Camera lenses of the late 20th century and certainly in the 21st yield good images at all openings. The rule about setting to the next-to-widest opening isn't required any more. If. however, you find optical defects, specially near the edges of the frame, stop down one notch. * storage - The card already in the camera will collect your star pictures. They'll be among your regular pictures. Be sure to have enough free space for your starfield shoot. The cameras will show how many more pictures you can store at the instant size and quality. * flash - Set to 'off'. In some settings of other parameters this may be forced to off. The flash will not light up your celestial target! What is more important is that it can be very annoying if it pops in your eyes on a dark night. * self timer or delayed shutter - Set to 'on'. Digital RF cameras usually do not have a socket for remote shutter release. Others require an expensive accessory cable. The self timer lets the camera settle still after handling it, even on a tripod. The delay is several seconds. The camera indicates that the timer is running on its display or a lamp on the body. leave hands off of the camera during the delay. * file format - Set to 'JPEG' or 'JPG' if there is a choice. Most simple cameras may have only the JPG format. This is not the ideal form for advanced astrophotography but is the easiest to work with for starfield pictures. Do NOT use a peculiar format, which many older cameras default to and compel the use of its own dedicated software. * white balance - Set to 'auto'. Opinion differs widely on this parm. One thinking is that setting it to 'tungsten' or 'incandescent' will darken the sky background by shifting the camera spectral response away from the main wavelengths of luminous graffiti. This seems plausible in as much as luminous graffiti tends to sit in the mid range of human sensitivity. An other thinking claims that 'daylight' is best to render the stars in their more natural colors by spectral class. Try all three modes and compare the results. * color space - Few simple cameras have this feature, which is the method of laying down the colors into the image. Set it to 'RGB' or 'LRGB'. Unless you really know chromatographic techniques, don't play with color space. * backlight compensation - Set to '0' at first. If possible, try the '+' values to gain additional exposure time. This feature may be forced to '0' when certain other parms are changed. * quality or resolution - Set to the 'best' quality and largest image size. Smaller image sizes bundle adjacent sensor pixels to act as one. You want as many pixels as possible in action, minding that a star's impression will sit on only a couple pixels. A high resolution means a larger filesize for the image. * viewing screen - Set to 'off' to reduce stray light around the camera and conserve battery strength. The screen may show the progress of a long exposure, like a countdown, which can be handy. Shield the screen with a dark card or cloth if there is a concern about its light getting into the lens. The screen may also be amazingly brilliant under night vision, which is then ruined. * color balance - Set to 'normal' or 'natural'. The 'vivid' or 'enhanced' mode can impress excess noise into the image. This is a different parameter from white balance, which shifts the spectral response of the camera to accommodate different scene illuminations. * color mode - Set to 'color' or 'B&W'. Some astronomers prefer black-&-white (gray scale) to simplify the manipulation of the image later. They work with only one dimension or channel of information rather than the four for a color picture. For casual starfield pictures, leave the setting at 'color'. At least the foreground will have some liveliness in it. * noise reduction or long-exposure cleanup - Set to 'off'. You have to dig into the technical details of the camera, perhaps from tests and reviews, to learn what this feature actually does. In some cameras it cleans up the image by removing tiny white specks. If so, you may end up with a picture with no stars! * zoom - Stay within only the OPTICAL zoom. Do not go into DIGITAL zoom. The latter does NOT enlarge the focal image for more detail but merely captures a smaller central portion of the sensing cell and stretches it to full frame size. The viewing screen somehow signals the shift from optical to digital zoom. Keep your eye on it. * battery - Have LOTS of spare batteries in room temperature storage for extra cold or hot or humid nights. Digital cameras eat thru batteries quickly, even for regular picture-taking. The viewing screen consumes battery strength and may be best turned off. * external power - If you're near electric use the external power adaptor. This may require an extension power cord. For field work, you have to rely on batteries. One scheme is to rig up a car or boat battery with a suitable power adaptor. ---------------------------------------------- Notice the extra features, deriving from the electronic nature of the image capturing process, provided in a digital camera. Film cameras had no where this range of flexibility! Make SURE after you finish taking star pictures to reset the camera back to regular picture-taking mode! Switching it to an all- auto mode may do this. Forget this restoration and your regular pictures will be completely ruined by grossly wrong parameter values.
