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Copyright 2000-2009 Michael A. Covington.
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The content of this page was last revised 2009 March 30.

Errors remaining in the 2000 reprint

(p. 111 second full paragraph) "who can reach can reach" should be "who can reach".

(p 182 second formula) "p = 1 - (log s / log t)" should be "p = 1 + (log s / log t)".
(Thanks to Lee Lumpkin for pointing this out.)

Plate 11.1 -- Both pictures were flipped left to right. Fixed in the 2002 reprint, finally!

Errors in the first printing (corrected in the 2000 reprint)

(p. 22 col. 1) After "one good source of information" add "is the".
(p. 45) Fig. 4.14 is upside down.
(p. 48) "Pacific Standard Time" should be "Pacific Daylight Time".
(p. 53 Fig. 5.9) The eclipse of 2003 May 31 (in Iceland and Greenland) is annular, not total. The Antarctic eclipse mentioned in the caption is in 2003, not 2002. (Thanks to Adam Stephens for these corrections.)
(p. 54) Fig. 5.12 caption should say "February 16".
(p. 74 bottom of col. 1) "as large as 10 degrees" should be "as large as 19 degrees".
(p. 75 middle of first col.) "at least 1000 mm" should be "at least 100 mm" (with space between number and mm).
(p. 71 Fig. 6.6) At the left bottom edge of the figure, some text has been cut off. The line that leads to nothing should lead to a caption saying: "2.000 inches O.D., 24 threads/inch."
(p. 110 Table 7.1) In the note, change "rises at midnight" to "is highest in the sky at midnight." Thanks to Renato Langersek for this correction.
(p. 131) After "1/40 mm" add "(i.e., plus or minus 1/80 mm)" and change "40" in the formula to "80". Results will then agree with Table 8.4. (Thanks to David Randell for this correction.)
(p. 134 Fig. 8.19) The bright star at the lower right is Canopus, not Achernar. (Thanks to Phillip Hosey for this correction.)
(pp. 131, 314) "Pulstar" should be "PulsGuide." ("Pulstar" is another version of the same product marketed by Celestron.)
(p. 153 Table 9.2) The Nikon FM10 does have mirror prefire.
(p. 232) Figure 12.19 was taken at f/2.8, not f/8.
(p. 249) Formula should be:
Pixels per arcsecond = Focal length / (206265" × pixel size)
Thanks to James Ellis for pointing this out.
(p. 260) Delete "×" at beginning of second line of left-hand column. For a more substantial correction to this formula, see below.
(p. 288) For a more accurate lunar rate see the note to p. 288 below.
(p. 313) Misplaced accent mark in Senillé.
(p. 327) "Flop, mirror" should refer to p. 143, not 153.
(p. 330) Indent the 2 lines following "periodic error."
(Plate 3.1) "Michael" should read "Michelle."
(Plate 10.1) Picture is flipped left-to-right. (Thanks to Ulrich Beinert for pointing this out.)
(Plate 11.1) The lower image is flipped left-to-right. The upper image is correct. (Thanks to John Pane for pointing this out.) [In a misguided correction, the 2000 reprint got them both flipped.]

All were corrected in the 2000 reprint.

Updates and additions

(p. 10) 20-second fixed-tripod exposures work better under a town sky than in the country. The reason? Country skies are too dark; a bit of skyglow from city lights helps push the film into its responsive region. My 20-second exposures taken on Kitt Peak showed plenty of stars, but the background was pitch-black and the Orion Nebula was less visible than on comparable pictures taken in town.

(p. 25) Kodachrome 200 is a promising film for meteor photography because of its severe reciprocity failure. It catches meteors as well as any other 200-speed film, but sky fog does not build up; you can go ten minutes or more at f/2.8 even in a town sky.

(Rumors of its imminent discontinuation were apparently incorrect; for the moment, Kodachrome 200, though not Kodachrome 200 Professional, is still on the market.)

Infrared film is reportedly even better for meteors. Several observers have reported great success using fast infrared film (Kodak High Speed Infrared, Ilford SFX 200) to photograph meteors because meteors are bright in infrared light, there is little sky fog at infrared wavelengths, and the strong reciprocity failure of the film helps prevent sky fog buildup.

[Update] Whether meteors are bright in the infrared is disputed. As incandescent objects, they must be -- but in fact they also have strong emission lines, some of which are in the blue and green areas of the spectrum. My personal recommendation is to use infrared film but not use a red filter. The film will then pick up the whole visible spectrum as well as infrared, and the strong reciprocity failure of infrared film is advantageous. For more information see Ed Majden's meteor spectroscopy page and the online photographic handbook of the International Meteor Organization.

(p. 26) Kodak Recording Film 2475 was finally discontinued in 1999. This film was introduced in the 1960s for CRT recording (photography of oscilloscope traces). Except for its extended-red sensitivity, it was inferior to T-Max P3200 for practically all purposes.

(p. 26) The Geminid meteor shower is particularly easy to photograph because most of the meteors are slow and bright and the radiant is relatively high in the sky all night; you don't have to wait until after midnight to get a decent meteor rate. One first-magnitude meteor every five minutes is typical.

Ordinarily, in any shower, only the brightest meteors get photographed. Fast film and a dark country sky are helpful.

(p. 27) The Leonid meteor showers of 1998 and 1999 were disappointing in the United States but better elsewhere; visibility depends on what part of the earth happens to be turned toward the cloud of particles when we enter it. Very fine Leonid showers are expected to continue through 2001 or 2002, with better visibility from the United States. Most Leonids are swift and relatively faint; a dark country sky is very helpful.

(p. 38) You may be wondering why the exposures for the full and gibbous moon differ sometimes by a factor of 2 and sometimes by a factor of 4. The answer is that the exposures are rounded to the nearest standard shutter speed. Before rounding, they differ by a factor of 2.5. Occasionally, one of them gets rounded down and the other gets rounded up, making them differ by a factor of 4 instead of 2.

(p. 45) I can confirm that Kodak T400 CN, Ilford XP-2, and Kodak B+W Plus 400 are excellent films for lunar photography. They tolerate overexposure well, so you can make a vibration-free "hat trick" exposure even if the calculated exposure is two or three stops less. They are also almost grainless.

(p. 47) Although the umbra should theoretically be uniformly dark, in fact it is somewhat darker in the center.

The reason is as follows. If the earth had no atmosphere, the umbra would be uniformly pitch-black. In fact, however, the earth's atmosphere bends light into the umbra, resulting in the red glow of a total lunar eclipse. This glow is brighter, and often redder, at the edges of the umbra than in the center.

