Color control 1361 Gamut

Like film, computer monitors and printers reproduce colors, not by regenerating the spectrum of the original light source, but simply by mixing three primary colors. This works only because the human eye has three types of color receptors. Creatures could exist - and indeed some human beings do exist - for whom the three-primary-color system does not work.3 By mixing primaries, your computer screen can stimulate the eye's color receptors in any combination, but not at full strength. That is, it...

Processing from linear TI FFs

If your astronomical image processing software doesn't support your camera's raw files, you can work with linear TIFFs instead. A linear TIFF file is one that has not been gamma-corrected - the pixel values in it are linearly proportional to the numbers output by the camera. The reason we want TIFFs is that (unlike JPEGs) they contain the full detail of the image. The reason they need to be linear is so that dark-frame subtraction EJe jit ew Plot ess Rlier Qokjr JJfmitm Help Figure 12.12. After...

Networking everything together

The typical modern DSLR astrophotographer uses a laptop computer for control, a webcam for autoguiding, and a computerized telescope. Here are all the connections that have to be taken care of Figure 10.7. Strategy for adapting a serial or parallel port cable to a non-Canon DSLR. Use a voltmeter to identify the + and sides of switch. Power for the telescope, the computer, and the camera (preferably all separate). USB or serial connection from the computer to the...

Sensor specifications

DSLR manufacturers do not release detailed specifications of their sensors. Accordingly, the sensitivity curves in Figure 11.5 (p. 134) reflect a certain amount of guesswork. What's more, the raw image recorded by a DSLR is not truly raw. Some image processing is always performed inside the camera - but manufacturers are extremely tight-lipped about what is done. For example, Canon DSLRs apparently do some kind of bias frame subtraction on every image, and Nikon's star eater speckle-removing...

Practical Amateur Astronomy Digital SLR Astrophotography

In the last few years, digital SLR cameras have taken the astrophotography world by storm. It is now easier to photograph the stars than ever before They are compact and portable, easy to couple to special lenses and all types of telescopes, and above all, DSLR cameras are easy and enjoyable to use. In this concise guide, experienced astrophotography expert Michael Covington outlines the simple, enduring basics that will enable you to get started, and help you get the most from your equipment....

Mirror vibration

Much of the vibration of an SLR actually comes from the mirror, not the shutter. If the camera can raise the mirror in advance of making the exposure, this vibration can be eliminated. Mirror vibration is rarely an issue in deep-sky work. It doesn't matter if the telescope shakes for a few milliseconds at the beginning of an exposure lasting several minutes that's too small a fraction of the total time. On the other hand, in lunar and planetary work, my opinion is that DSLRs aren't very...

Sharpening

One of the wonders of digital image processing is the ability to sharpen a blurry image. This is done in several ways, and of course it cannot bring out detail that is not present in the original image at all. What it can do is reverse some of the effects of blurring, provided enough of the original information is still present. The simplest way to sharpen an image is to look for places where adjacent pixels are different, and increase the difference. For example, if the values of a row of...

What is the magnification of this picture

Non-astronomers seeing an astronomical photograph often ask what magnification or power it was taken at. In astronomy, such a question almost has no answer. With microscopes, it does. If you photograph a 1-mm-long insect and make its image 100 mm long on the print, then clearly, the magnification of the picture is x 100. But when you tell people that your picture of the Moon is 1 35000000 the size of the real Moon, somehow that's not what they wanted to hear. Usually, what they mean is, How...

F ratio and image brightness

The f -ratio of a telescope or camera lens is the ratio of focal length to aperture One of the most basic principles of photography is that the brightness of the image, on the film or sensor, depends on the f-ratio. That's why, in daytime photography, we describe every exposure with an ISO setting, shutter speed, and f-ratio. To understand why this is so, remember that the aperture tells you how much light is gathered, and the focal length tells you how much it is spread out on the sensor. If...

Portable electric power

Although my main observing sites are blessed with AC power, most amateurs rely on batteries in the field, as I sometimes do. For much more advice about portable electric power, see How to Use a Computerized Telescope. Here I'll review the basics. Most telescopes operate on 12 volts DC and can be powered from the battery of your parked car. Doing this can be risky what if you drain the battery and the car won't start when you're ready to go home It's much better to use portable lead-acid...