Common features ------------- All four sample cameras had at least SOME manipulation of the picture taking procedure but NONE offered complete manual operation. In this method, each parameter can be set independently of the others, like on a film camera. I started by leaving alone several parameters, or letting them be set on their own when manipulating other parms. They were: --------------------------------------------------------------- * meter - Turned off by selecting the sky scene * focus - Forced to infinity by selecting the sky scene * shutter - 15 and 30 seconds on Panasonic. * ASA or ISO - Forced to some low value by selecting sky scene. * burst or multishot - Set to 'off' or 'single'. * aperture - Set to widest open at lowest zoom. * storage - Existing cards, images erased after offloading. * flash - Set to 'off'. * self timer or delayed shutter - Set to 'on' or to '10 sec' when a delay time was offered. * file format - All cameras made only JPG files. * white balance - Set to 'auto'. * color space - None of the cameras had this feature * backlight compensation - Forced to '0' with sky scene * quality or resolution - Set to the 'best' and largest size * viewing screen - Left on to see progress of the shoot * color balance - Set to 'normal' * color mode - Set to 'color' * noise reduction or long-exposure cleanup - Nikon seems to do something with the image before writing it to storage. The others did not have this feature. * zoom - Widest, lowest, zoom * battery - Loaded new batteries * external power - Not needed ----------------------------- The game for me was to find such combinations of the constrained choices that best allow for starfield pictures. In this game, it turns out the Panasonic, with a simple two-step setting of scene and shutter, gave the best all-around parameters for astrophotography. All the cameras allowed independent setting of the picture quality and image size. I chose the 'best' quality and largest image size. The images varied among cameras from 4 to 7 megapixels. All cameras delivered only compressed JPGs of 1 to 3 megabyte filesize. It was real tough to do up a comparison chart of cameras because each brand has its own names for the features and similar features work in very different ways. The sections here for the four cameras is about the best I could think of for now.
Kodak DX6340 ---------- The best setting appeared to be 'landscape' to pin the focus at infinity and '4 second' for the shutter. Other settings were either locked out or set to default value. The ASA bumped down to 100, altho in other modes it could be set up to 400. The pictures were all too dark by too-short exposure. I suppose a series of pictures can be stacked, but I did not try this.
Nikon Coolpix 4600 ---------------- This camera had the lowest degree of manual operation. The best I found was the 'Night' scene mode, but the shutter seemed to do its own thing, hanging open for several seconds with no apparent way to adjust it. The ASA was increased to an unknown high value. The instructions mention an in-camera processing for pictures taken at slow shutters before writing them to storage. It doesn't explain this processing, but is COULD be a dark-frame subtraction like on Nikon's digital SLRs. The pictures were, erm, lousy.
Panasonic LZ7 ----------- The combination yielding the best results of the four cameras was 'starry sky' and '15 seconds'. In the 'starry sky' mode, the shutter can be set to 15, 30, or 60 seconds. The instructions say specificly that 'starry sky' is for, erm, a starry sky and other night scenes. I later reduced the exposure to 8 seconds to shorten the star trails, yeilding rounder dots for the stars. Since the shortest shutter offered was 15 seconds, I covered the lens with a dark cloth after 8 seconds. Setting this mode opens the shutter adjustment panel. Once set it remains set until deliberately changed, making a series of pictures simpler to take.
Samsung S760 ---------- The settings were 'night' and '8 seconds'. The ASA appeared to increase for the night scene modes. The pictures were a bit inconsistent but approached those of the Panasonic camera.
Attachments --------- Many simple cameras have no threads on their lenses for filters. If not, filters are taped over the lens. If you get new filters, get a set just a little bigger than the lens snout. They are easier and more secure to tape to the lens than filters that are much larger. Most simple cameras have no lens shade, even as an accessory. Make one from a length of cardboard tube, like from toweling. You may blacken the inside with a black felttip marker. In daylight examine the field of view with the lens shde in place. If necessary cut the tube shorter so if doesn't intrude into the field of view for the zoomn setting you're using.
