Introduction to Astrophotography (for Terrestrial Photographers) Greg Marshall, Wa'chur'ed Observatory

Transcription

1 Introduction to Astrophotography (for Terrestrial Photographers) Greg Marshall, Wa'chur'ed Observatory

2 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.

3 Introduction to Astrophotography (for Terrestrial Photographers) Before jumping into how to do it, let's review some of the equipment and terminology

4 Equipment & Terminology Types of Telescopes: The Refractor

5 Equipment & Terminology Types of Telescopes: The Reflector (Newtonian is most common)

6 Equipment & Terminology Types of Telescopes: The Catadioptric (Schmitt Cassegrain is most common)

7 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.

8 Equipment & Terminology Types of Mounts: The Altitude/Azimuth ( Alt-Az ) Mount

9 Equipment & Terminology A Special Case Worth Mentioning, the Dobsonian Telescope

10 Equipment & Terminology Types of Mounts: The Equatorial Mount (German EQ Mount or GEM is most common)

11 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

12 Equipment & Terminology Camera on Tripod: Tip: Reverse mounting allows higher pointing angle with many tripods

13 Equipment & Terminology Attaching a Camera: Piggyback

14 Equipment & Terminology Attaching a Camera: Afocal Camera Support

15 Equipment & Terminology Attaching a Camera: Prime Focus Camera Adapter

16 Equipment & Terminology A DSLR Camera at Prime Focus:

17 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

18 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

19 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)

20 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...

21 Equipment & Terminology Alternative Equatorial Mount: The Barn Door Tracker

22 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

23 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

24 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

25 Night Landscape Short Exposure

26 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 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

27 Night Landscape Long Exposure Christopher J Picking

28 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

29 Wide Field / Constellation Greg Marshall

30 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.

31 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. * To combine the images you can just use Photoshop, but better results can be achieved using a free program called Deep Sky Stacker. It involves some complex parameters, but most of them can be ignored. Instructions for using DSS can be found online.

32 Special Events Roger Marcoux

33 Special Events * Unusual events in the sky always garner interest * Some are wide angle targets: * Auroras ( Northern Lights ) * Meteor showers * Others require moderate magnification, but not long exposures: * Lunar Eclipse * Solar Eclipse * Most comets (see example exception) * Various transits * Following are some examples

34 Special Events: Auroras Roger Marcoux Roger Marcoux

35 Special Events: Meteor Showers

36 Special Events: Lunar Eclipse Roger Marcoux Greg Marshall

37 Special Events: Solar Eclipse CAUTION! Roger Marcoux Luc Viatour.

38 Special Events: Comets Roger Marcoux Roger Marcoux

39 Special Events: Conjunctions Roger Marcoux

40 Special Events: Transits Roger Marcoux

41 Special Events * How to do it: * CAUTION: Use appropriate filter for any solar photos or observing * Identify event (astronomy mags, internet search, news) * Select best location (consider terrestrial foreground) * Use camera on fixed tripod for wide angle shots * Targets requiring a telescope can be done afocal * Meteor Showers: * Use very fast, wide angle lens * Set camera for maximum sensitivity (wide open aperture, high ISO) * Find best background exposure by trial * Repeat this exposure until you capture something * Most dramatic image is looking toward the shower source, BUT... * You may have better luck capturing a meteor in a darker part of sky

42 Planetary / Lunar - Afocal Matija Pozojevic

43 Planetary / Lunar Afocal * Afocal means the camera is pointed into the eyepiece of a telescope * Rather limited, but easy to do a good way to get your feet wet * Exposure must be quite short a fraction of a second, typically * Works best on the moon because it is so bright How to do it: * Acquire a telescope! A schmitt-cassegrain telescope (SCT) is best for this type of astrophotography because it provides a lot of magnification in a small package. A tracking mount is very helpful, but it can be either an equatorial or alt-az mount. * For the best results, also obtain an adapter to attach the camera to the telescope. These are available from any astronomy store and are inexpensive. If one is not available, use a tripod. You will have to point and focus the telescope first, then move the camera into position over the eyepiece. * You will have to experiment with focal length, focus, and exposure to get it right. You might also need to try different eyepieces in the telescope. A common problem is vignetting.