(p. 52 bottom) The duration of totality in solar eclipses actually ranges from zero to seven and a half minutes. If totality is shorter than half a minute, though, the eclipse probably isn't worth traveling to observe.

(p. 57) When using a sun filter on a Newtonian, be sure to cap the back end of the telescope; otherwise light will come in around the mirror.

(p. 62) Barry Gordon estimates that shadow bands were visible at only about 10% of the eclipses he has observed. They probably require a very clear sky, which is what I had when I saw them at the May 1984 annular eclipse.

(p. 69) It may seem odd that "f/8" is an abbreviation for "f = 8", but it is. All I can say is that usage is confused, and, on a camera lens, "f/8" actually means (in my notation) "F/8", i.e., aperture is focal length divided by 8.

(p. 65) Most of Europe had cloudy weather for this eclipse -- and, paradoxically, the clouds caused a few eye injuries! People stared at the partially eclipsed sun when it was dimmed by clouds. Unfortunately the clouds did not always reduce its brightness to a truly safe level.

The moral: Even a cloud is not necessarily a safe sun filter.

On the whole, however, eye injuries were quite uncommon. Needless fear and panic were, however, common, and the public (at least in Britain) received confusing advice from authorities on TV.

(p. 69) Many newer Newtonians are designed with photography in mind and do not require modification for prime focus photography. To check, aim the telescope at the moon, remove the eyepiece, and hold a piece of paper just outside the eyepiece tube, moving it back and forth until a sharp image of the moon appears on it. If you can get the image at least 60 mm from the end of the eyepiece tube, you have enough back focus.

(pp. 69-71) Lenny Abbey reminds me that with observatory telescopes, "prime focus" means "focus of the main mirror, with no secondary mirror." (Really large telescopes allow you to put film or a CCD there.) What I have been calling "prime focus" would be called "Newtonian focus," "Cassegrain focus," etc.

Amateur telescopes do not normally have removable secondaries, but the Celestron Fastar does; in that case one must distinguish between prime focus (sensu stricto) and Schmidt-Cassegrain focus.

In this book, "prime focus" usually means whatever you get by removing the eyepiece, without removing any other optical elements.

(p. 73) Commercial Schmidt-Cassegrain telescopes may perform somewhat better than Rutten and van Venrooij's computations indicate. Both Celestron and Meade reportedly perform some minor "adjustments" to the figure of the secondary, though the details are proprietary. (Meade tells me explicitly that they cannot tell me whether the secondary is spherical. However, George Paltoglou writes that he has had a Celestron 14 secondary tested, and it is spherical to a high degree of precision.) Note also that Rutten and van Venrooij's analysis ignores the effects of diffraction; star images that are not maximally compact in the spot diagrams may nonetheless be diffraction-limited.

(p. 75) (corrected) A very interesting adapter for afocal coupling of camcorders, digital cameras, and even full-size SLRs is the LE-Adapter, which grips the eyepiece and attaches to the camera by its filter ring.

See also an afocal bracket available from ScopeTronix and a very versatile one available from Broadhurst Clarkson and Fuller.

(p. 76) If you want to do afocal coupling with a digital camera, you need a tiny exit pupil because the lens aperture is often very small (< 1 mm). I've had some success imaging the moon with a Sony Mavica camera, an f/10 telescope, an x2 Barlow, and a 9-mm long-eye-relief (Vixen) eyepiece.

[Update] Newer digital cameras have larger entrance pupils (up to 5 or 7 mm, like your eye) and work well with any eyepiece that gives adequate eye relief. Experiment to find the best camera position. Turn the flash off, lock the focus on infinity, focus the telescope while looking at the camera's screen, and use the self-timer so that vibrations from pressing the button will die down before the picture is taken. This is a very good way to image the moon and planets. On the moon, auto exposure works well; on the planets, manual adjustment is generally necessary, but easy to do by experimenting.

(p. 80) After testing a number of negative projection setups on the moon, both teleconverters and Barlows, I found I had internal reflection problems with all of them. Positive projection with an eyepiece works better. In planetary work, internal reflections are unlikely to be a problem with any setup.

[Update] That was true with my older telescope (Meade LX3), but not with a Meade LX200 of the same aperture and focal length. The internal glare stops are better in the new model.

A strong advantage of negative projection is that it flattens the curved field of a Schmidt-Cassegrain. Positive projection increases the curvature.

(p. 83) Meade and Celestron x 0.63 compressor lenses are considerably more sophisticated than the simple compressor analyzed by Rutten and van Venrooij. They flatten the field and correct off-axis aberrations. I enjoy visual observing with a compressor lens and 32-mm eyepiece; the whole setup is much lighter and compact than a 2-inch-diameter diagonal mirror and 50-mm eyepiece would be.

Further notes about compression:

• It requires a lot of back focus. Only catadioptrics and some refractors can do it.

• Inherently, compression flattens the curved field of a Schmidt-Cassegrain.

• When you try to do the calculations, you may get results that don't agree with actual experience because:
  • The focal length of a Schmidt-Cassegrain changes appreciably with focusing. For instance, when you put in the Celestron x0.63 reducer and refocus, you actually increase the focal length about 50%. The reducer is actually x0.4 but the telescope gains some additional focal length from the change in mirror separation.

  • Small changes in lens-to-film distance make a big difference. It's best to make actual measurements by photographing stars of known separation and measuring the image on the film to determine the focal length. On a Meade 8-inch f/10 telescope with a Celestron x0.63 compressor and an Orion off-axis guider, I get f/5.6, not f/6.3.

• The x0.3 compressors marketed for use with CCDs will not reach focus with a film camera.

(p. 84) The Rayleigh limit is more formally defined as the separation of two stars whose Airy disks are separated by the radius (not the diameter) of one Airy disk. I thank Anton Jopko for pointing this out.

Since the eye works over a wide range of wavelengths, there is no single Rayleigh limit for visual observing, and as seen by the eye, the two stars are better separated than the Rayleigh limit for 550 nm would predict. Thus the star images do appear to be "just touching" even though their 550-nm Airy disks, strictly speaking, overlap.

(p. 87) Use a plain Beattie Intenscreen, not the kind with a split-image prism (which is not useful for astrophotography). The one I find handiest is the plain screen with a grid printed on it. The grid makes it easier to focus a Varimagni Finder or other magnifying device. See also www.intenscreen.com.

(p. 89) When using the crosshairs focusing technique on a Schmidt-Cassegrain telescope, you may be momentarily confused by the fact that the image normally shifts sideways unpredictably as you focus, because the mirror does not move with perfect smoothness. An add-on zero-image-shift focuser (on the back of the telescope) is helpful.

(p. 90) I'm told the SureSharp is no longer available, but more knife-edge focusing devices have appeared on the market. Noteworthy are Richard Shell's Stiletto focusers. These use camera-lens-to-C-mount adapters -- the same adapter that is used to put an SLR camera lens onto a TV camera -- so that the focuser can mount right in place of the camera body. Such an adapter would also be a good starting point for a homemade knife-edge focuser.

(p. 95) See note to p. 143 below concerning drive vibration.

(p. 97) Some more advice about weather and atmospheric steadiness:

• Try observing under a wide variety of conditions; don't give up too easily. Conditions are never good if you don't set up the telescope.
• In particular, don't let a few wisps of cirrus stop you from attempting lunar and planetary work; the air is often unusually steady then.
• Allow a long time for the telescope to equalize with the surrounding air; two hours is not too long.

(p. 110) The actual contrast of features on Jupiter and Saturn is quite low; your washed-out looking pictures are accurate, and it takes some effort to get high enough contrast and color saturation to show planetary features clearly. See Andrew T. Young, "What Color is the Solar System?" in Sky and Telescope, May 1985, pp. 399-403.

(p. 114) Another time-honored way to piggy-back is to mount the camera on the counterweight of a German-style equatorial mount.

(pp. 118, 133) More tips on polar alignment:

• Declination drift and field rotation are maximum at points 90 degrees apart. What this means is that you shouldn't just check drift in the area of the sky where you'll be photographing. Instead:
  • If you'll be photographing high overhead, check drift low in the east or west.

  • If you'll be photographing in the eastern or western sky, check drift high overhead.

• Assuming you have good declination setting circles (i.e., they read exactly 90 when the telescope is pointed exactly along its polar axis), then you can use them to check your polar alignment. Compare the true declination of stars (which you can get from a star atlas, or from the on-board computer if your telescope is computerized) with the reading of the declination circle. Then:
  • If stars near the meridian show the wrong declination on the setting circles, the polar axis is too high or too low.

  • If stars low in the east or west show the wrong declination on the setting circles, the polar axis is misaligned to the left or right.

  In either case, the cure is to move the mount so that the telescope will be aimed at the star while its correct declination is shown on the setting circle.

Not much, because of field rotation. Suppose you're at latitude 40 north, photographing an object on the celestial equator, due south of the zenith. The field will then rotate 0.2 degree per minute, so your longest possible exposure for a completely sharp photograph will be 30 seconds.

Of course, 30 seconds may be plenty for lunar and planetary work, and it may be interesting to do 30-second piggy-back exposures with a long, fast telephoto lens (such as 180-mm f/2.8). Clearly, though, for deep-sky photography, you need a wedge or field de-rotator.

You can use a field de-rotator if you are photographing through the telescope. However, only the wedge will help you with piggy-backing. Also, tracking is not as smooth with the de-rotator as with a properly aligned equatorial mount; after all, you are relying on twice as many motors, and as I understand it, the LX200 periodic-error correction does not operate in alt-azimuth mode.

(p. 135) The periodic-error correction on some units of the Celestar 8 Deluxe does not work correctly because of an embarrassing programming error -- it records and plays back for 4 minutes, whereas the actual gear period is 5.41 minutes. An updated microcontroller chip is available from Celestron.

(p. 136 and elsewhere) Lest there be any confusion, I want to make it clear that color print film is exactly the same thing as color negative film.

(p. 139) Light pollution: An even simpler argument to put before non-astronomers is this:

• Do you have lampshades on the lights in your living room?
• Why?
• Have you ever thought about doing the same thing for outdoor lights?

(p. 143) I recently encountered an off-axis guider that wouldn't focus. Normal procedure is to focus the image in the camera, and then focus the guiding eyepiece by pushing it in or pulling it out. In this case, even pushing it all the way in, I couldn't reach focus.

The solution to the mystery was to move the camera body farther back. On this particular guider (made by Orion) there were far more T-threads than actually necessary to hold the T-adapter, and I had screwed the adapter too far in. I backed off on it about 5 mm, then tightened the retaining ring around it, and all was well.

(p. 143) Mirror flop vs. drive flop: Not everyone agrees that mirror flop is a real problem. What is clear is that many fork-mount clock drives have about 1 degree of "drive flop," i.e., free movement in right ascension; that is, even with the R.A. brake locked, you can move the telescope back and forth. If the weight of the telescope shifts during a long exposure, the telescope will move suddenly.

Drive flop can generally be cured by mechanical adjustment; once it's cured, be sure not to try to move the telescope in R.A. with the brake on, or the problem will come back.

With many mounts, the problem is slack in the gears. Orange-tube Celestrons can be adjusted as described in the instruction book -- all you do is look at the bottom, loosen the screws holding the motors, and push the motors closer to the main gear. If the system is too tight, though, they will suffer excessive wear.

With my Meade LX3 telescope, the slack came from something a little more complicated. To fix it, here's what you have to do:

     (1) Remove the base from the fork arms;
     (2) Remove the screw that is under the little round cover at the top of the base;
     (3) Remove the drive motor and worm gear (by removing the bottom cover).

Then the whole drive base comes apart. The reason for the slack is that 3 screws, the screws that hold the clutch plate to another plate, are loose and sliding back and forth in their holes. All I had to do was tighten them. I also applied a bit of lubricant all around the big gear, and then ran the drive for several days (indoors of course) to distribute lubricants and help "wear it in." Don't run it forever -- the motor has a finite life.

(p. 143) Vibration from stepper-motor drives (and even from AC motors) can also be a problem. The vibration usually manifests itself only under very specific conditions when the tripod or telescope tube becomes resonant at the stepping frequency (something like 30 Hz). The symptom is that stars become elongated in an east-west direction.

A small change in the length of the tripod legs or the position of counterweights often solves the problem. If the problem is persistent, increase the tension holding the worm gear against the big spur gear.

(p. 145) On the rare occasions when I observe alone at a remote location, I bring along a cellular telephone and call my wife every hour or so; she knows that she should send help if the calls stop coming and she can't reach me. In 1999 an American amateur observing alone was seriously injured when he fell on a patch of ice, hit his head against the body of his truck, lost consciousness, and almost bled to death.

(p. 146) Here are two more ways to deal with headlights that come on automatically when you put your car in gear:

(1) Depress the parking brake very slightly, one or two clicks, not enough to actually engage the brake, before starting the engine. (On my Oldsmobile, once the lights come on, depressing the parking brake will not turn them off.) This should enable you to drive away from the observing site. Release the parking brake completely before reaching highway speeds.

(2) Tape something over the headlights. (Standard practice at Kitt Peak National Observatory.)

Backup lights and brake lights can be disabled by removing fuses. It is important to experiment with your car in advance to find out how to disable all lights, before going to a star party from which you will need to drive away before dawn.

(p. 152) People have asked me why I didn't say anything about Canon EOS cameras. See note to p. 155-156 below!

Many of the older manual-focus Canons (F-1) are quite suitable for astrophotography and are renowned for their high quality.

Please remember that there are hundreds of good SLRs that are usable, at least to some extent, for astrophotography but are not mentioned in this book because they are not among the ten or twenty that best fit the requirements.

For the full story on how to use Minolta Maxxum cameras with T-adapters and lenses that lack autofocus circuitry, see Minolta's documentation A few models (2xi, 3xi, SPxi) cannot be used this way; most of the others require special switch settings which are described in the documentation.

(pp. 152-153) Despite what modern camera makers seem to think, mirror lock or prefire does make pictures sharper, even pictures taken with an ordinary 135-mm telephoto lens and a tripod. See Herbert Keppler, "SLR: For sharpest images, do you really need mirror lockup?" in Popular Photography, June, 1999, pp. 18, 20, 22, 24, 64.

(pp. 154-155) The eye relief of the Olympus OM-4(T) can be extended appreciably in the following manner. Get an eyeglass maker to make a -2.0D plastic lens that will press-fit into the plastic frame of the camera eyepiece. (Do not use an add-on lens holder; it will take up precious space.) You will probably want to order a rectangular plastic lens and then sand the corners to fit, leaving a bit of extra space at one corner so you can pry the lens out later.

Insert the -2.0D lens into the frame, taking great care not to bump it into the existing eyepiece window, which is fragile. Then turn the eyepiece focusing knob all the way counterclockwise and leave it there (or adjust for a sharp image). You will then have about 5 mm more eye relief than with the unmodified OM-4(T). That's nothing like a Nikon F3HP, but it's just enough, at least for me.

Alternatively, you may want to try the Canon eyepiece extender (see below).

(p. 154-155) The Olympus OM-2000 and Nikon FM10 consumes battery power during long exposures because the "underexposure" LED in the viewfinder remains on. Remedy: Remove the battery. (See note to page 157, below, about the "Cosina quadruplets" -- the others may be afflicted similarly.)

(p. 154-155) The Olympus OM-2 (not OM-2S(P)) is subject to a strange quirk -- an LED inside the camera can light up and fog the film during long exposures. This is apparently a rare problem. The LED serves to limit the length of automatic exposures; it comes on after a time delay and fools the photocell. (They didn't have microprocessors back when this camera was designed.) Under very obscure circumstances this LED will fog the film. Remedy: Remove the batteries.

(p. 155) Olympus has announced discontinuation of the OM Series in March 2003 (31 years after its introduction), with some products becoming unavailable sooner than that. Actually, 31 years is a very long lifetime for a line of SLR cameras. The only one I can recall that has lasted longer is the Minolta manual-focus line, now almost 40 years old and still going.

(p. 155-156) Nikon SLRs: Since writing this material I have bought and enjoyed using a Nikon N70 (F70) for terrestrial photography. For astronomy, it's far from ideal but also far from useless. On the plus side, it has a very bright fine-matte viewfinder screen and can time exposures as long as 30 seconds. On the minus side, it has no mirror lock or prefire, and it consumes battery power continuously through the exposure; still, one $11 pair of batteries is good for more than 15 hours of time exposures, so the battery expense is less than $1 per hour. This camera is very handy for 20-second fixed-tripod exposures of the constellations. Its big brother, the N90 (F90), consumes considerably less battery power in long exposures because it does not keep its LED illuminated.

The N70 (F70) can be triggered electronically by simply shorting together the two pins in its cable release connector. They are 2.5 mm apart and a standard computer connector fits them, though it does not screw on the way the Nikon connector does. On B, the shutter remains open as long as the contacts are shorted.

The F70 (N70), F90 (N90), and F100 take AF and AI, but not pre-AI, lenses. The N50, N60, and N80 require AF lenses; they don't work with manual-focus lenses at all because they rely on electronic rather than mechanical aperture coupling.

...Further to above, since getting into Nikons I've taken the plunge and bought a Nikon F3HP body and a 180-mm f/2.8 Nikkor ED IF AF lens. The latter is a superb lens, completely free of color fringing or any other aberrations visible on slides. (Some photographers complain that it has no personality; unlike other long fast lenses, it does not introduce any optical special effects!)

Plunging deeper, I've also bought a Nikon 300-mm f/4 AF ED IF lens, and it, too, is superb. There are actually two varieties of this lens, AF and AF-S; the latter is newer but has a weaker tripod socket. You want the AF (non -S) version, even though it is supposedly a less sophisticated optical design. It's excellent, and it's bigger in diameter than the other, which probably means there's less vignetting (none that I've noticed).

The manual 300-mm f/4.5 ED lens is also excellent; the non-ED version is probably quite respectable, but not state-of-the-art.

I still think the Olympus OM-1 is the best camera for lunar and planetary work, but because of the "stovepipe" (DW-4) finder, the F3HP wins when dim images of deep-sky objects are involved. Here are some notes on it:

• "F3HP" denotes the F3 body and high-eyepoint (long-eye-relief) prism finder. There are also a few F3s in circulation with other finders attached. The F3T is a titanium-bodied F3HP.

• Time exposures should definitely be taken on "T", not "B". On "B", the camera draws 18.5 mA from the battery throughout the exposure. If you want a mechanical "B" setting, use a Nikon FM or FM2 or a Nikon F2.

  Actually, using the electronic "B" setting may not be prohibitive. A fresh DL1/3N lithium battery costs $5 and will power the camera for at least 5 hours of time exposures, possibly twice that.

  Taking a time exposure on "T" sounds awkward but is actually easy. You can start the exposure with a cable release (which requires a bit of battery power) or by pressing the mechanical shutter release lever (which does not, and which requires so little force that it generally does not introduce vibration). To end the exposure you must turn the shutter speed dial from T to another setting (which will obviously be B, right next to it). When working with a very long focal length (over 300 mm) I hold a black card in front of the camera during these manipulations. At shorter focal lengths, vibration is not a problem.

• Mirror lock is easy to actuate, requiring only light pressure on a small lever. The Olympus OM-1 and Nikkormat FTN require a good bit more muscle effort.

• The variety of focusing screens is bewildering -- and is greater than Nikon admits!

  The Beattie Intenscreen has concentric Fresnel lines that are visible, though not seriously obtrusive, at high f-ratios. The Nikon B screen has a plain matte (non-Fresnel) area at the center and may be easier to use, though not quite as bright. The Nikon D screen is plain matte all over and is very bright and easy to use; however, it does not work well with wide-angle lenses, so you cannot use it for general photography. See Jerry Lodriguss' comments on Nikon focusing screens.

  Nikon F4 screens fit the F3 just fine, and are brighter and have a finer-grained matte surface. They have two small marks to indicate the autofocus area; these are irrelevant on the F3, which is why Nikon says they're unsuitable for this camera.

  In particular, the Nikon F4 "B" screen is an overall fine-matte screen very much like the Beattie Intenscreen, finer-grained than the F3 "B". It can be used with all lenses. The two autofocus marks help you find the extra-fine-grained spot in the center, as well as helping you focus a focusing magnifier.

  There are several other fine-matte Nikon screens. The "D" is smoothest, but it vignettes with some lenses. The "F" is a new one that I'm still evaluating; it, too, is extra-smooth but vignettes with short lenses at high f-ratios.

  As if that weren't enough, Nikon F and F2 focusing screens also fit the F3, though they lack the tab that facilitates insertion and removal. (My camera arrived with a Nikon F screen in it.) Used Nikon focusing screens are abundant on eBay (search for "Nikon screen") and at KEH Camera Brokers.

• The Nikon DW-4 6x right-angle finder is, to my considerable relief, usable with eyeglasses on as long as you remove the rubber eyecup. The serious astrophotographer might consider buying a used F3 with a DW-4 but without the usual high-eyepoint prism finder.

  Recall that Nikon reckons magnification differently than Olympus; 6x on a Nikon is just 1.2 times the normal magnification of the OM-1. With the Varimagni Finder, the OM-1 renders what Nikon would call 12.5x magnification. That's plenty! The DW-4 is handy, but it should not be considered a really high-magnification device.

• I'm thinking about way to put an Olympus Varimagni Finder on a Nikon F3HP. Optically, the two are a great combination; you even get extra eye relief with the Nikon. When I make the requisite bracket or adapter, I'll post plans for it. Then I may not use my DW-4 any more...

  But in favor of the DW-4, it gives a very bright image. The Varimagni Finder has a small (2 mm) entrance pupil and does not take in as much light as your eye would. The DW-4 doesn't introduce any such restrictions.

• Overall vibration on the F3 seems to be very well damped, though I don't have complete test results back yet.

A wealth of Nikon information is available at www.nikonlinks.com.

(p. 155-156) [Note added 2004]:
In the original book I completely ignored Canon SLRs because they were entirely autofocusing. The Canon EOS (Electro-Optical System) cameras use electronics to control not only the focus but also the aperture of every lens.

However, this doesn't mean you can't use them for astrophotography. They compete closely against recent-model Nikons and have similar advantages, such as (in some models) interchangeable focusing screens and viewfinders. The long-focal-length autofocus lenses are as mechanically rugged as Nikon's and have very similar optical designs and performance.

Some of the film EOS cameras, such as the EOS 3, minimize battery consumption during long exposures. (I have not yet assembled full information about this.) Film EOS bodies are available secondhand at very low prices because Canon photographers are switching to digital bodies, which take the same Canon EF lenses.

More Canon data is here.

(p. 157) I recently got a good look at Contax SLRs and a Yashica FX-3, which takes the same lenses. The Contax S2 carries on the Olympus OM-1 tradition perhaps better than any present-day Olympus; it is solidly built, has a long-eye-relief finder, and has mirror prefire but not mirror lock. The Yashica is similar in features but much less expensive. Both of these give you access to excellent Zeiss lenses.

(p. 157) The Olympus OM-2000, Nikon FM10, Yashica FX3 Super, and Ricoh KR-5 Super II are the "Cosina quadruplets," made for their respective manufacturers by Cosina. These are solidly built, low-cost SLRs.

As far as I can determine, all four have mirror prefire (despite some Nikon documentation that says the FM10 doesn't), and all except the Nikon will accept the Olympus Varimagni Finder. Only the Olympus has a spot meter; only the Olympus and Nikon allow double exposures. For more on Cosina's role as a manufacturer of cameras for other companies, see Popular Photography, Nov. 1997, pp. 20-30.

(They may actually be sextuplets. In some parts of the world you will see very similar cameras, also made by Cosina, marketed under the Phoenix and Vivitar labels.)

Further to above: As of January 2000, Ricoh has discontinued all 35-mm cameras for the U.S. market. Several varieties of KR-5 are still in stores.

(p. 157) Having compared an Olympus OM-2000 and an OM-1N side by side, making identical exposures of the moon with the same telescope, and using mirror lock or prefire on both, I have sadly concluded that:

• Some cameras, including the OM-2000, produce a lot more shutter vibration than others, such as the OM-1N.
• Mirror lock or prefire is unimportant unless you also have a low-vibration shutter.
• The vibration is random or circular and looks like bad focusing. I am becoming convinced that many focusing problems are actually vibration problems.

The OM-2000 feels as if it's producing less vibration than the OM-1N. However, this must be because it produces less vibration after the shutter closes -- at which time vibration doesn't matter. Or perhaps the OM-1N's vibration is mostly in a front-to-back direction, with little ability to shake the telescope.

Everything I have said about the OM-2000 presumably applies also to the Nikon FM10, Yashica FX-3, and Ricoh KR-5, very similar cameras all made by Cosina.

I am going to continue testing to see if I can find out anything else. Please take this only as a provisional report.

(p. 158) The experts at Essex Camera Repair do not recommend periodic cleaning, lubrication, and adjustment; they say to do this to a camera only when it starts to show problems.

(p. 159) The Olympus Varimagni Finder actually fits a great variety of cameras. During one camera show I tried a lot of cameras and found that it fits:

• All Olympus OM SLRs, including the OM-2000 (you knew this already);
• All Minolta 35-mm SLRs, including Maxxums;
• All Pentax SLRs, from Spotmatic through K-1000 and ME to newer models;
• All Canons with rectangular eyepiece frames, including EOS (but not the F-1 or F-A);
• All Yashica and Contax SLRs with rectangular eyepiece frames (not the Contax RTS III);
• Ricoh KR-5 Super II, Ricoh Singlex TLS, and possibly other Ricohs [but not all units of the Super II; I thank John Russo for this information];
• Various older cameras, such as Mamiya/Sekor 500 DTL, Bell and Howell Auto 35, Konica Autoreflex, and even Kodak Instamatic Reflex.

I've heard of some attempts to make a longer-eye-relief version of the Varimagni Finder. It can be done by substituting a suitable eyepiece (probably an 18mm orthoscopic or a 10x microscope eyepiece) for the original one; you will have to do some machining to make a suitable focusing mount. With the original eyepiece, the rubber eyecup does not bend back out of the way; removing it is advisable if you wear glasses.

Pentax makes a right-angle finder with variable magnification, 1x and 2x. Naturally they can't comment on use of it with non-Pentax cameras, but it probably fits the cameras listed above. I'd really like to hear from anybody who has tried it!

(p. 160) Still more SLR hints:

Most SLRs leak some light through the eyepiece. This is significant only when you are photographing the sun with a filter, performing reciprocity tests with a heavy filter, or duplicating slides; in those situations, there's a lot more light behind the camera than in front of it. Some of my reciprocity tests were ruined by this effect. Also, light entering the eyepiece can affect the exposure meter. Some Nikons have an eyepiece shutter; Nikon supplies a black plastic cap for the others. Olympus should but doesn't; you can use a piece of black tape.

Although Olympus doesn't make an eyepiece cover, the Pentax eyepiece cover fits Olympuses as well as many other cameras -- basically all the cameras that take the Varimagni Finder (see above).

With some SLRs, mirror prefire works with "B"; with some, it doesn't. It's a moot point because you can hold a black card in front of the camera while opening the shutter.

Canon makes an eye relief extender ("eyepiece extender") for its EOS Rebel series of SLRs. It does not fit directly on Olympus or Nikon SLRs but can probably be modified to fit their eyepiece frames. It introduces noticeable barrel distortion.

(p. 169) Since this book was written, the Meade ETX 3.5-inch (9-cm) telescope has taken the world by storm. It offers unusually good performance for the price, with excellent optical quality, a built-in flip mirror for switching between camera and eyepiece, and a computer-controlled equatorial mount and clock drive. Unlike Schmidt-Cassegrains, Maksutovs (including this one) do not require frequent readjustment of the collimation.

However, the drive motors of the ETX are, I'm told, not really smooth enough for serious photography. I know of a number of people who are using ETXs for piggy-backing, but high-quality photography through the telescope probably requires a better mount and drive. The Celestron Ultima and Meade LX200 series are excellent; I wish Meade would make a 5-inch version of the LX200, using the ETX-125 tube assembly.

The jury is still out concerning the ETX-125 (the 5-inch ETX); early units were mechanically unreliable and Meade re-engineered them.

Initially, when Celestron came out with the NexStar 5 they forgot to offer a piggy-back camera bracket for it. I am told that such a bracket is now available, as item 93601. I have not tested a NexStar, but the older C5+ works very well in all photographic modes.

(p. 169) It is clear from more recent reviews that the original models of the Celestron NexStar 5 and 8 are not designed for photography, though they will do piggy-backing acceptably. See the review of the NexStar 8 in Sky and Telescope, November 2000.

[Update] That limitation does not apply to the NexStar 5i and GPS models. They are fine for photography.

(p. 169) If you want to transport a Schmidt-Cassegrain telescope by car, you don't need a special carrying case -- just strap it into the back seat with the seat belt, perhaps with a blanket around it.

(p. 171) There are reliable reports that both Celestron and Meade are producing better optics now than they were before the mid-1990s. Newer telescopes very often are better, at least in many cases.

One systematic difference between Meade and Celestron is that, except for the low-end LX10, Meade telescopes are heavier. This improves steadiness but reduces portability. The Celestar 8 is the lightest-weight 20-cm SCT, followed closely by the Meade LX10; then comes the Celestar 8 Deluxe, which weighs about 27 pounds (12 kg) without the tripod; and all the other Meades are heavier, up to about 46 pounds (21 kg) for the LX200 GPS.

(p. 176) See note to p. 26 on Kodak Recording Film 2475.

(p. 180) It is disputed whether reciprocity failure actually makes film less sensitive to ionizing radiation. Greater reciprocity failure does seem to be associated with better keeping properties, though, for whatever reason.

(p. 180-181) These formulae for estimating reciprocity failure make an assumption that has not actually been tested. They assume that the reciprocity curve is the same shape for all films, and all that varies is the slope.

For the most part, this seems to hold true, but I am getting scattered reports that some films hold up well in 2-minute exposures but fail in 30-minute exposures. That would mean that for such films, the hump in the curve in Fig. 10.7 is flatter and extends farther to the right, but then drops off more sharply.

Note that my tests include 10-minute as well as 2-minute exposures. In case of a discrepancy the 10-minute results should take precedence.

(p. 183) Reciprocity tests similar to these are being done by Robert Reeves, who posts his results on the Web at http://www.connecti.com/~rreeves/filmtest.htm. His results are generally similar to mine, though not identical. This is to be expected because there is considerable batch-to-batch variation as well as experimental error.

(p. 183) Some additions to the reciprocity test chart

Color slide films:
Fuji Provia 100F (RDP III), tested May 2000, p > 0.95, red 3, blue 1, color shift yellow.
same, preflashed 4 stops below mid-gray, red 8, blue 3.
Kodak Elite Chrome 200, tested June 2000, p > 0.95, red 5, blue 1, color shift yellow.
same, preflashed 4 stops below mid-gray, red 6+, blue 3.

Color negative films:
Fuji Superia X-tra (800 speed), tested June 2000, p = 0.80, red 6, blue 3, color shift cyan.
Kodak Supra 400, tested June 2000, p = 0.85, red 6, blue 3, color shift not significant.
Kodak PJ400, tested June 2000, p = 0.82, red 6, blue 3, color shift not significant.

Black-and-white films:
Ilford Delta 3200, tested November 2000, p = 0.65, red 4, blue 1.

(p. 186) John B. Marling is better known to his friends as Jack Marling.

[Update] Lumicon (Dr. Marling's company) went out of business in 2002, but its product line was taken over by Parks Optical; the new Lumicon is reachable at www.lumicon.com.

(p. 187) Choice of film: If there is one thing I can't say often enough, it's that you must buy your film by name, not by speed. One 200-speed film is not like another. (Test Kodachrome 200 against Elite Chrome 200 in any exposure lasting more than a second or two, and you'll see what I mean.)

(p. 188) Kodak E100VS film is a new color slide film quite similar to E100 but has higher color saturation and higher contrast. My early tests indicate that it is excellent for both deep-sky and planetary work. The mass-market version is called Elite Chrome 100 Extra Color.

Note added 2003: Ektachrome 100G is another big improvement, better than the ones just mentioned, and with finer grain.

(p. 188) Fuji Provia 100F (RDP III) has almost no reciprocity failure in a two-minute exposure, though I observed a color shift toward yellow (which is not a bad thing). It has reasonably good red response, though not as good as Elite Chrome 200.

What stands out, though, is its unusually fine grain, which gives the pictures a smoothness you won't want to give up once you've experienced it.

Note added 2003: Fuji Provia 400F and Fuji Sensia II 400 are two practically identical films that may beat Elite Chrome 200 (E200)at its own game. This film is very slightly grainier than E200 but twice as fast, and it has very good reciprocity characteristics. It also has somewhat more contrast. It has more red response than earlier Fujichrome 400 films, though not quite as much as E200.

Note added 2004: A detailed comparison between Fuji Provia 400F and Kodak E200 has been performed by Jim Misti. He finds Provia 400F better for galaxies, E200 better for anything involving emission nebulae.

Note added 2004: Kodak Elite Chrome 100 has just been re-engineered to match Kodak E100G, which is much finer-grained than the previous E100S and E100SW, while retaining excellent red sensitivity and reciprocity performance. It may be the best astronomical color slide film ever marketed -- for a while! Elite Chrome 200 is slated for a similar reformulation that will make it much finer-grained, and then it will be our first choice again.

Kodachrome 25 is has been discontinued by Kodak. Kodachrome 64 and Kodachrome 200 remain available.

(p. 189) Contrary to earlier announcements, some Kodak documents now say that Elite Chrome 200 film does reach speeds of 400 and 800, respectively, with a standard 1-stop or 2-stop push. For a 3-stop push they recommend exposing at EI 1000 (not 1600, which is 3 stops above the original speed).

(p. 189) Fuji Provia 400F is a promising new product that I have not yet tested. Claimed advantages include high speed, fine grain, and reduced reciprocity failure.

(p. 189) [Note added in 2001] The new Kodak Portra color print films have severe reciprocity failure and poor red response. This makes them unsuitable for astronomy, though very good for portraiture (their intended application).

[Updated 2002] The Kodak Portra line has been updated and has much better reciprocity characteristics than before, but considerably less sensitivity at 656 nm (hydrogen nebulosity). Supra is no longer available in its original form.

(p. 189) [Updated 2002] Kodak PJM and PJ400 film were replaced by Kodak Supra 400, apparently quite similar to them but finer-grained. But Kodak then changed the design parameters to remove the sharp peak at 656 nm that made the films so good for recording hydrogen nebulae. No currently available Kodak film is a good substitute for PJM; the nearest is Fuji Superia 400.

Tests by Robert Reeves in 1998 showed that Fuji Superia 400 and 800 films outperformed the competitors. They were very sensitive to red nebulosity (unlike Fuji slide films, which generally aren't). However, in the spring of 1999, Fuji Superia was reformulated and apparently lost much of its red (hydrogen-alpha) sensitivity.

[Updated 2002] And in 2001 or 2002 it was reformulated again -- I don't quite know where things stand now. The problem is that films with a strong peak in the deep red tend to make human skin look too pale. What's good for a nebula is not good for a portrait.

(p. 190) According to data sheets, the latest version of Ilford HP5 Plus film has a useful amount of sensitivity at 656 nm (hydrogen-alpha), and the new Ilford Delta 3200 (actually a 1000-speed film that is highly pushable) is quite sensitive to that wavelength. (Other general-purpose black-and-white films do not respond to hydrogen-alpha.)

Both of these films, especially the latter, can provide a quick and easy way for a beginner to photograph nebulae. Unlike color films, they also respond well to oxygen at 500 nm.

(p. 190) Kodak Black & White Plus 400 Film (BWC) is another new chromogenic (process C-41) film with very low reciprocity failure, but also rather low contrast. I have used it briefly for lunar photography. Ilford XP2 has been reformulated; I have not yet tested the new version.

And in mid-2001, Kodak Portra 400BW was added to the family. It, too, produces black-and-white images with standard C-41 color processing. Extremely low reciprocity failure is claimed; I haven't tested it.

Minilabs are often reluctant to process these films because of simple ignorance. "We don't do black-and-white. If you use black-and-white your pictures will come out purple." And so on...

(p. 191-192) Ektachrome Professional Infrared Film (EIR) turns out to have severe reciprocity failure, especially in the infrared-sensitive (red-colored) layer. I photographed some star fields and nebulae with it, and everything came out blue-green because there was very little response to infrared in a long exposure. This film is very contrasty. It may be of value in planetary work, but exposures should be bracketed at 1-stop or even half-stop intervals.

Contrary to Kodak's recommendation, you do not have to load and unload your camera in total darkness; subdued indoor light is safe enough. However, be very careful not to have it processed in a machine that uses infrared sensors, or the film will be fogged red.

(p. 197) For more about Kodak HC-110 Developer, see my HC-110 resource page.

(p. 196) I didn't say anything about developing tanks. I generally use a Paterson plastic tank. Old, battle-worn plastic reels are hard to load because of friction; new ones work better. Do not let wetting agents such as Photo-Flo dry on plastic reels.

But I may be about to switch back to a metal tank and reels. I've just discovered Hewes metal reels, which are much easier to work with than the usual American-made ones.

Apart from generally more solid construction, I think the secret is that instead of a clip to hold the end of the film (usually slightly crooked or off center), they have two prongs that fit into perforations and hold it perfectly straight. Thus you don't have to fight with it!

Relevant web addresses: www.hewes.co.uk (manufacturer); www.calumetphoto.com (one of many US vendors).

(p. 199) Kodak no longer recommends using Xtol developer diluted 1:2. Apparently, when it is used at that dilution, it is unduly sensitive to aging or partial exhaustion.

I have been getting very good results with undiluted Xtol. Full data sheets are available from Kodak.

The formula of Xtol, or something close to it, is published as part of U.S. Patent 5,853,964, available online free of charge from the U.S. Patent Office.

Another Xtol note: To get the powder to dissolve, work with water at the upper end of the temperature range (85 F, 30 C). Distilled water can be warmed easily in a microwave oven.

And another: If the contents of the Part A packet are caked or discolored, you have a bad batch that may not work satisfactorily; return it to Kodak. This was a known problem with some early production runs but it keeps popping back up.

For still more about Xtol, see my Xtol resource page.

(p. 201) I have been getting scattered reports that the shelf life of photographic paper is much less than it used to be, due to changes in manufacturing made on environmental grounds. Paper more than 2 years old is not necessarily trustworthy.

(p. 206) The formulae for the older E-4 process have now been published by Kodak as Publication CIS-111. The newer E-6 process is chemically similar but operates at a higher temperature,

(p. 207-210) It's possible to use your enlarger to duplicate slides. Ron Jegerings describes his setup in Photo Techniques, May/June 2000, pp. 59-61. The slide goes in the enlarger in place of the negative, and the enlarger projects it into the 35-mm SLR camera body.

As you might guess, connecting all the parts together requires some cleverness. If you have an EL-Nikkor enlarging lens, you're in luck. Add a Nikon 2640, 2641, or 2654 reversing ring to the front of your enlarging lens (consult the Nikon catalog to pick the right one). Your lens now has the same 39-mm Leica-standard threads on the front as it has on the back.

Next, you need whatever would be needed to mount an enlarging lens on your camera. Close-up photographers often use an enlarging lens on the front of a bellows, which may have T-mounts or camera-lens mounts. For example, Nikon makes an adapter for putting 39mm-thread enlarging lenses on the front of a Nikon bellows; it will work equally well for this purpose. Or you can use a "Leica T-flange" (which would be used to put a 39-mm-thread lens on the front of a T-mount bellows) and a T- adapter for your camera.

Last, you need some extension tubes to provide space between the enlarging lens and the camera body. When you get the system working, try the enlarging lens both forward and reversed; for 1:1 copying, some lenses perform better reversed.

(p. 209) If you want to make duplicate slides that look just like the originals, try Kodak's new EDUPE film, to be available in mid-2001 in 36-exposure rolls. It is designed for exposure with blue photofloods or a color enlarger head. If using photofloods, you will need a set of color printing (CP) filters. Recommended filter packs are specified in the instructions.

(p. 222-224) Some scanner hints:

• To reduce scanner artifacts, you can scan the same slide several times in different positions (right side up and upside down, even sideways if you're working with the central area) and then combine the images.
• If your scanner wants to overexpose a dark slide, here's a trick: Put a light slide in, choose "preview," and then change slides for the final scan. If you don't tell the software you've changed slides, it may not recompute the exposure.

(p. 224) If a slide is too dark, you can't scan it. I've tried! All you get is a pattern of scanner artifacts. That's why, even with scanners available, dark slides are best processed by duplicating them in a slide duplicator. Underexposed negatives, on the other hand, are thin and scan relatively well. That's one reason for using negative film if you do your final processing digitally. See also note to p. 10.

With dark slides, it helps a great deal if your scanner will let you adjust the brightness of its light source. Published specifications generally don't tell you whether you can. The Nikon Coolscan III (LS-3000) provides this valuable feature, and I am getting much better scans with it than with my previous Minolta QuickScan 35 Plus. The brightness adjustment is called "analog gain," but it in fact varies brightness, not amplification.

To get the most out of any film scanner, use VueScan, a low-cost software package from Hamrick Software. Even if you don't purchase VueScan, read Hamrick's up-to-date comments about the performance of various scanners. He will tell you more about how they work than the manufacturers will.

VueScan gives you low-level control and lets you combine multiple scans into a single image with large dynamic range.

(p. 232) If you are using an inkjet printer, beware of glossy photo paper, which has a thin gelatin-like coating on top of a water-resistant base. Although prints made on this material look like real photographs, they have a subtle problem that I call afterblur -- over a period of days to weeks, or even longer, the inks continue to spread out, and tiny star images on a dark background simply disappear!

This is a worse problem with some printers than with others. It can be avoided by using matte-surface paper. The prints then look less photographic, but the stars are much easier to see.

(p. 243) On digital cameras see note to page 76.

(pp. 244-246) Note that field-of-view calculations are covered in greater depth on pp. 73-75. If you find this section hard to understand, read that one.

(p. 259) Jeff Conrad points out that in order to agree with the principles of astronomical photometry, the last two formulae on this page (interconverting m'' and B) should say 10.0 instead of 9.0. This is without correction for absorption of light by the earth's atmosphere. The formulae, as published, incorporate 1.0 magnitude of correction, which may be too much — except that planetary work is often done under hazy conditions.

where d is the apparent diameter in arc-seconds.

But an even simpler way to write the corrected formula would be:

leaving out all mention of π/4.

(p. 286) If you experiment with variations of this circuit, note that the IRF510 MOSFET requires 12-volt gate drive in order to turn on fully. Despite its claimed 4-volt threshold, it does not actually turn on completely with only 5 volts on the gate. Thus, if you drive it from a 5-volt microcontroller or logic circuit, use a logic-level MOSFET such as the IRL510 or IRL530 instead.

(p. 288) Lunar rate tracking must take into account the effects of parallax, since the observer is not at the earth's center. While the moon moves eastward in its orbit above us, the rotation of the earth also whisks us eastward, making the moon seem to move closer to the sidereal rate than it actually does.

The "average" lunar rate given in my table is geocentric and therefore not accurate for any given telescope, though it is much better than tracking the moon at the sidereal or solar rate.

Calculation of the effect of parallax is discussed by E. S. King (1931, p. 164; note that in King's formula, the moon's equatorial horizontal parallax is given in arc-minutes, not arc-seconds as King says).

For average conditions with the moon high in the sky, the following rates are a reasonable compromise:

Lunar rate:
  As fraction of solar rate, 0.978
  For 60-Hz motor, 58.69 Hz
  For 50-Hz motor, 48.91 Hz
  Crystal frequency, 4.807 MHz

There is no point in striving for great accuracy, since the rate varies quite a bit with the position of the moon in the sky.

I have developed a microcontroller program that generates all these frequencies -- along with sidereal rate, two-phase drive waveforms, and other niceties -- from a single 4-MHz crystal. It is known as "Alcor" and the programmed chip is available from Dontronics. For details see http://www.ai.uga.edu/~mc/alcor.html.

(p. 291) Kodak Technical Pan Film has been discontinued. For some notes by Ohad Drucker on his experiences using outdated Technical Pan (which apparently has a very long shelf life), click here.

(p. 301-306) The Kodak Ektapress films (PJ400, etc.) have been discontinued, replaced by the new Kodak Supra films, which are similar but have not yet been extensively tested for astrophotography. Data sheets are available from Kodak.

(p. 307) Kodak Wratten filters are now distributed by Tiffen.

(p. 313) Chasseur d'Images is on the Web at www.photim.com.

(p. 313) Darkroom User (an excellent British magazine) has changed names; it is now Camera & Darkroom. The address is the same.

(p. 315) SBIG has moved to 147-A Castilian Drive, Santa Barbara, CA 93117, phone (805) 571-7244.

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