ISO or ASA -------- In chemophotography this, either term, rated the efficacy of the film to convert impingent photons into an image. The higher the value, the quicker an impression is made for a given influx of light. It also allows capturing images of fewer photons, for night or other low-light situations. ISO is the current term, 'International Standards Organization', while ASA, 'American Standards Association', is the legacy one. Both assign a number to each formulation of film. Because the number relates to how rapidly an impression is formed by photons, it is also called the film 'speed'. Over the years the ASA rating settled out to 50, 100, 200, 400, and higher is binary steps. The numbers are arithmetic so a 400 ISO film is four times 'faster' than a 100 ISO film. It captures the equal strength of image in 1/4 the exposure. Digital cameras collect light on a sensing cell with electronics to convert the photons into a numerical value or count for each pixel. This count is linearly proportional to the number of photons collected during the exposure. The pixel counts are arranged into a computer file in proper rank and file. When the computer plays the file the pixel counts are rendered into a replica of the scene that illuminated the pixels, all with proper brightness and mix of colors. The sensitivity of the cell is tens of times more efficient than film in converting photons to image. When the digital camera's ISO is selected, the counts from the sensing cell are amplified to match the ISO level, like the volume of an audio signal. Like for audio amplification, static and distortion can intrude for too high a ISO setting. Pixels under high illumination can saturate, reaching the maximum count they can hold. For astrophotography, setting the ASA to the highest value is not a good rule. Leaving it one or two steps below the fastest rating will significantly lessen the noise and prevent saturation. The ISO can be selected for EACH FRAME. It is put back to the level for regular picture-taking without disturbing the astrophotos. Switching the camera to 'auto' sets up every thing for ordinary snapshots. Make you DO this, else your ordinary picture will come out, well, awful.
Bracketing -------- This is the practice of shooting several pictures at slightly different exposures to increase the chance of getting at least one good one. On a film camera this is commonly done by changing the f- number or shutter speed for each picture, To change shutter or other parms in a silicon camera you have to dig into the menus, taking time from the shoot and impairing night vision. The other problem is that handling the camera after each picture upsets the aim and framing of the scene. It's just too clumsy to bracket altho as sound photography practice you should. One way out of this situation is to set the shutter for the LONGEST exposure of the shoot. Have at ready an opaque cloth or cover. Once the camera starts the exposure, count off orangutans for the first, shortest, exposure, When time is up, hold the cover over -- but not touching! -- the lens and let the exposure finish on its own with no more light entering the lens. Go to the next shoot, now counting off the time for the next longer exposure. Continue until the final shot is down with no cover for the longest exposure, Yes, this sounds like an early 20th century way of taking pictures, but it still works.
Trail length ---------- Unless you are deliberately shooting star trails, or must take a really long exposure to capture meteors, you must keep the exposure time short. Else the trails get unattractively long. Bear in mind that lengthening the exposure after trailing sets in does NOT record fainter stars. The photons of a given star track across the sensing cell, pixel after pixel, and do NOT accumulate into one set of pixels. Trail length is a function of exposure and target declination, The longer trails are closer to the celestial equator. Most of us shoot based on the equatorial trails, declination zero. This is plausibly because Orion, straddling the equator, is so typicly a target. As the Earth rotates, it carries the stars thru 1/4 arcminute (at O degree declination) per second of time. A 15 second exposure moves the star, and makes it trail, thru 3 arcminutes or 1/10 Moon diameter. This table gives trail length for assorted exposure times: ---------------------- exposure| trail length --------+----------------- 1 sec | 15 sec, 0.25 min 10 sec | 150 sec, 2.5 min 15 sec | 225 sec, 3.8 min 20 sec | 300 sec, 5.0 min 25 sec | 375 sec, 6.3 min 30 sec | 450 sec, 7.5 min 40 sec | 600 sec, 10 min 60 sec | 900 sec, 15 min ------------------------- Depending on your tolerance for trails and the zoom of your lens, the length of trail may be acceptable. In the end, the ideal goal is to expose as long as practical to impress as many photons into one set of pixels on the sensing cell, without making too long a star trail.