44 Planetary / Lunar Prime Focus Emil Kraaikamp

45 Planetary / Lunar Prime Focus * Our solar system provides only a few targets that are suitable * If you have a telescope and computer, it is inexpensive to add what you need * Uses a webcam to capture 1,000s of frames of video * Software processed results much better than any single frame * Commonly used software is free How to do it: * Assuming you already have a suitable telescope (ideally an SCT of at least 8 inch aperture) and a laptop computer, you need to acquire a webcam and an adapter to attach it to your telescope. The lens on the webcam is removed (most of them use a standard thread size) and replaced with the adapter. The camera can then be inserted into the telescope's eyepiece holder and you're ready to shoot! * You will capture a video of your target, which will then be processed by the software.

46 Deep Space Greg Marshall

47 Deep Space * Deep space means areas well outside our solar system * A huge number of targets of varying sizes and brightness * They generally fall into one of four groups: - Nebulae (areas of dust and/or gases that either emit or reflect light within galaxy) - Star clusters (globular or open groups of stars within galaxy) - Galaxies (very distant, very large) - Galaxy clusters (even more distant and large) * Dimmer objects can take hours of exposure * Tracking requirements vary with magnification, but generally very high * Guiding is often necessary

48 Deep Space How to do it: * Equipment: - Telescope on tracking equatorial mount (small to medium refractors are most common) - DSLR or astronomy-specific digital camera (the latter is usually a CCD camera) - A second camera (guide camera) with high speed capability for tracking - Either a separate telescope for guiding or a device that allows both cameras to use one 'scope - A laptop computer with guiding and capture software - Power supplies for all the above - A wide variety of other accessories * Capture Process: - Using one of several methods, carefully align the mount with Earth's axis of rotation (just pointing it roughly at Polaris is not good enough) - Point the 'scope at the target, which is usually done via computer control and also requires some calibration of the navigational system - Start the guide camera and calibrate the guiding system on a star near the target - Critically adjust focus (check focus throughout the process it will change with temperature) - Determine best exposure by trial, avoiding saturation as much as possible - Set parameters for capture (including number of frames) and start capture

49 Deep Space How to do it (continued): * Capture Process: - In addition to the image frames, you will need various calibration frames: + Dark frames (same exposure as image frames, but with lens capped) + Flat frames (mid-level exposure of a flat white source) + Bias frames (shortest possible exposure with lens capped) - Multiple frames of each of the above are needed. They will be used in the next step * Calibration Process: - The purpose of the calibration frames is to reduce (as much as possible) the inherent defects of the image sensor and optical system. Bias frames represent the offset imposed by the electronics when there is no signal at all from the sensor. The dark frames represent the offset generated by the sensor over the exposure time in the absence of any light. Both of these are subtracted from each image frame. The flat frames represent the unevenness of illumination in the telescope. It can also remove the appearance of dust specs on the sensor. - Calibration seems complicated, but the software handles it somewhat automatically and at least one such program, Deep Sky Stacker, is free.

50 Deep Space How to do it (continued): * Align and Stack Process: - The same software that does the calibration will also analyze the calibrated images to correct for small changes in the position of stars from one frame to another, then combine ( stack ) them into a single image with lower noise and more significant bits per pixel. * Post-Processing: - The stacked image allows you to apply extreme stretching of the image (actually, this is dynamic range compression) without becoming too noisy. The dim regions are greatly increased in brightness while the brighter stars stay the same. - Additional post-processing steps are similar to conventional digital photography. However, special attention must be given to minimizing noise. Color accuracy, on the other hand, is relatively unimportant because there is no natural color to these objects even through a telescope, very little color is apparent to the human eye.

51 Common Problems Tracking: * Magnification needed for small objects also magnifies the mechanical errors * The typical motor drive system uses a worm gear to slowly move the telescope * Gears are never perfect and exhibit Periodic Error or PE * Tracking mounts vary widely in the amount of PE * Only the very best mounts can be used at >25X for >10 seconds * Simple correction models the error and applies inverse correction to motor * This is called PE correction, or PEC, and can greatly improve the performance of a mount * Can only correct for the errors of the worm gear - other gears are different * Guiding corrects (albeit imperfectly) for all error sources * A second camera that operates with short exposures * The position of stars in the guide image is analyzed by a computer program * Program sends commands to the mount to bring the stars back to their original position * Can also be done manually, but this is (literally) a pain in the neck