Camera control

Canon 400d Energy Astrophotography

10.2.1 Where to get special camera cables The following pages describe a number of homemade cable releases and computer interface devices. Please do not build these circuits unless you understand them thoroughly and can use test equipment to verify that they are assembled correctly. It would be a pity to damage an expensive camera while trying to save money on accessories. Makers of camera control software can tell you where to get the special cables if you can't make them for yourself. One...

Info

Didymium glass (Hoya Intensifier filter) cuts through skyglow from sodium-vapor streetlights. Each image is a single 3-minute exposure of the field of Zeta and Sigma Orionis with a Canon Digital Rebel (300D) at ISO 400 and a 180-mm lens at f 5.6. Note reflection of bright star caused by filter (arrow). Figure 11.8. The Horsehead and other faint nebulae captured under light-polluted skies 10 miles from central London. Stack of 20 3-minute exposures and 23 2-minute exposures with...

The classic M42 lens mount

M42 Screw Mount

Canon DSLRs with adapters have given a new lease on life to many classic lenses with the M42 (Pentax-Praktica) screw mount. Here M42 means metric, 42 mm and has nothing to do with the Orion Nebula, Messier 42. Figure 7.11. Classic M42 (Pentax-Praktica) screw mount. Note 42x1-mm threads, aperture stop-down pin, and manual auto switch (for diaphragm actuation, not autofocus). Figure 7.11. Classic M42 (Pentax-Praktica) screw mount. Note 42x1-mm threads, aperture stop-down pin, and manual auto...

Parfocal eyepiece

For rough focusing only, and for centering deep-sky objects that are too faint to show up on the SLR screen, a parfocalized eyepiece is handy (Figure 8.6 c). That is an eyepiece that you have adjusted to focus at the same position as your camera. Once you've found an eyepiece that is approximately correct, you can fit a parfocalizing ring around it to control how far it goes into the eyepiece tube. To match a DSLR, you'll probably need to fit the eyepiece with an extension tube, easily made...

Removing gradients and vignetting

A gradient is a difference in brightness from one side of the picture to the other it generally occurs when you're photographing close to the horizon or the Moon is in the sky. Vignetting is underexposure at the edges of the picture, usually caused by the limited diameter of the light path. Both of these can be corrected by software in fact, MaxDSLR and similar packages can usually correct them automatically (Figure 14.8). Of course, if the vignetting originates in the camera, flat-fielding is...

Calibration frames

Calibration frames are recordings of the camera's errors, made so that the errors can be corrected (Table 12.1, p. 147). One form of calibration, dark-frame subtraction, was introduced in the previous chapter here we deal with the rest. As you already know, a dark frame is an image taken with the lens cap on, so that no light reaches the sensor. Its purpose is to match the hot pixels (and partly hot pixels) of an image of a celestial object, so that they can be subtracted out. The two must...

Nebulae red response and filter modification

Figure 11.5 sums up a lot of information about the wavelengths of light to which cameras respond. Reading from the top, the first thing you'll notice is that color film has a gap in its response between green and blue, and DSLRs don't. This is important because the strong hydrogen-beta and oxygen-III lines from emission nebulae fall in that gap. That's one reason nebulae that look red on film may come out blue with a DSLR. Dldymlum glass (red) intensifier Dldymlum glass (red) intensifier Figure...

Filters to cut light pollution

Now look at the middle part of Figure 11.5, showing the transmission of various filters. Some of them are quite effective at cutting through the glow of city lights. The poor man's nebula filter is didymium glass, which blocks the brightest emissions of sodium-vapor (orange) streetlights. Didymium glass looks bluish by daylight and purplish by tungsten light. It contains a mixture of praseodymium and neodymium. Glassblowers look through didymium glass to view glass melting inside a sodium...

Overcoming sky fog

Like astronomical CCDs and unlike film, DSLRs make it easy to subtract a reasonable amount of sky fog from the picture. This is partly because their response is linear and partly because it is easy to stack multiple exposures. In the suburbs, a single 15-minute exposure might come out overexposed, but five 3-minute DSLR exposures are easy, and you can combine them and set the threshold so that the sky is relatively dark. The results are not quite equivalent to what you'd get under a dark...

The Nikon star eater

Another way to get rid of hot pixels is to check for pixels that differ drastically from their immediate neighbors. The rationale is that genuine details in the image will always spill over from one pixel to another, so if a pixel is bright all by itself, it must be hot and should be eliminated. The Nikon D40, D50, D70, D70s, D80, and related DSLRs do this automatically, in the camera, even before saving the raw image, and it has an unfortunate side effect it eats stars. That is, sharp star...

CCD and CMOS sensors

Electronic image sensors work because light can displace electrons in silicon. Every incoming photon causes a valence electron to jump into the conduction band. In that state, the electron is free to move around, and the image sensor traps it in a capacitive cell. The number of electrons in the cell, and hence the voltage on it, is an accurate indication of how many photons arrived during the exposure. Modern sensors achieve a quantum efficiency near 100 , which means they capture an electron...

The Canon Angle Finder C

Viewfinder focusing is much easier and more accurate if you add magnification. The best device for this purpose, whether your camera is a Canon, Nikon, Figure 8.2. Canon Angle Finder C fits almost any SLR with a rectangular eyepiece frame -even a Nikon. Figure 8.2. Canon Angle Finder C fits almost any SLR with a rectangular eyepiece frame -even a Nikon. Olympus, or Pentax, is the Canon Angle Finder C (Figure 8.2). This is a magnifying right-angle viewer that comes with adapters to fit...

Types of coupling

Direct Astrophoto Focus

Figure 5.2 shows, from an optical viewpoint, how to couple a camera to a telescope. Optical details and calculations are given in Astrophotography for the Amateur and in Table 5.1. Figure 5.2. Ways of coupling cameras to telescopes. Piggybacking, direct coupling, and compression are main modes for deep-sky work. Not all of these modes work equally well, for several reasons. First, DSLRs excel at deep-sky work, not lunar and planetary imaging. Accordingly, we want a bright, wide-field image....

The camera as your logbook

Digital cameras record a remarkable amount of information about each exposure. This is often called EXIF data (EXIF is actually one of several formats for recording it), and Windows itself, as well as many software packages, can read the EXIF data in JPEG files. Photoshop and other utilities can read the same kind of data in raw files. From this book's Web site (www.dslrbook.com) you can get a utility, EXIFLOG, that reads a set of digital image files and generates a list of exposures. This fact...

How lens designs evolve

Most of us have seen dozens of diagrams of lens designs without any explanation of how to understand them. Figure 7.12 is an attempt to make lens diagrams comprehensible. It shows how nearly all modern lenses belong to four groups. Looking at the historical development, you can see that the usual way to improve a design is to split an element into two or more elements. Thus the three-element Cooke Triplet gradually develops into the 11-element Sigma 105-mm f 2.8 DG EX Macro, one of the sharpest...

How well can you do with an altazimuth mount

Sbig Efinder

I hinted at the end of Chapter 4 that you can do deep-sky work successfully with an altazimuth mount. The trick is to make relatively short exposures and stack them with rotation. The reason you must rotate and stack with software is that even if the individual images are sharp, there is cumulative rotation from one to the next. Over 10 minutes you get 10 minutes' worth of rotation, but if it's divided up Figure 9.9. The author's piggyback autoguider, consisting of an SBIG ST-V and a homemade...

Major manufacturers Canon

Many astrophotographers have settled on the Canon Digital Rebel, XT, XTi EOS 300D, 350D, and 400D and their successors. These are low-priced, highperformance cameras. One reason Canon leads the market is that Canon is the only DSLR maker that has specifically addressed astrophotography, first with tutorials published in Japan1 and then, briefly, by marketing a special DSLR for astrophotography the EOS 20Da . Also, because Canon SLR bodies are relatively compact, you can use other brands of...

Photo editing

For general-purpose photo editing, the software that comes with your camera may be adequate, especially if you pair it up with relatively powerful astronomy software. What you need is the ability to crop and resize pictures, adjust contrast and color balance, and make prints. The king of the photo editors is of course Adobe Photoshop, which is one of the basic tools of computer graphics many scientific add-ons for it have been developed. But Photoshop isn't cheap. An alternative is Photoshop...