Luminous graffiti --------------- A sky that is gray with luminance will force you to shorter exposures, possibly below the limit for capturing a good population of stars. The longer the exposure, the more photons from the sky impinge on a given pixel on the sensing cell. They accumulate to a higher photon count, graying the sky at that point. This happens because the luminance of the sky is tied to the horizon and ground. It does not trail with the diurnal motion of the stars. It, for the camera in a fixed aim, builds up on the sensing cell while the star images slide across it. The result is that the star images are veiled, potentially into oblitteration. Image processing can remove this excess photon count. This is a faculty essentially lacking in film photography, except by truly heroic and fanatic techniques. Even for silicon imaging it is a chore. It is best to minimize the grayed pixels in the first place. Shoot only on the nights with abated luminance. The principal human-based cause of a luminous sky is luminous graffiti, also called 'light pollution'. This is the artificial twilight created by reckless and excess discharge of photons into the sky from outdoor lighting. You ideally should take star pictures from a site of subdued luminance, but you will be commonly restricted to the place you're at with little discretion to move to a darker one. You must assess the sky brightness and select those nights of the least imposition of luminous graffiti. Many lightings close after hours or on weekends Others turn on seasonally, like for yearend celebrations. In no case what so ever must you disable or damage an other person's lighting! This applies to private ones like yard lights and public ones like street lamps. Doing so exposes you to horrible civil and criminal liability, in addition, you will lose credibility as an astronomy point of presence in your community. For serious infringement of your quiet and peaceful enjoyment of life, apply at your civic and social affairs agency or the actual operator of the lighting. They can, and likely must, give due consideration to your complaint and realize at least some remedy.
Natural luminance --------------- Other sources of sky luminance can be natural. One is the 'white night' effect at high latitudes. Beyond +/-50 degree latitude the sky does not get completely dark, there being no end of twilight. Avoid this season, centered on the summer solstice, for dark starfields. Noctilucent clouds, still poorly understanded, seem to prevail in the summer months beyond the mid latitudes. They have been seen, and photographed!, in the +/-40 to 50 degree latitude band, even during geometric night time. Aurora is both a curse and blessing. It lights up the sky to ruin starfield pictures and it in itself is a prime, and rare, target of picture-taking. They can not be firmly predicted. The best is to follow the solar influx of electrons, as monitored by ground and satellite stations, and be outside when these readings are favorable. The main difficulty in photographing aurorae is that they are kinetic lights. They move swiftly ao that an exposure suitable for a starfield will smear the aurorae. This is why aurora pictures look so vague and blurred. It can be quite firm and definite in shape by eye. The Moon when large washes out the sky and hides the fainter stars. This is specially true for unnoticed haze or thin cloud. You may have to endure a large Moon if she performs an interesting show like a lunar eclipse or occultation. Be prepared for suboptimal pictures.
Temperature --------- The ambient air temperature and internal heat of the camera can produce static or noise in your pictures. This is disguised in ordinary pictures due to the brightness and texture of the scene. For star pictures this static appears as spurious flecks and dots that can resemble real stars. The newer models of camera are far better at preventing image noise and yours may be quiet enough to have only a few flaws in the picture. These can be dotted out in a picture editor if you are ambitious. For many such specks, you better do dark-field subtraction. This is a technique beyond the depth of my piece here. It involves taking a picture with the lans cap in place to exclude light from the sensing cell. Any 'stars' in the image are due to pixel excitation. This 'dark frame' is digital negated or subtracted from the target frame in a suitable image processing program to suppress these false stars. It's not usually possible to alter the air temperature. You should be mindful, like for visual stargazing, to avoid obvious heat sources like machinery, pipes, and chimneys. The sources of camera heat are the sensing cell, batteries, and viewing screen. If possible, wait about 10 seconds between shots to let these cool a bit. Try turning the viewing screen off if you can shoot without its displayed information. Some astronomers band or tape a cold pack, from picnics, to the camera. This may sound extreme, but recall that a real CCDgraph is cooled to -30C or lower by a thermocouple running in reverse before shooting pictures.
Weather ----- Of course, when ever an astronomer has a project to begin, the weather turns against him. This happened at yearend 2009 with cloud, haze, snow. For weeks I could not attempt sky pictures with the borrowed cameras. My friends let me hold their cameras over the year's end, normally filled with major picture-taking activity. Maybe they got new ones for the holidays? Like for film astrophotography, silicon sky pictures can not be taken in haze or thin cloud with thought of good success. Roving clouds with clear dark sky between them can be dangerous. A cloud can slide into your camera field and ruin a good shot. An other obstacle, commonly omitted from other astrophotography tuition, is snow. When the grund is blanketed by snow, the sky can be far more illuminated than without snow. Snow reflects upward any light shining on it, even sky-riendly lamps. What is peculiar for New York is that snow tends to STAY WHITE for as long as it persists on the ground. It no longer turns gray and black from collecting industrial pollution. The reflective snow can last thru many clear nights, holding photo efforts at bay. I spent the dead time reading the instruction books and test reports. I also manipulated the cameras indoors to see what the menus looked like and the ease, or otherwise, of navigating thru them. I shot pictures of a darkened room, with shades and door closed. I figured for the first trials on the sky I could shoot from an open window rather than from the ground. This allowed for a good night to be exploited under a frigid windy air. It was also easier to work between camera and computer without doing doors, stairs, coats. One gross advantage of silicon pictures is that I will not waste film or wait for the film processing. The digital image is pulled off to my computer and erased from the camera's storage card.
First attempts ------------ The first break in weather, both for sky clarity and air mildness, was on 2010 January 9. I spotted Mars rising in the east with Castor and Pollux above him. The Big Dipper was low in northeast upright on its handle. I tried each of the four cameras in the mode that seemed best for long exposures at infinity focus. I propped each camera on my window sill to aim at Mars and then at the Big Dipper. I used the self-timer to trip the shutter. I waited until the camera finished before replacing it with the next camera. I uploaded the pictures to my desktop computer by a storage card reader. When the camera card is slotted into the reader, it becomes an other disc with its own letter. I copied the pictures to the harddisc and examined them in Paint Paint Pro.
Reviewing the pictures -------------------- The picture on the camera viewing screen will probably look blank. You see the foreground but perhaps only the very brightest celestial targets. The screen compresses the full size image into a tiny, both logical and physical, area. On the computer screen, even at full-screen size, the picture may still look pretty empty. Again it's the compression of a large pixel dimension into a smaller screen area. Enlarge or zoom with the picture editor until you just start to see the individual image pixels. The picture becomes blocky like mosaic seen closeup. Then back off one notch of zoom. The stars each occupy a few pixels and may at first escape notice. By slowly panning and scrolling the stars are more easily spotted. They are distinct from blemishes by their short trails aligned with their declination circles. I found it is easier to inspect the image in subdued room lighting. Not a fully dark room, which makes the computer screen too harshly glaring. A room at night with ordinary lamps is dark enough.
Field distortion -------------- When you try to spread the curved celestial sphere onto a flat sensing cell, there is distortion. It's exacta mente the same as for any cartographic work. On the sky this distortion can be very ugly because you know what the star patterns look like for real. On the ground, a map can be strongly out of shape without you realizing it because you don't see what the territory it covers really look like. Nowayears, this deficiency is fading rapidly. You can pull an aerial or satellite picture of the territory from Internet and compare it with the map. There are several distortion patterns, but for the simple cameras discussed here it may be hard to learn which prevail to your model. Unless you are into astrometry or are making a star globe from your pictures, it's well to leave the distortion as is. There is one distortion, a natural one, you can actually use for astronomy demonstration. Photograph a constellation when it's rising or setting and again when it's near the meridian. Overlay the two and see that its stars are displaced by atmospheric refraction! This becomes noticeable in the overlapped images below about 15 degree altitude and is a maximum of about a half degree at the horizon.
Distortion -------- Modern lenses do away with the common gross distortions of the old days. These included coma, astigmatism, chromatic aberration, spherical aberration. It's rare to find, even in the cheaper digital cameras, these defects in an unacceptable degree. In starfield pictures there could be some minor instances of these distortions in star images near the edges of the frame. For the causal level of picture-taking they may be ignored. For astrometry and photometry, they can corrode the measurements into rejection. For field and point distortion the easiest work-around is to keep the main target near the center of the frame. This may require zooming out to shift widely separated targets, planets along the zodiac, closer angularly to the center. In my case, because I used the cheaper camera models, the mechanism that moves the lens, like to focus or open the cmaera, is a bit sloppy. Altho the focus was set to infinity, the loose parts at times slipped ever so sligthly to blur the stars. This slippage can happen by merely handling the camera to set it up for the next shot, without touching the lens settings. The image lost its crispness. My solution was to take three pictures of each scene and pick out the overall best-focused one. I played with much better models of dsigital fixed-lens camera and found their lens movement far more robust and smooth. I did not yet get the chance to test any ogf these models for star pictures.
Results ----- Of the particular four cameras I tried, the Panasonic LZ7 worked the best. It gave pleasing -- not great or wonderful -- images with its 'starry sky' and '15 second' setting. 60 seconds washed out the sky and made the star trails grossly too long. 15 seconds seemed at first a bit too short with too few stars captured. After a few more nights of experimenting I found the 8 seconds, with the cloth trick, did in deed capture about as many faint stars as likely possible. The Kodak DX6340 was the poorest performer, with a under-exposed sky. It's 'landscape' and '4 seconds' were able to capture only the brightest of stars in a way-too-dark background sky. The other two cameras were a bit peculiar. The Nikon gave so-so images with its still-automatic 'night' setting. The Samsung gave images between the Panasonic and Kodak. All images had little noise, having a smooth gradation of illumination (due to some luminous graffiti). Under a high zoom in Paint Shop Pro, there were small spots, resembling the texture of the cosmic microwave background, but they did not seriously degrade the picture quality. Stars were easily recognized for having short trails aligned with their declination circles. This actually was a bonus because stars of surprisingly dim magnitude could be spotted in apparently vacant parts of the sky. The magnitude limit was between 4-1/2 and 5th magnitude in high sky and 3-1/2 near the horizon, just over rooftops. Neither the Beehive nor the Rosette cluster showed up because their individual stars were fainter than 5th magnitude and did not impress onto the image. The brightest Beehive star, epsilon Cancri, is about 6-1/2 magnitude.
Postcapture processing -------------------- I prepared for my experiments by installing Registax and GIMP. I ended up not using them yet. The global enhancements thru a general- purpose picture editor were sufficient. I already had Paint Shop Pro, but any full-featured software is all that's needed. Paint in Windows and Photo Editor in Office, in a dire pinch, will accomplish some postcapture processing. Of the initial bunch of about ten images from the four cameras, i picked two as the best. Both were from the Panasonic LZ7, one of the Big Dipper and one of Mars and Cancer. All pictures opened without choking in Paint Shop Pro. The large image size overflowed the monitor screen. I zoomed out to get a more or less full screen panel. A low glow dome sat in the northeast, under the Big Dipper but the high sky was pleasantly clean and dark. It still, after two decades of aggressive darksky agitation, amazes lower-level advocates that New York City has remarkably dark skies. Eye transparency attains 5th magnitude on the best nights, 4-1/2 on the normal nights. The histogram, a plot of pixel value against number of pixels looked quite good. Just about all pixels were on the low end of the scale because just about all are in dark sky or foreground. The high end had a spike. These were the stars and also lamps and windows. The Panasonic can show the histogram on the viewing screen but not the others. I examined the histograms in Paint Shop Pro. I first converted the images to gray-scale. This removes several degrees of anarchy for further processing. For future tests I may switch the color mode in the camera to 'B&W'. I next boosted the contrast to compress the low end in the histogram and, hopefully, make the sky darker against the light points of the stars. By trying various degrees of contrast increase, I came up with a good compromise between stars on a darkish sky and stars smothered out of sight in a black sky. I saved the final images under new names.
Finishing the picture ------------------- The last thing in processing was adding labels for many of the stars with the 'text' feature of Paint Shop Pro. I compared the picture against a planetarium program set to the same date and hour. A last touch was a title along the bottom of the picture. I adopted the practice of putting in one lower corner of the image the name, location, date, hour of the picture. I do not use the camera's date stamp function. The lettering is plain grotesque for star pictures. This continues my long-standing strategy of writing the photo specs in the mount of chemoslides. Do this soonest after collcting the pictures!! It is so very easy to lose track of date and hour even only a couple days after the shoot. Because I now generally take several pictures over many minutes, I note just the hour, skipping the minutes. I do not at first put the main subject, usually constellations in the image yet. That comes later, after I thoroly inspect the picture. The subject is noted in the opposite lower corner. After doing this for the first few runs of skyscape picture-taking I no longer give the image a subject. The star labels within the image are enough. Should you label any objects in the scene? It's up to you. I do label stars with their abbreviated Bayer or Flamsteed names. I tend to label enough to delineate the constellation figure but there are lots more stars I leave unmarked. I do not label any landscape features, they so far being houses on my street. For the title and labels I use a small typesize to lessen interference with stars near them. In full frame view on the computer or a paper printout, the labels are tiny but legible. When I zoom in to scrutinize the stars the labels are a bit indelicate. I do not draw the catch-figures of the constellations or the boundaries. These are not seen in the sky. I also don't add compass rose or scale of distance. I take a minimalist approach to cluttering up the picture.
Tripods ----- I at first just set the cameras on my window sill with books and small tools to shore them into proper aim. For any proper sky photography you better have a tripod. This is a generic term for a stable steady stand to aim and support the camera. It doesn't have to be the three-legged pyramid with the camera perched on top. It can be books or doorstops on a table, sandbag or beanbag, pole with camera strapped to it with bungie cords. As long as the camera can be aimed at the target and then stay still for the picture-taking, the support is adequate. On one occasion at a convention I photographed a Venus-Jupiter conjunction with my film camera. I strapped it to a lamp shade in my hotel room with a extra shirt! the picture came out quite well! It seems that every simple digital camera has a tripod socket, even those that only allow snapshot pictures. This socket has a 1/4x20, oldstylr spec, thread for standard tripod heads. Being that digital cameras, even the SLRs, are so much lighter than equivalent film cameras, the tripod may be of a lesser strength and weight. This makes them easier to carry and pack for travel. There are awful models that let the camera quiver and wobble. Get one that is in fact a sturdy build. There is one ridiculous handling of a tripod I see in ads and in show and store displays. The tilt handle, the longest of the set on the tripod, is virtually always sticking back into the astronomer's chest! In this position not only is there obvious discomfort, the tilt angle is often limited to about 60 degree altitude. The handle bumps into the tripod legs or a limit notch on the tripod head. Turn the scope or camera around so the tilt handle aims AWAY from you, into the sky. This both makes life a lot more comfortable for you and gains a far greater altitude range. The handle could intrude into a wide-angle camera field. In such a case, swop it for a shorter handle from an other axis. One refinement I now use is a laptop wedge. This is a plastic tray with an adjustable tilted rack. A laptop sits on it to vent warm air from under it. Vent slots on a laptop are typicly on the bottom where thay are blocked by setting the instrument flat on a table. A smaller model of wedge is for calculettes, this time to tilt its display and keys for easier use. The camera sits snugly on the rack, adjusted for the altitude of the scene. The wedge then fits snugly into the window sill for a stable support.
Field orientation --------------- A tripod allows for a much more free alignment of the camera field with the stars than makeshift supports. Depending on the scene to hand, you aim the camera on the tripod to one of two orientations: portrait or landscape. They refer to aligning the sides of the picture against the stars. Landscape lines up the frame verticly and horizontally. The scene looks like the eye view, best for twilight or where the foreground is an important element of the scene. In portrait alignment the frame is set north-south and east-west in the celestial coordinate grid. The scene resembles a star atlas plate with north at the top and east at the left (for northern astronomers). The landscape orientation is achieved by just about any tripod with a tilt-&-pan head. For portrait orientation you need a triaxial tripod. In this design, of which the Star-D and Tilt-All were the archetypical examples, the lowest axis pointed the upper part of the head along the polar axis of the sky. The other two axes then moved the camera in right ascension and declination. Portrait is also favored for operating small telescopes on the tripod. It makes finding and following targets a lot easier. Only one knob is loosened to nudge the scope along. The ASCII diagrams here help visualize the two methods: ------------------------------------------------------- T +-------------------+ (toward NCP) x | @ , * *. ,| N / \ L | @@ ( * | R /* . \ W | @@@ , * @@ | / . \ |=@@@@=======@@@@===| x .* . . \ +-------------------+ \ # * x B \ *. . / landscape with foreground (@, E \ . / =), stars (*,.), and Moon (() \ / S x portrait with stars (*, .) and a nebula (#) -------------------------------------------------------------- Camera handling ------------- In low light you can not see your camera's buttons and markings. You have to operate the camera by feel, with clues from the viewing screen. However, you may have turned off the screen to remove its bright light and save battery power. Shining a pocket torch on the camera ruins your night vision. It also adds an other item to lose, break, or go out of order. . Practice working your camera in a darkened room or with eyes covered. Do so on soft ground, your bed or sofa, in case you drop the camera. Include unpacking, loading batteries and card, fitting to the tripod, taking it down, packing it away. Look for tactile clues, like texture and shape. Be careful with any loose small parts, Dropping them may force you to do without them, possibly aborting your shoot. Buttons needed for the shoot may be emphasized with a bit ot tape. Others that should not be pressed may be covered with larger tape. Masking tape is good because it has a definite rough texture and can be removed later without marring the camera. On the occasions when you want the viewing screen turned off, try making all the proper settings first and then turn off the screen. This presumes a thoro understanding of the interaction among the parameters, where altering one may shift an other without warning. IF the viewing screen is in an awkward location to look at directly, have handy a small mirror. Inspect the screen reflected in the mirror, it being, ern, mirrored. Learn how to read mirror-writing.
Aiming the camera --------------- It is as tough to aim a digital camera at a sky target as for a chemocamera. Some digital cameras do not have optical finders and the viewing screen will likely show almost no stars at all on it. You have to sight along the top of the camera, just like for the film camera. Recall that in the tiny finder of a film camera, just about nothing in the sky was discernible. For tripods, extend the legs so you work the camera standing up or properly seated. Don't contort yourself to work the camera on too short a stand. Comfort is a virtue in astrophotography! Things are a lot easier if there is landscape in the picture. Frame the shot against that. In twilight you may gauge the aim by the sky glow. For targets in high sky with no landscape or twilight to guide you, the aim can be rather erratic. If there is some physical structure on the camera against which to band or tape a sighting tube, try that. The tube can be cut from the cardboard core of toweling. A sipping straw will be much too narrow and its interior will reflect too much light into your eyes.
Choosing an RF camera ------------------- It can be tough in a store to select the suitable model for astrophotography. The instruction book may not be available, it is clumsy and distracting to read it at a counter, and other customers want to be served. For sure the clerk will not understand what you want to do with taking pictures of the stars in the sky, not those at the stage door of the television studio. He may be sincere but blissfully ignorant of celestial photography. And he can't spend all day just with you. As a first cut, and this is not really a solid rule, if several choices within the same brand are offered, skip the lowest level and start inquiring from the mid level models. After all, my Kodak cost only $100 as a used camera and the Samsung cost its owner only some $60 (by memory from a year or so ago) as a closeout item. You could be blessed to be in the league of other astronomers, like from an astronomy club, who can lend you their cameras or bring them to a rally of starfield photography. Let them show you the settings and results. In the end a good RF camera suitable for starfield photography should, in yearend 2009, cost no more than $200 and likely as low at $100. These are for new items, not used. If your friend's camera which you want for astrophotography is a year or more old, you can seek out a used specimen at the larger photography emporia or camera repair shops. You may also seek the current equivalent model with the same desired features.
Conclusion -------- This run was only a first cut at digital starfield photography. However, it proved that with some ordinary digital range-finder cameras entirely adequate and pleasing skyscapes are feasible. It also showed the wide differences in manual control allowed among brands. For selecting a new camera that will be used also for starfields, careful examination of the manual capabilities is essential. The instructions and tests have to studied carefully and the camera has to be played with a bit to see what can be set on it. Of the cameras I had the Panasonic was the easiest to set for astrophotography, requiring only two actions. The scene is set to 'starry sky' -- which seems by the instructions intended for astrophotography! -- and the shutter is set for one of the 15-30-60 second speeds. Focus is locked at infinity and the lens is opened to its widest aperture. The others (leaving out the Nikon) needed drilling thru several menus to get at the best settings.