Astrophotography Handbook for DSLR Cameras
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Astrophotography Handbook for DSLR Cameras Michael K. Miller Oak Ridge, TN Quick Start Guide The three most common forms of wide-angle astrophotography with DSLR cameras adjust red values DLSR cameras Lens Modes ISO, Post as necessary for RAW preferred NR noise reduction aperture, processing proper exposure shutter speed Star trails Full frame better Wide angle M mode, ISO 400, Combine Point N for circles 16-50mm Manual focus Widest f-stop, frames in Remote shutter e.g., 24 mm at in 30 min.- 2h Photoshop release (hold or lock (16 mm for a crop in multiple 20 -30s down release) Liveview, (see slide 30) sensor camera) shots (camera on CL or intervalometer set NR OFF continuous low), Star trails on 20s (LONG) or BULB setting for exposure time intervalometer Stars/Milky Full frame better Wide angle M mode, ISO 400-1600, Normal Way Remote shutter 16-50mm Manual focus Widest f-stop, release e.g., 24 mm at in 10-20s max. (16 mm for a crop Liveview, Single shot, S Stars sensor camera) NR ON The moon Crop factor Longest focal Spot meter, ISO 200, Normal cameras better length: Spot focus, f/5.6, Remote shutter 400-500mm Auto focus ~1/1000s, Moon release on moon Single shot, S The explanations for these settings are discussed in the following slides Tripod is required for all these celestial objects; remove lens filters, use lens hoods 2 Types of Astrophotography Wide-field, or landscape, astrophotography - photographs of the night sky revealing the stars and galaxies, including the Milky Way, that are acquired with DSLR and other cameras with wide-angle lenses with focal lengths shorter than roughly 35 mm. Time-lapse astrophotography - an extension of wide-field astrophotography where many exposures are taken over time and then combined to make time-lapse videos and star trails. Solar system – images of the planets, moons, and the sun of our own solar system mostly photographed through telescopes, but a super telephoto lens on a DSLR cameras can also give good results for some of these objects. Deep space – images which are taken with a telescope of distant galaxies and nebulae, i.e., objects beyond our own solar system. This presentation is only applicable for DSLR cameras directly photographing the universe. It is not intended or applicable to cameras attached to telescopes. All exposure settings (ISO, aperture, shutter speed) quoted herein should be taken as a starting point and refined as required for proper exposure. The blue background “How-to” slides have detailed suggested instructions 3 The night sky The earth spins on its 23° axis once every 24 hours. Therefore, the sky rotates anti- clockwise about the Celestial North Pole (CNP) in the Northern Hemisphere. Different parts of the celestial globe are seen at different locations around the world and at different times and time of year. View from Oak Ridge, TN 4 The night sky • The sun. • The moon. • Light pollution • Clouds • Haze – which can smear out the brighter stars The earth spins on its 23° axis once every 24 hours. Stellarium and many other portable smartphone apps enable simple identification of stars, constellations, planets, etc. in the night sky at specific times and locations. 5 Twilight • Civil Twilight Civil twilight is approximately the limit at which solar illumination is sufficient, under clear weather conditions, for terrestrial objects to be clearly distinguished. • Nautical Twilight The length of twilight depends on latitude During nautical twilight, sailors can take reliable star sightings of well-known stars, using a visible horizon for reference. The end of this period in the evening, or its beginning in the morning, is also the time at which traces of illumination near the sunset or sunrise point of the horizon are very difficult, if not impossible, to discern (this often being referred to as "first light" before civil dawn and "nightfall" after civil dusk). • Astronomical Twilight In some places, especially those with skyglow (the diffuse glow that can be seen over populated areas), astronomical twilight may be almost indistinguishable from night. Most casual observers would consider the entire sky fully dark even when astronomical twilight is just beginning in the evening or just ending in the morning, and astronomers can easily make observations of point sources, such as stars. 6 Twilight The Photographer's sunset moonrise Ephemeris (TPE) is one tool that can calculate moonset the different types of twilight, and sun and moon rises and sets for different dates and locations. On Sept. 17, 2016 at the sunrise TMO, Astronomical twilight starts at 5:56 AM and sunrise is 1h 27 min. later at 7:23 AM Sunset is at 7:41 PM and Astronomical twilight is 1h 26 min. later at 9:07 PM. But it is close to a full moon that night. photoephemeris.com 7 Factors influencing the night sky Even deep into Astronomical Twilight or Night, there are still several factors to take into account for star gazing. Although the moon emits a lower Skyglow intensity of light than the sun, it will still dominate the night sky if present. after moonset and before moonrise or during a New moon are optimum Light pollution (photopollution) - brightening of the night sky caused by Melton Hill Dam, Oak Ridge street lights and other man-made sources that is prevalent in all towns and cities. Clouds and rain which obscure the sky • Haze which can smear out the brighter stars. Not advisable to setup over pavement or buildings. Viewing over grass or water will help avoid heat currents (thermals) that will degrade the image. Interference from clouds 8 Local Light Pollution Maps International dark sky location Pickett’s State Park 4605 Picket Park Hwy (Hwy 154) Jamestown, TN 38556 Pickett’s SP Norris Dam SP Lilly Bluff SO TAO good bad The International Dark-Sky Association (IDA) is a jshine.net/astronomy/dark_sky/ non-profit advocacy group involved in the movement to reduce light pollution. 9 Night Vision Night vision is the ability to see in low light situations, such as astrophotography. It takes most people about 20 minutes to become completely dark adapted after entering a totally dark place, e.g., photographic darkroom, or underground cave. There is significant light outdoors at night in except in a certified dark sky viewing location. It is inadvisable to check or set your camera settings with a regular white flashlight, as you will ruin the night vision for you and anyone near you. Rhodopsin in the human rods in the eye is insensitive to the red wavelengths of light, so red lights and/or red googles are used to help preserve night vision. Red filters are available for some flashlights, and red LEDs are incorporated into many LED headbands. It must be dim (not see light at 1/2s @ f/2.8, ISO 1000). Turn LCD screen brightness on camera to dimmest setting Put black tape over LED lights on camera and AF light. 10 Eye versus Camera The human eye can detect approximately 6000 stars DSLR cameras can detect ~5 million stars with a larger brightness range. 20 second exposures (under optimum conditions) should be able show all the stars plotted on Sky Atlas 2000.0 10 - 20 minute exposures* will chart all objects seen with an average amateur telescope. Spectral response of the eye is from ~390 nm to almost 850 nm Spectral response of a Nikon D810 is from ~420 nm to ~700 nm Therefore, the camera’s sensor cuts off the H UV, violets and deep reds and infrareds. However, the universe emits radiation over a much larger range of wavelengths. *if the rotation of the earth is compensated for 11 Exposure and Camera Settings In order to accurately expose the image of stars on the camera, the 3 factors that control exposure, shutter speed (exposure time), aperture, and ISO setting have to be properly selected. This is often a compromise! Use RAW files. 1. Normally, the widest aperture of the lens is selected to gather as much of the available light (photons) as possible. However, some wide angle lenses have poor resolution and bad field curvature wide open (i.e., the focus at the edges of the image may not be the same as at the centre). Coma may also be observed at wide apertures. So the aperture may need to be changed from wide open by 1 or 2 stops to f/2.8 or f/4. 2. The ISO setting does not change sensitivity of the sensor to photons, but changes the amplification of the signal from the sensor at the expense of dynamic range and sensor noise. Although the camera LCD monitor and the recorded image gets brighter with higher ISO settings, no additional faint stars or nebulae are recorded! Noise reduction can be applied when recording images to improve the signal-to-noise ratio. 3. The exposure time (shutter speed) can be selected to correctly expose. Many cameras have a limit on the shutter speed, e,g,. 30s. This can be increased using the BULB [or TIME] setting or by taking multiple exposures and combining them in post processing. More importantly, the earth rotates with respect to the universe, so the stars become streaks at longer times, which increases as the focal length of the lens increases. To overcoming this issue, a tracking equatorial mount can be used. Due to the long exposures, any camera movement or vibrations must be eliminated by mounting the camera on a sturdy tripod and using a remote shutter release, etc. 12 Dynamic Range of Sensor In the 400-6400 ISO range that is often used in astrophotography, most DSLR cameras have very similar dynamic range performance.