52 Common Problems Focus: * Seems trivial, but all of these factors work against you: 1. Even bright stars are difficult to see through a viewfinder 2. Stopping down the aperture is not possible with a telescope 3. Since a star is effectively a point source of light, any error in focus will be readily visible 4. Focus will probably change with temperature through the night * Infinity position on a lens will almost never work * Check focus through images captured by the camera * A DSLR with live view mode can be used to adjust focus on a bright star * Adjust focus on any star and then move to any other object in the sky. * Computer analysis of sharpness is possible * With suitable hardware it is possible to auto focus on even fairly dim stars * AF typically takes one or two minutes - much faster than manual focus procedures. * A mask can be placed in front of the lens/telescope to produce diffraction patterns * Helps in manually determining best focus

53 Common Problems Optical Aberrations: * Stars are effectively point sources of light * Every defect is visible in the captured image! * With refractors the biggest problem is chromatic aberration (CA) * Astrophotography stresses CA much more than most terrestrial photography * Only the very best camera lenses (some Canon L series lenses) are effectively free of CA * In refractor telescope design CA is the main consideration after aperture and focal length * 'Scopes range from crude achromats to apochromatic triplets with ED elements * The cost of a refractor rises very quickly with size and number/quality of ED elements. * On the other hand, very few telescopes produce a flat field * Telescopes are designed primarily for visual use, which does not require a flat field * Astrophotographers usually add a field flattener between the telescope and camera. * Reflecting telescopes do not suffer from CA * They have more trouble with other aberrations * Very recent developments in reflector optics have greatly reduced these problems

54 Common Problems Light Pollution: * Most Americans now live within 100 miles of a major city * Light pollution fogs the black level, limiting exposure and dynamic range * Varies with location, atmospheric conditions and direction of the light pollution source * Filters to reduce light pollution ( LPR filters) can be of some help * They block light at wave lengths corresponding to street lights (mercury and sodium) * However, most light comes from incandescent, for which filters are of no help * Many targets are bright enough to be photographed under moderate light pollution * Examples: Andromeda galaxy, Orion Nebula, Pleiades star cluster * Dimmer objects may require a trip to an area with darker skies. * Narrowband imaging uses filters that pass only wave lengths of common elements * Most common in deep space are hydrogen, oxygen, and sulfur * Filters almost completely blocks light from man-made sources * Limits the selection of targets to emission nebulae * I use this technique at my home observatory, which is 20 miles north of Portland.

55 Common Problems Hydrogen Sensitivity: * Hydrogen is the most common element in the known universe * Strongest emissions are in the very deep red Hydrogen-alpha band * This wavelength is just beyond the range of most digital cameras * The native sensor is sensitive well into the near infrared range * Camera manufacturers add a filter to block the near IR and UV * Camera filters do not have a sharp cutoff, so some sensitivity to H-alpha remains * Results can be greatly improved by removing the filter * Replace it with one that passes H-alpha with minimal attenuation * If you are skilled you can modify a DSLR yourself * Or you can have your camera modified by a professional for $200 to $400. * Cameras made specifically for astronomy do not have this problem, of course.

56 Common Problems Image Noise: * Astrophotography is all about beating down the noise level * High ISO setting and long exposures result in more noise in the image * Stretching the histogram amplifies the noise. * Begin reducing the noise by determining the best ISO setting and exposure level * Then increase the number of sub-exposures * Double the number of exposures to add 1 bit of significance to each pixel (approx.) * Another way to reduce noise is to cool the sensor * A decrease of 7 degrees Celsius will typically cut the noise level in half * Cameras designed for astronomy have built-in cooling mechanisms * I typically run my camera at -15C in the summer and -25C in the winter. * DSLRs always have color sensors * A good deal of light is lost to the dye-based color filters used in such sensors * Cameras designed for astronomy can be found in both color and monochrome versions * A monochrome sensor is significantly more sensitive than a color one * Color images can be captured using dichroic filters that pass about 95% of the light in band * This means having to capture at least three sets of sub-exposures (red, green, and blue) * A luminance (full band) filter is also often used, since this allows even shorter exposures * Sharpness is defined by the luminance frames - RGB data can be filtered to reduce noise.

57 Common Problems Diminished Capacity: * The process of capturing deep space images is complicated and error-prone * Our ability to correctly execute the necessary steps is greatly diminished by: - Working in the dark - Working long past our bed time - Often working in very cold conditions * Many of us have automated the process as much as possible * Some people sleep while their system captures images of multiple targets * My system is designed for partial remote operation * I can control much of the operation from inside my home or heated trailer * However, I don't have enough faith in my gear to sleep while imaging

58 Example Images The following astrophotos were captured and processed by myself. Most were done in Some were done at my home observatory and others at the site of the Oregon Star Party, east of Prineville.

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85 Introduction to Astrophotography The End Thank You! For more information visit: