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Introduction to Astrophotography (For Terrestrial Photographers)

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Introduction to Astrophotography (For Terrestrial Photographers) Introduction to Astrophotography (for Terrestrial Photographers) Greg Marshall, Wa'chur'ed Observatory Introduction to Astrophotography (for Terrestrial Photographers) * An astrophoto is any photograph in which the night sky is a significant part of the composition. * Misconceptions: - That a “powerful” telescope is required (extreme magnification) - That only scientists/professional astronomers make such images * Light gathering power of telescope and long exposure are important * The problem: Earth won't stop turning while you take your pictures * Last month, fully 50% of the images on NASA's APOD were non-telescopic. In this presentation we will see that gorgeous astrophotos can be captured by just about anyone. We will also see that once hooked on astrophotography, one can spend a lifetime (and an equivalent fortune) pursuing more complex forms of it. Introduction to Astrophotography (for Terrestrial Photographers) Before jumping into how to do it, let's review some of the equipment and terminology Equipment & Terminology Types of Telescopes: The Refractor Equipment & Terminology Types of Telescopes: The Reflector (Newtonian is most common) Equipment & Terminology Types of Telescopes: The Catadioptric (Schmitt Cassegrain is most common) Equipment & Terminology Types of Telescopes: Which is best for you * Refractors are the most expensive choice, but remember that we do not need a large telescope for astro photography. A small refractor is usually a better choice than a large reflector or catadioptric at the same price. * Refractors are usually shorter focal length, which eases many of the problems found in astro photography. * We will see later that the exception to this rule is that lunar/planetary photography requires the high magnification of an SCT. * Newtonian reflectors have an added problem in that the weight of the camera is to the side of the scope, making it difficult to balance and handle. Equipment & Terminology Types of Mounts: The Altitude/Azimuth (“Alt-Az”) Mount Equipment & Terminology A Special Case Worth Mentioning, the “Dobsonian” Telescope Equipment & Terminology Types of Mounts: The Equatorial Mount (German EQ Mount or “GEM” is most common) Equipment & Terminology Types of Mounts: Why do we use the Equatorial Mount? * Most targets are so far away that their motion is insignificant, BUT... * Earth is rotating, so everything appears to be moving. * An EQ mount has 2 orthogonal axes. * By aligning one axis with Earth's rotation, * And then turning it in the opposite direction (preferably by a precise motor), * We cancel the motion and get stable images. * The unchanging location of each object can then be specified by the angles of the 2 axes. * The polar axis is called “right ascension” (RA) and the other is called “declination” (DEC) * RA is given in hours, minutes, and seconds. * DEC is given in degrees, minutes, seconds Equipment & Terminology Camera on Tripod: Tip: Reverse mounting allows higher pointing angle with many tripods Equipment & Terminology Attaching a Camera: “Piggyback” Equipment & Terminology Attaching a Camera: Afocal Camera Support Equipment & Terminology Attaching a Camera: Prime Focus Camera Adapter Equipment & Terminology A DSLR Camera at Prime Focus: Equipment & Terminology Choosing a Camera: The “Point & Shoot” * Use for Night Landscapes and “Afocal” telescope shots * Ability to do long exposures is not necessary for most shot types * Remote control allows you to use longer exposures without shaking * Best if “auto” features can be turned off, especially auto focus. * High pixel count does not help and may be worse because of noise Equipment & Terminology Choosing a Camera: The DSLR * Use for all kinds of astro photography * Almost always lower noise and more flexible than a P&S camera * Most can do up to 30 second exposure internally * Longer exposures require remote control or computer connection * High pixel count does not help and may be worse because of noise * Most high-end features are of no use in astro photography * “Live View” and articulating display are VERY helpful in focusing * Generally needs significant amount Of setting up for AP use * Can be modified for H-alpha sensitivity Equipment & Terminology Choosing a Camera: The CCD astro camera * Use for all kinds of astro photography * Much more expensive than DSLR, with these advantages: * Internal cooling reduces noise * Higher precision A-to-D conversion * H-alpha sensitivity * Many available with monochrome sensor (higher sensitivity) * And some disadvantages: * Requires a computer to operate * Not useful for non-astro photos * Can be difficult to fit with camera lenses (designed for use with telescope) Equipment & Terminology Choosing an Equatorial Mount * You might not need one, but if you do... * Prices range from a few hundred $ to “astronomical” levels * Most basic model with RA motor drive is adequate for “short tele” lens and wider * Several models in the $500 to $1500 range offer full GOTO and are adequate for small to medium refractors (less than 1000mm focal length) * Check weight limit spec and try to stay under half (excludes counterweights) * For a low cost alternative... Equipment & Terminology Alternative Equatorial Mount: The “Barn Door Tracker” Equipment & Terminology Alternative Equatorial Mount: The “Barn Door Tracker” * This simple device can be constructed for a fraction of the cost of a GEM * When well made and properly aligned, it can exceed the quality of a low-end GEM * RA axis can be driven by hand, with an AC motor, or stepper motor * Mounts on a standard tripod * The angle is limited and must be reset when the end of the gear is reached * Does not (usually) provide any coordinate indication * Point “hinge” at Polaris * Camera can then be pointed anywhere Equipment & Terminology Camera Lenses: * With P&S cameras you are stuck with one lens * A DSLR can use a telescope as a lens, but they are often too long in focal length * Modern lenses (even zoom lenses) often have very good optics * Here are some things to remember about how such lenses are different from a telescope: * Chromatic aberration is usually higher (compared to an ED doublet or triplet) * Often need to stop it down 1 or 2 stops from wide open to get good quality * The aperture is never perfectly round, so it may create diffraction spikes * Although a lens may have a focus scale, you cannot trust that “infinity” will be correct * You need to study test reports to determine which apertures (and zoom setting, if applicable) produce the sharpest images * On the plus side, camera lenses will almost always provide “flat” focus, while most telescopes require some kind of corrector Types of Astrophotos 1. Night Landscape – Short Exposure 2. Night Landscape – Long Exposure (Star Trails) 3. Wide Field / Constellation 4. Special Events 5. Planetary / Lunar – Afocal 6. Planetary / Lunar – Prime focus 7. Deep Space 8. Scientific Imaging Night Landscape – Short Exposure Night Landscape – Short Exposure * Earth-based foreground is main subject, night sky objects are part of the composition * Exposure must be kept relatively short: < 20 seconds @ 18mm focal length * Therefore, high sensitivity is necessary * Astounding results can be achieved under the right circumstances. How to do it: * Find the location and set up the shot (on a tripod) before sunset * Consult star charts (or software) to determine what celestial objects will appear * Shoot at various times through the night as the background changes * Use wide angle lens at fastest aperture possible * Use the highest ISO you can * Since you will have plenty of time, take multiple shots with different settings * If you like, you can “paint” the foreground with a flashlight during exposure Night Landscape – Long Exposure Christopher J Picking Night Landscape – Long Exposure * Long exposure causes the appearance of “star trails” * The motion is circular, with a center near the North Star (Polaris) * A complete circle is scribed by each star in 24 hours (15 degress per hour) * Do multiple exposures, then combine the exposures into a single image How to do it: * Set up the shot before dark – same as for short exposure night landscape * Think about how arcs will appear – counterclockwise around Polaris * Use low to moderate ISO * Set aperture for optimal sharpness (Usually around f/8) * Determine exposure duration (do not let stars saturate) * It is much easier if you have some mechanism to automate the exposures. * Be careful not to bump the tripod or camera throughout the exposure period Wide Field / Constellation Greg Marshall Wide Field / Constellation * Low magnification of wide field allows use of much less sophisticated tracking * Example: 1 minute exposure, 35mm lens, low-end GEM * Could be done with a “barn door” tracker * Motion can be provided either by a motor or the human operator How to do it: * Acquire a basic tracking device and attach your camera to it using a ball head * Align the axis of rotation with Polaris. For very basic alignment, just point the mount north as accurately as possible and point it up to an angle equal to the latitude of your location (approximately 45 degrees here in Oregon). It might be easier to wait until it gets dark and visually align the axis with Polaris. Turn on the mount's tracking motor. * Point the camera at the desired target (without moving the mount), making sure that no Earth-bound objects are included in the shot. Wide Field / Constellation How to do it (continued): * Exposure can be determined in the same way as for landscapes, but be aware that most of the image will probably be very dark, while the stars are very bright. You need to be careful that the stars (or at least most of them) do not saturate from over exposure. * Use the widest aperture that will produce a sharp image and the highest sensitivity that will produce a low-noise image. * Use “raw” capture mode if possible. This will provide lower noise images. * Take multiple exposures of each target. The total exposure time (sum of individual “sub-exposures”) can be anywhere from a few minutes to a few hours.
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