Astrophotography  for  Beginners

 

NOTE: This article is excerpted from a  copyrighted manuscript (Astronomy for Beginners) written by me before CCD cameras became the standard for astrophotography. It is therefore focused on film photography with only a few mentions of CCD. However, the reader should note that current (~2004-2005) digital cameras, even the point & shoot variety, can also be used for some types of astrophotography and with very little change in procedure. Certain other chapters in the original manuscript may be cited in the text below but are not yet available on this website. All rights are reserved and reproduction without explicit permission is prohibited. 

Last updated 16 May 2005.

 

Introduction

Having a picture of a colorful nebula or star cluster is one of the best rewards for a long cold night spent tracking stars. (Being able to impress your friends with a picture of the wonders of space isn't bad either.) In this article we'll explore getting started in astrophotography with film.

Taking photos that you and your friends will enjoy is definitely within the reach of the beginning amateur. You may get the impression from reading magazines and more advanced books that taking astrophotos is difficult and requires expensive equipment. However, you don't need expensive equipment, you don't even need a telescope; you can get great pictures with just a camera and a sturdy tripod. Four important rules for getting started in astrophotography are: 

1. Read Covington (see Links). I assume that for general astronomy you already have a copy of The Backyard Astronomer's Guide by  Dickinson & Dyer (see Links) and a good guide to finding your way around the night sky. For getting started in film astro-imaging,  Michael Covington's Astrophotography for the Amateur is a great beginner's book and worth every cent. Several other excellent books on imaging are available (see Links) for reading after you've digested Covington. If you are plunging right into CCD astroimaging, you should definitely read Ron Wodaski's book, The New CCD Astronomy.

2. Shoot lots of film (frames, if you're using a digital camera).  Film (and digital memory) is cheap compared to most of the other expenses of life so don't hesitate to take several shots of the same thing; bracketing exposures (taking several shots with different exposure times) is very cheap insurance against a lost opportunity.

3. Experiment. Try new things, even if they break all the other rules laid down here and in other books; as long as you don't overload the mechanical parts of your setup or burn up your system with too much sun, there is very little you can do that is dangerous.

4. Keep records of everything you do.    Remember to keep notes of every exposure and review those notes when you get your images back—this is how you'll learn what works and what doesn't.

In this article I discuss the equipment and methods used for capturing astronomical images and some of the easiest and most beautiful astronomical targets for beginners. This will get the image onto the film or digital sensor. Once you have the image, an astrophoto often requires a bit of tweaking to bring out the best features. Another article (not yet available on this website) will deal with computer-based methods for getting the most out of your images and doing it at minimum expense.

If you're a confirmed observer without a camera you may want to skip this article. On the other hand — what have you got to lose? Why not borrow or buy a cheap used camera? Who knows? If you give astrophotography a try you just might discover you love it!

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Astrophotography Techniques & Targets

There are many different ways of capturing images in a permanent and reproducible form. For images of astronomical objects, traditional film cameras and charge-coupled device (CCD) cameras are the most practical methods at present. CCD cameras designed specifically for astrophotography can be expensive ($900 and up—way up!), require the use of a computer, and have a significant learning curve. In this article, we'll mention them only briefly (see the box below: CCD astrophotography).

Astrophotography with Film

Almost any through-the-lens reflex camera can be used for film astrophotography. Furthermore, the most interesting method for astrophotography through a telescope requires removing the camera lens. Thus, a 35 mm reflex camera with a removable lens is by far the most practical and popular astro camera. It doesn't have to be the latest and greatest electronic Nikon or Canon, however. In fact, the older mechanical models are better for our purposes than the fully electronic marvels that are being produced now. Almost any brand (Canon, Leica, Minolta, Nikon, Olympus, Pentax, Yashica, etc) will do, but it must have a time or bulb exposure shutter setting for long exposures. (Most removable-lens 35mm reflex cameras have such shutter settings.) You will need a few special accessories for attaching the camera to your telescope when you take photos through the scope. If you happen to have a medium format reflex camera (one that uses 120 or 220 roll film) you can use that and get superb results — many advanced astrophotographers use medium format. Unfortunately, the number of suitable astro films available in 120 format is very small.

CCD and CMOS digital astrophotography. CCDs — not film — are used by professional astronomers. Many advanced amateurs have at least one CCD camera that has largely or completely replaced their film cameras. A major advantage of CCD and CMOS digital sensors — in addition to their great sensitivity — is their linear response to dim light. They have no "reciprocity failure" (see box below: Hypered Film). Other advantages are immediate feedback (was the image a good one?) and their somewhat reduced insensitivity to light pollution. If you have a videocamera or a digital camera, you already have one type of CCD/CMOS camera. The major differences between these daylight digital cameras and astro CCD cameras are the higher sensitivity and lower image "background noise" of astro CCD cameras at low light levels. The low noise level is achieved by electrically cooling the CCD chip to low temperatures, a relatively expensive proposition. In astrophotography, video and consumer type digital cameras are really only suitable for imaging the sun (with appropriate filters, of course!), the brightest planets, and the moon. If this group of subjects is a major interest, then a digicam can be a very useful imaging device — as shown by many excellent images on the web. The more expensive digital cameras (denoted DSLRs, for digital single lens reflexes) have removable and interchangeable lenses, so they can be used exactly like the film single lens reflex cameras while taking advantage of the digital process. They are (mostly) cheaper ($800 to $8000)  than cooled astro-CCD cameras ($3000 to $8000) but carry with them the disadvantages of single shot color and lack of built-in cooling. 

 

Breaking the speed limit: fast and slow films

It's a lot darker at night than during the day, right? Even a huge galaxy like the Andromeda galaxy (M31) can't provide many photons per second for exposing your film — it's more than two million light-years away. So how do you get enough starlight into a camera to affect film that is designed for daylight? Most telescopes have focal ratios of f4.5 to f10. You need fast film or long exposures; there's no other way. Frequently, you'll need both the fast film and relatively long exposures. For nebulas "long exposures" means 15 to 30 minutes with a really fast film, and  up to 2 hours exposure for a slow or medium speed film. I recommend that you start off with the fastest film you can find in order to keep the exposure down to 15–20 minutes. Longer exposures are difficult to accomplish successfully until you've had some practice.

Either color or black and white film can be used in astrophotography but starting with color will provide a sampling of some of the colors of stars and nebulas. For piggy-back photos (discussed below) with a fast camera lens (f1.8-2.0) a 400-800 speed film is fast enough. For your first attempt at through-the-scope imaging, get the fastest standard 35 mm color film you can find. A print film with an ISO rating of 800 to 1600 is good (versus ISO 50–200 for ordinary daylight snapshot films). For example, Kodak  Royal Gold 1000, Fujicolor NHG-II 800, or Konica Centuria 800. (ISO is a speed rating; it has the same significance of the old "ASA" rating.) These films can be processed by any standard one-hour film processing lab, so no special handling is needed. However, you may not find these films on the shelves of the local pharmacy. Try your local photo specialty shop or  the larger mail-order photo outfits (see Links). Avoid films that claim "4-layer technology" such as the "new, improved" Fuji Superia and Kodak Supra series. The fourth layer in these films results in an unpleasant color shift when exposed for the long periods needed for astrophotography.   Kodak and Fuji both make slide films in the ISO 800–1600 range that are fairly easy to obtain and give good images of nebulas with 15 to 30 minute exposures. A technique called "pushing" can be used to increase the apparent speed of most color and black and white negative films (see box: Push processing, in Chapter 11). Note: the very fast Konica film, SR-G3200 previously recommended in these pages has been discontinued by the manufacturer.

The disadvantage of any very fast film is its coarse grain: your color negatives cannot be enlarged beyond 8x10 inches without showing a lot of grain. That's all right; an 8x10 enlargement of your own shot of the Orion Nebula is a mighty attractive thing to have on your wall! Furthermore, there are digital processing techniques for reducing grain (see Chapter 11). Don't buy a lot of really fast film; after 5 or ten rolls of it, you'll want to experiment with slower, finer grain films and longer exposures. I've had good results with Kodak professional films in the 400-640 ISO range (old Supra 400),  Konica Centuria 800, and Fuji 800 NHG color print films. Kodak Royal Gold 400 has also been recommended. These films are not truly  fine-grained but by  "stacking" two or more images of the same object, the grain can be averaged out quite effectively  (described in the link in the next paragraph). For slide films, I have had good results with Kodak EliteChrome 200, Kodak Ektachrome E200, and Fuji Provia F 400 film. Among the films that are not good for long-exposure astrophotography are Kodak Supra 800, any of the Kodak Portra films, and Fuji Superia films (see above). 

The need for high speed film does not apply to filtered sun, or unfiltered moon shots. Compared to stars and nebulas, the moon is a very bright object and ordinary 1/60 to 1/250 second exposures work well even with a film of ISO 100. A good rule with astrophotography shots is to bracket your exposures, i.e., take at least three exposures of the object: one at three times the estimated correct exposure, one at the estimated exposure, and one at one-third the estimated exposure. For example, if you think 1/100 is the perfect exposure duration, take a 1/30 second shot, then the 1/100 exposure, and follow up with one at 1/250 second. Better to take more frames than to lose a great image. Obviously, this is not practical if the estimated exposure is 2 hours. For such dim objects, it may pay to take several pictures with the same duration. Advanced digital techniques permit combining multiple images with a reduction of grain and noise (for example, see http://www.astropix.com/HTML/J_DIGIT/TOC_DIG.HTM).

A reputedly good, very fast black and white film is Kodak T-Max 3200. I have no personal experience with it, but reportedly this film can be push-processed with reasonably good results up to a daylight speed of 12,500 or even 25,000! Ilford also produces a very high speed black and white film that can be push-processed to similar speeds. You definitely won't get that kind of speed with astrophotos (see box below: Hypered film) but one of these films is worth an experiment if you need short exposures (eg, you're not using a tracking mount) and you don't need very fine grain.

At the other end of the speed spectrum was Kodak Technical Pan 2415 film. Kodak stopped production in 2005, much to the grief of astro-images and black & white terrestrial photographers. This was a much slower black and white film with ISO about 25–200 unhypered, depending on development, for astrophotos. When hypered, its speed approached 800 ISO. However, it has extremely fine grain, high contrast, and extended sensitivity in the red (for nebulas), properties that make it a favorite of really serious amateur astrophotographers.  You may still be able to buy it from the Lumicon (listed in the Links). However,  you will have to develop it yourself or take it to a professional lab.

Hypered film. Photographic film has a peculiar habit of becoming less and less sensitive to additional light as exposure time is prolonged. A full explanation of this phenomenon (called reciprocity failure) is complicated, but one important result is that, under dim light conditions with many minutes exposure, doubling the exposure time does not result in a proportionate increase in image density. This contrasts with the film's behavior in daylight, in which twice the exposure does produce the expected increase in density. In the dim light of astrophotography, doubling exposure time from, say, 15 minutes to 30 may result in only negligible increase in image density. The bottom line is that at very low light levels, ISO film speeds don't mean as much as they do in daylight. In fact, the "true speed" at 100 minutes exposure may be one tenth of the speed at 1 sec exposure (or even less).  Many years ago astronomers discovered that treating film in various ways before exposure could partially reverse reciprocity failure, i.e., film could be made hypersensitive to dim light. Film treated in this way is therefore called hypered film. Some expert amateurs hyper their film but it's not necessary if you stay with fast or medium speed films. If you decide you want to try it, you can obtain commercially hypered film as well as hypering equipment from Lumicon (see Links for address).

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Other general suggestions

To reduce vibration, always use a cable release to trip the shutter. Buy a cable release that has a self-locking plunger so that it doesn’t have to be held down for the full 30 to 90 minutes of exposure time. The self-locking types have a built-in catch or thumbscrew that holds the plunger once it's depressed and remains there until released.

With single lens reflex cameras, the vibration caused by the mirror swinging in the camera can ruin a shot so if your camera has mirror-lockup capability, lock the mirror in the up position before taking the exposure. If—like most cameras—it doesn't have mirror lockup, use the delayed shutter release timer so that pressing the cable release flips the mirror and starts the timer: ten seconds later, when the shutter actually opens, all the vibrations should have damped out. For exposures of more than a minute or so, use the "hat trick." Place a black piece of cardboard (or a hat) over the objective of the scope. Open the shutter of the camera. After ten seconds, gently remove the cardboard (or hat) and start timing the exposure. At the end of the exposure, replace the cardboard or hat and then close the shutter.

If you have a light pollution filter (Chapter 2, Table 2-4), you'll find it most useful in photographing nebulas. Very often the limiting factor in nebula shots is the maximum duration of exposure that you can use without getting too much light pollution fogging of the film. With a pollution filter you may be able to double or even triple the exposure time before getting significant fogging. This is because the polluting light has specific wavelengths that are cut off by the filter. Photographs of galaxies and star clusters cannot usually be improved as much but may still be helped by the addition of a filter.

If you store your film in the refrigerator (a good idea), let it come up to the outdoor temperature before opening the cannister, otherwise moisture may condense on the film.

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Taking the Pictures

Imaging the sky with a fixed camera and no telescope

The experience that got me into astronomy was taking pictures of the Hyakutake and Hale-Bopp comets. I was totally uninformed about the movement of comets relative to the stars and had no equipment for tracking, so I simply set up my camera on a tripod and shot away at the comets using a variety of lenses, films, and exposures. To my delight, some of the pictures turned out quite well (for example Hyakutake, Hale-Bopp). Take a look at almost any issue of Sky & Telescope or Astronomy magazine and you'll find that many of the photos are views of constellations or other wide areas of the sky made with standard camera lenses, not telescopes. (The ones with a great amount of detail are made with the camera tracking the stars on a piggy-back mount that uses the telescope's tracking drive, which we describe later.) Very attractive shots can be made without a telescope, just using fast film and a reasonably fast lens with the camera mounted on a sturdy tripod.

For pictures with a fixed camera, try any film from  ISO 800 to 1600. Use a 50–80 mm f/1.8 to f/2.8 lens and 20 seconds to one minute exposure. You can get great pictures of major constellations and comets this way. The longer the exposure and the faster the film and lens, the more stars will be recorded. Unfortunately, with a fixed camera you can't go much longer than the above exposures without getting star trailing; the apparent movement of the stars during the exposure (actually, of course, it's the movement of the earth that's being recorded). Furthermore, the apparent motion of stars across the sky at the celestial equator is faster than at the poles, so constellations directly overhead and nearer the equator will be limited to shorter exposures than objects near the celestial pole. Most books suggest that for a 50mm focal length lens, exposure should be limited to 12 sec for objects on the celestial equator, 20 sec at 45 degrees declination, and 50 sec near the poles. For a 200mm telephoto lens these suggested limits are cut to 3 sec, 5 sec, and 12 sec, respectively. If you don't enlarge the image too much you can get away with somewhat longer exposures.

A very attractive way of turning the earth's movement into an advantage is to maximize the star trails effect, i.e., take as long an exposure as the darkness and your patience permit. This requires a stable tripod and is best done with a slower film (ISO 50–200) to reduce film fogging by light pollution. A light-pollution filter can also help reduce fogging of the film (see above), but may also reduce star brightness. If you have a clear view of Polaris, you can get perfect semicircular trails. If you're lucky, you may catch a meteor crossing the field of view. Remember that the stars "move" at 15 degrees per hour, so you need very long time exposures for long trails, e.g., 6 hours for a quarter-circle.

If you have a medium or long telephoto (200–400 mm focal length), you can get some great shots of the moon over some favorite area of landscape. The moon is so bright that the main problem is avoiding overexposure on films faster than ISO 100. Try 1/125 to 1/500 sec at f3.5 on ISO 100 film for a beautiful full moon. If you have an extender for the tele lens (one of those 1.4x to 3x couplers that fits between the camera and the lens) you may get the moon image large enough to recognize some larger lunar features. (To fill a 35 mm film frame with the moon requires a very long lens—or your telescope—with at least 1000 mm focal length.) 

Pictures with a camera piggy-backed on the telescope

Piggy-back pictures are taken with a standard lens on the camera and the camera mounted on top of a telescope, not at the eyepiece end. An alternative is a "barndoor tracker" (see box: Barndoor trackers). The purpose of the piggy-back or barndoor arrangement is to borrow the star-tracking capability of the mount to get nice sharp untrailed star images. The difference between these pictures and the ones taken on a tripod is that much longer exposures (up to several hours) can be used if the camera is accurately tracking the motion of the stars; longer exposures permit much dimmer objects to register on the film. With an f2.8–3.5 lens on a piggy-backed camera, use 1–2 minute exposures to capture the brighter stars. Exposures of 5 to 15 minutes on ISO 800 film will show many dim stars and some of the larger, brighter nebulas. Occasionally one of the planets is close to the moon and you can get a stunning shot of the two together with an ordinary telephoto lens (for example, Venus, Saturn, Aldebaran, and the Moon) .

Barndoor trackers. You don't need a telescope for piggy-back photos; some great multi-minute shots have been taken with barndoor trackers. These devices are home-made platforms that connect a camera support to a tripod-mounted base with a hinge. A screw device rotates the support on the hinge at the same rate as the stars are (apparently) rotating above the earth. In simple barndoors, the screw is turned by hand; the fancier ones have motor-driven screws. The barndoor is a special form of equatorial mount and must be aligned with the celestial pole just like a regular equatorial mount. The Backyard Astronomer's Guide (Dickinson and Dyer, Links) has a discussion of barndoor trackers and a photo of a hand-driven version. A web site with good photos and construction details for barndoor trackers is:   http://www.albany.net/~zardoz/barndoor.html.

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Pictures through the telescope — variations on a theme

There are several ways of taking pictures through a telescope. Covington describes them all in Astrophotography For The Amateur, with diagrams and the methods for figuring out magnification, etc. In brief, the three most common through-the-scope methods are: afocal (leaving the eyepiece in the scope and the lens in the camera); prime focus (removing both the eyepiece and the camera lens and mounting the camera directly on the eyepiece tube of the scope), and eyepiece projection (placing an eyepiece in the extender tube between the telescope focusing mechanism and the lens-less camera).

Afocal photography requires a bit of juggling to get things lined up because the telescope and camera are (usually) not physically coupled together. However, for a camera with a fixed lens, it may be the only way. This is usually the case for consumer type video camcorders and digicams (which rarely have removable lenses). Focus the telescope by eye as usual and focus the camera lens at infinity. Then place the camera (mounted either on the telescope or on a sturdy tripod) so that it is aligned with the eyepiece. If the camera is a through-the-lens reflex type, you will see exactly (if somewhat dimly) what the film will see, so focusing and framing is easier than with a nonreflex camera. This method is more difficult than the prime focus method but can be surprisingly effective with modern digicams on the moon and planets.

With prime focus photography, the camera is substituted for the telescope eyepiece. Put another way, the telescope replaces the regular camera lens, becoming a really long telephoto lens. (Ordinary photography with regular camera lenses is prime focus photography and follows the same rules.) This method has several advantages. First, the field of view for the camera-telescope combination is predictable and similar to that for a normal eyepiece-telescope combination. Second, the number of glass surfaces between the scope's primary and the film is minimized, thus maximizing the light that reaches the film and reducing reflections and distortions. Finally, a very solid connection can be made between camera and telescope, so the camera will track precisely with the scope during a long exposure. For mounting the camera on the eyepiece end of the scope, you will need a mounting tube and a camera "T adapter." The tube connects the telescope to one end of the T adapter; the other end of the T connects to the lens mount of the camera. Astronomy stores and mail-order firms have extensive assortments of these adapters for 35 mm cameras; just tell them you need a mounting tube and T adapter, give them the name of your telescope and your camera make and model, and they can usually provide the necessary gadgets for $30 to $50. (Note that digital SLRs are mounted on telescopes with the same adapters, because they use the same lens-mounting fixture.)

For eyepiece projection photography, you alter the connection used for prime focus photography. This mode of operation requires that an eyepiece (but not the camera lens) be present in the optical path between the primary mirror or lens and the camera. To accomplish this you need an extendable camera mounting tube that also accommodates an eyepiece and clamps it firmly in place within the tube. The purpose of eyepiece projection is to provide higher magnification. This is very important for pictures of planets because most telescopes at prime focus give a tiny image of the planet. For example, a typical SCT telescope of 2000 mm focal length yields a film image of Saturn only about 0.44 mm (less than 1/32 inch) in diameter! With a short focal length eyepiece in the tube, the image can be magnified to a 1/2 inch or more (see below). However, precise focusing is much more difficult with eyepiece projection. Saturn and the other planets are definitely a challenge — even for the experts!

Which method to use? The size of the object to be photographed determines which of the above methods is best. As shown in Table 10-1 below, an ordinary 50 mm focal length camera lens is adequate for most constellations but even a 5000 mm telescope yields a very small image of Saturn. A simple formula determines the image size produced by an optical system of a given focal length when used in prime focus mode:

film image size (millimeters) =

object size (arc seconds) x focal length (millimeters) / 206265 arc seconds

For a large nebula like M42 in Orion (roughly 60 arc minutes [3600 arc seconds] in diameter) and a 2000 mm focal length scope:

image size = 3600" x 2000 mm focal length / 206265" = 35 mm diameter

Since a 35 mm film frame is approximately 35 mm long but only 24 mm wide, M42 would be too large for the frame. However, one can use an optical reducer lens (also known as a compressor or telecompressor lens) on most telescopes to reduce the focal length. The typical reducer for Schmidt-Cassegrains is a "0.63x reducer," meaning that it reduces the focal length of the scope to 63 percent of the unmodified value. (Reducers have the added benefit of decreasing the focal ratio by the same amount, ie, an f10 scope speeds up to f6.3.) With a reducer of this type on a 2000 mm focal length SCT scope, the focal length is reduced to 1260 mm and the image size of M42 becomes 35 mm x 0.63 or 22.5 mm—just about right for the 35x24 mm frame. Obviously, a reducer would not be useful for photographing Saturn; you need to increase the apparent focal length by means of eyepiece projection.

Calculation of image size is a bit more complicated when using eyepiece projection. First find the magnification (M) of the projected image:

M = (S2 – F2) / F2

where S2 is the distance from the eyepiece lens to the film and F2 is the focal length of the eyepiece. Thus a 14 mm eyepiece with a 140 mm eyepiece-to-film distance would give M = 9. Then calculate the focal length (F) of the telescope-eyepiece combination from

F = M x F1

where F1 is the focal length of the telescope primary. Thus, for a 2000 mm Schmidt-Cassegrain and a magnification of 9 with the 14 mm eyepiece,

F = 9 x 2000 = 18,000 mm.

Now the image size on the film will be 9 times larger: for Saturn, 0.44mm x 9 = 4 mm — a lot better than 0.44 mm! Unfortunately, the image will also be a lot dimmer because the focal ratio has been increased in the same way (from f10 to f90).

From a list of sizes of astronomical objects and the above equations, it's possible to generate a table of the focal lengths needed to achieve an acceptable image size of each (Table 10-1). Note that the apparent size of the diffuse objects (nebulas, galaxies) depends somewhat on the exposure: objects appear larger with longer exposures because more of their wispy outer portions show up on the film.

Table 10-1. Approximate target and image sizes of some typical photo targets for different focal length lenses (camera or telescope). Image size is calculated using the formula given in the text (see Which method to use?) Remember that a sharp negative or slide can be enlarged 5 to 15 times, depending on film grain. Under target size, d = degrees, m = arcminutes, s = arcseconds. Under image size, "Too large" = larger than 35 mm, "Too small" = less than 0.1 mm. (Note that the field of view for 35mm film is 6.8x10.4 degrees at 200 mm fl, 1.4x2.1 degrees at 1000 fl, and 17x25 arcminutes at 5000 mm fl.)

   

Image size (in millimeters) for the following focal lengths

Target

Target size

50 mm

200 mm

1000

5000

Big Dipper

29 d

25

Too large

Too large

Too large

Cygnus constellation

40 d

34

Too large

Too large

Too large

Double cluster in Perseus

1 d

0.9

3.5

17

Too large

Horsehead nebula area

10 m

0.15

0.6

2.9

15

Jupiter (alone) or Saturn

40 s

Too small

Too small

0.2

1.0

Jupiter (with four moons)

8 m

0.12

0.5

2.3

11

M13 (Hercules cluster)

18 m

0.26

1.1

5.2

26

M31 (Andromeda galaxy)

185 x 75 m

2.7 x 1.1

10.8 x 4.4

Too large

Too large

M33 (Pinwheel galaxy)

67 x 42 m

1 x 0.6

3.9 x 2.4

19 x 12

Too large

M42 (Orion nebula)

65 m

0.95

3.8

19

Too large

M45 (Pleiades)

2 d

1.7

7

35

Too large

M57 (Ring nebula)

2.5 m

Too small

0.15

0.7

3.6

M8 (Lagoon nebula)

45 m

0.65

2.6

13

Too large

North America nebula

3 d

2.6

10.5

Too large

Too large

Orion constellation

33 d

29

Too large

Too large

Too large

Sagittarius teapot

21 d

18

Too large

Too large

Too large

Sun or full moon

30 m

Too small

1.8

9.1

Too large

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PHOTO TARGETS

Constellations, Star Trails, and Star Fields

Without a doubt, Orion and Sagittarius are the hands-down winners for constellation pictures. The star patterns are familiar and there are lots of stars in the field of the main constellation outlines. Cygnus and the Big Dipper are close runners-up in interest and ease of recognition.

Star trails are usually made with the camera pointed at the north celestial pole but there is no reason why interesting pictures cannot be made in other quadrants of the sky, especially if some interesting foreground or horizon is included. Great pictures combining star trails with comets, meteors, and the aurora have been published.

A special category of star trail picture is the "brushed trail" photo. This method starts the exposure with precise focus for a fixed period of time (say 15 minutes), then continues with a slight defocusing (which produces a broader trail on the film) for another 15 minutes, then progresses to a further defocusing (for a still broader trail), and so on. In addition to producing a uniquely beautiful image, this technique brings out star colors better than any other. The method can even be automated, as shown in an article in the September 1998 Sky & Telescope magazine.

Star fields are areas of the sky, usually in the Milky Way, that are particularly rich in stars and nebulas. The Cygnus and Sagittarius regions are particularly good targets. Photos of star fields are usually made with a regular lens (28 to 100 mm focal length) on a camera piggy-backed on an accurately tracking mount. Exposure in the 15 to 30 minutes range at f4-5.6 on a fast film (ISO 400–1000) is usually sufficient for an interesting image. Use a light pollution filter unless the sky is very dark.

Clusters

The brightest and largest (and therefore the easiest to photograph) clusters are open clusters, e.g. M45 (Pleiades) and the Hyades cluster in Taurus. These clusters lie in the borderline area of size; Pleiades is small enough to capture with a 1000 to 1250 mm focal length but Hyades is too large; a long telephoto camera lens is required for it. Globular clusters are more distant and therefore smaller and fainter. M13 in Hercules is the best example. Globular clusters make excellent targets but require longer exposure and extremely critical focussing and tracking: the tightly packed masses of stars are reduced to a fuzzy mush if focus and tracking are not precise.

Deep Sky Objects (Nebulas, Galaxies)

The red emission nebulas (Orion, Lagoon, Swan, etc) are wonderful photo subjects. Some of the most beautiful astrophotos ever taken are pictures of the nebulas. What do you need to get the nebulas on film? In one word: photons! Nebulas are pretty dim objects; even in very large amateur scopes they are "dim fuzzies" to the eye. In an 8 inch f10 SCT scope, they are (except for the Orion nebula) very dim indeed. And we don't see color when pointing a scope at these targets because our eyes are not collecting enough photons to stimulate the color-sensitive cone cells in our retinas; only the black & white-sensitive rod cells are activated at the very low intensity; very long wavelengths of light are not detected even by the rods. In contrast, film collects photons just as long as we keep the shutter open; color film can also register the different wavelengths that we see as color when we view the final developed image. Thus, to photograph a dim object, you need long exposures—up to one or more hours for film of ISO speed 400 at f6-8. If the film or telescope you are using is very fast the exposure can be shorter, but it is difficult to image nebulas with less than 15 minute exposures, even with ISO 1600 film at f6-8. See Table 10-2 for some recommendations regarding exposures for various objects.

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Alignment and tracking. Long exposure times require that steps be taken to prevent star trailing. The first step is to make sure the telescope is properly tracking the sky on its right ascension axis. "Properly tracking" has two important meanings here. Number one, the scope must be polar mounted and properly aligned. An ordinary alt-azimuth mounting will not work unless you add a "derotator" device. Polar alignment requires that the telescope be mounted on a wedge or a German equatorial mount (GEM) to point the RA axis very precisely at the celestial pole. The second requirement for accurate tracking is that the inherent tracking errors of the telescope drive (and all drives have some error) must be corrected by manual or machine-assisted guiding adjustments. Even unusually precise (usually meaning very expensive) telescope drives will require such guiding corrections for any exposure longer than 10 or so minutes.

The requirements for successful long-exposure tracking are excellent alignment, followed by lots of patience in guiding! The basic principle in the most effective alignment method ("drift alignment") is to progressively refine your initial alignment by repeatedly correcting small errors. Guiding can be manual or automated and requires a separate guide scope fixed to the main scope or an off-axis guider on the main scope. Manual guiding involves viewing a guide star—while the film is being exposed—and entering (on the telescope control keypad) appropriate corrections that keep the guide star precisely centered in the viewing eyepiece. Automated guiding involves the use of a small CCD camera that monitors the guide star and automatically generates the appropriate correction signals. You can get detailed advice regarding precise polar alignment from the web pages listed in Links.

Sun, Moon

These objects are the brightest astrophoto targets. As noted earlier, the moon can be photographed with exposures similar to those used in daylight. In fact, very slow color films such as Fuji Velvia (ISO 40) or Kodachrome (ISO 64) can be used to get very nice fine-grained photos of the moon. The sun can only be photographed with a solar filter on the telescope because unfiltered solar rays focused by the telescope will promptly destroy a camera shutter at prime focus. The only exception to this is during the time of a solar eclipse when the sun is completely covered. Therefore exposure is determined by the density of the solar filter (Table 10-2).

A special consideration in moon photos of more than a few seconds is the special tracking rate required. Because the moon "moves" through the sky at a different rate from the sun or the stars, the standard sidereal tracking rate built into motorized mounts will not precisely follow it. Some mounts have several tracking rates: sidereal, solar, and lunar. (A few add a fourth rate, the so-called King rate, which accounts for the apparent accelerated rate of movement of objects near the horizon.) The special lunar rate should not be necessary for exposures shorter than about 1/30 second and only if the moon is a thin crescent will longer exposures be needed.

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Table 10-2. Some exposure times that give useful images. Exposure times assume no filters. With filters (eg, DeepSky) exposure time may often be doubled or more without excessive light-pollution fogging of film (see text). (s = seconds, m = minutes, h = hours). Data from various sources.

 

Object

 

Method, focal length, f ratio

Exposure

ISO 1000 film

ISO 200 film

Moon, full 1

Prime focus, 2000 mm, f10

1/500–1/1000 s

1/250 s

Moon, crescent, 25%

Prime focus, 2000 mm, f10

1/250–1/500 s

1/60 s

Sun

Prime focus, glass solar filter, 2000, f10

1/500–1/1000 s

1/100–1/250 s

Jupiter (no moons)

Eyepiece (20 mm) projection, 10,000 mm, f50

1/2–2 s

4–20 s

Jupiter (with moons)

Prime focus, 1250 mm, f6.3

1–3 s

4–8 s

Saturn

Eyepiece (20 mm) projection, 10,000 mm, f50

1–4 s

4–20 s

Venus or Jupiter conjunction with moon

50 or 100 mm lens, f2.8, fixed tripod

1/30–1/8 s

1/2–2 s

Cluster, Pleiades

Prime focus, 1250 mm, f6.3

15 m

20–30 m

Orion nebula, M42 or Lagoon nebula, M8

Prime focus, 1250 mm, f6.3

15 - 30 m

30 - 60 m

Cygnus or Sagittarius star field

Piggy-back, 50 mm lens, f1.8–2.8

10–15 m

20–30 m

Orion constellation or Big Dipper

Piggy-back, 50 mm lens, f1.8–2.8

1/2–2 m

2–10 m

Orion constellation or Big Dipper

Fixed tripod, 50 mm lens, f1.8

10–20 s

Too slow, trailing

Andromeda galaxy, M31

Prime focus, 1250 mm, f6.3

20–30 m

30 - 60 m

Sombrero Galaxy, M104

Prime focus, 1250 mm, f6.3

No data

60 m

Star trails

28 or 50 mm lens, f2.8, fixed tripod; slower films better (reduced sky fog)

Too fast

1–6 h with ISO 50 film

Auroras

28 or 50 mm lens, f1.8, fixed tripod

5–10 s

20–60 s

Comets2

Piggy-back or fixed tripod, 50–200 mm lens, f2.8

10 s–1 m

20 s–4 m

Meteors3

Piggy-back or fixed tripod, 24–50 mm lens, f1.8 (as fast as possible)

1–10 minutes

2–20 m

1 Or use 1/60 sec with Fuji Velvia (ISO 40) at f6.3.

2 Comets vary from very bright to very dim. Hale-Bopp, a very bright comet, gave good results with the exposures noted.

3 Because meteors can't be predicted, point the camera in the direction of the most frequent tracks and leave the shutter open as long as possible for your light-pollution conditions (try to avoid fogging the film). Take multiple exposures, using a wide range of exposure times.

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E-mail your comments to the author:  bkatzung@astronomy-images.com