Transcript
9/14/2016
Quick Start Guide The three most common forms of wide-angle astrophotography with DSLR cameras
Astrophotography Handbook for DSLR Cameras
adjust red values as necessary for proper exposure
DLSR cameras RAW preferred
Star trails
Full frame better Point N for circles
Star trails Stars/Milky Way
Remote shutter release (hold or lock down release) or intervalometer set on 20s (LONG) exposure time
Full frame better Remote shutter release
The moon
Moon
Crop factor cameras better Remote shutter release
ISO, aperture, shutter speed
Post processing
M mode, ISO 400, Manual focus Widest f-stop, 30 min.- 2h at in in multiple 20 -30s Liveview, shots (camera on CL NR OFF continuous low),
Combine frames in Photoshop (see slide 30)
Modes NR noise reduction
Wide angle 16-50mm e.g., 24 mm (16 mm for a crop sensor camera)
or BULB setting for intervalometer
Wide angle 16-50mm e.g., 24 mm (16 mm for a crop sensor camera)
Stars
Michael K. Miller Oak Ridge, TN
Lens
Longest focal length: 400-500mm
M mode, Manual focus at in Liveview, NR ON
ISO 400-1600, Widest f-stop, 10-20s max. Single shot, S
Normal
Spot meter, Spot focus, Auto focus on moon
ISO 200, f/5.6, ~1/1000s, Single shot, S
Normal
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
Types of Astrophotography
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The night sky
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.
The earth spins on its 23° axis once every 24 hours. Therefore, the sky rotates anticlockwise 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.
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
View from Oak Ridge, TN
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The night sky • • • • •
Twilight • Civil Twilight
The sun. The moon. Light pollution Clouds Haze – which can smear out the brighter stars
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).
The earth spins on its 23° axis once every 24 hours.
• Astronomical Twilight
Stellarium and many other portable smartphone apps enable simple identification of stars, constellations, planets, etc. in the night sky at specific times and locations.
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. 5
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Twilight moonrise
sunset moonset
sunrise
Factors influencing the night sky Even deep into Astronomical Twilight or Night, there are still several factors to take into account for star gazing.
The Photographer's Ephemeris (TPE) is one tool that can calculate the different types of twilight, and sun and moon rises and sets for different dates and locations. On Sept. 17, 2016 at the 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.
Although the moon emits a lower 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 street lights and other man-made sources that is prevalent in all towns and cities. Clouds and rain which obscure the sky •
photoephemeris.com 7
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.
Skyglow
Melton Hill Dam, Oak Ridge
Interference from clouds
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Local Light Pollution Maps
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.
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
The International Dark-Sky Association (IDA) is a non-profit advocacy group involved in the movement to reduce light pollution.
bad
jshine.net/astronomy/dark_sky/ 9
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Eye versus Camera
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.
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 H Therefore, the camera’s sensor cuts off the UV, violets and deep reds and infrareds. However, the universe emits radiation over a much larger range of wavelengths.
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.
*if the rotation of the earth is compensated for 11
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Dynamic Range of Sensor
Special Cameras Most stock cameras have infrared (IR) and ultraviolet (UV) blocking or hot mirror filters that cut off the ends of the visible spectrum so that the color balance is close to that of the human eye. The spectral response varies by the sensor in each camera, so some cameras are better than others for astrophotography (see hyperlinks below). H-alpha (Hα) is a deep-red visible spectral line with a wavelength of 656.281 nm that is emitted by many emission nebulae. For astrophotography, the transmission of the H line should be greater than 20%. For some cameras, it is possible to have the IR filter changed to extend its spectral response. In some other cameras, the sensor itself is the limiting factor. Note opening the camera will void the camera’s warranty, so best done by specialist firms.
In the 400-6400 ISO range that is often used in astrophotography, most DSLR cameras have very similar dynamic range performance. Mirrorless and compact cameras have lower dynamic ranges.
Nikon's D810a is designed for astrophotography Specialized IR cut filter captures the red tones of H emission nebulae - 4X increase in sensitivity to the 656 nm wavelength than a standard DSLR. 36.3MP FX-format CMOS image sensor with no optical low-pass filter captures staggering detail Shoot ultra-long exposures up to 900 seconds (15 minutes) Built-in Time-Lapse, Interval Shooting and unlimited continuous shooting The discontinued Canon 20Da also was specially made for astrophotography. It passes almost 70% of the H wavelength
DxOMark
Lowest ISO yields the best dynamic range and lowest sensor noise.
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See ClarkVision.com and kolarivision.com/articles/internal-cut-filter-transmission/
Focusing the DSLR Camera
Focusing with a Bahtinov Mask
Setting the correct focus is especially critical is astrophotography. Due to changes in atmospheric conditions, temperature, etc., most modern lenses are designed to “focus past infinity”. Therefore, you cannot simply turn the focus to the end of the range of focus travel. In addition, many lenses no longer have a hard stop at infinity ∞. For the moon with a long focal length lens, use standard spot autofocus. However, for stars in the night sky, autofocus is not effective due to the low light levels and small objects, so manual focus has to be used.
Bahtinov masks were invented in 2005 by amateur Russian astrophotographer Pavel Bahtinov to assist in focusing stars. 2 1
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Manually focus with liveview with the LCD screen magnification at maximum and a bright star in the frame – temporarily increase the ISO to ~5000 – turn focus ring on lens carefully and slowly from one side of the symbol to the other till you get the smallest spot size – note you will not see the stars until close to focus – reset ISO. • Manually focus on the stars through the viewfinder – or focus on a distant object during daylight and lock the focus ring with tape. Start around the ∞ symbol. • Use a Bahtinov mask and live view to align the 3 diffractions spikes (next page).
As stars are point sources of light, they should appear as sharp circular points when in focus. Cameras with tilt LCD screens or tethered cameras facilitate this process The depth-of-field at infinity is very large so any wide aperture can be used.
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focus too far
correct focus
focus too near
3 grids produce 3 angled diffraction spikes. As the focus is changed, the central spike appears to move from one side of the star to the other. Optimum focus is achieved when the middle spike (arrowed) is exactly centered between the other two spikes. Use bright star in center of frame, widest aperture, high ISO, & high magnification in Liveview to see, check with test images, remove afterwards. en.wikipedia.org/wiki/Bahtinov_mask
SharpStar2 Precision Focusing Tool by Lonely Speck lonelyspeck.com/sharpstar/ 16
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Red Intensifier Filter
Other Filters Solar Filters Threaded solar filters should be attached to the front element of lens ND 5 (ND100000) is the minimum strength for direct eye solar observation without damage of retina. F-stop reduction of 16 2/3 stops NOT FOR DIRECT EYE OBSERVATION ND 3.8 (N8192) or ND 4 (ND10000) are the lowest values for solar CCD sensor exposure without risk of electronic damage. F-stop reductions of 13 and 13 1/3 stops, respectively.
hoyafilter.com blocked
If it is not possible to completely distance yourself more than ~60 miles from the light pollution coming from cities, the Hoya Red Intensifier, or Didymium, filter can filter out the yellow-orange portion of the spectrum from ~575 nm to ~600 nm. This yellow-orange part of the spectrum that is blocked is the color of most sodium vapor lamps but also blocks any stars emitting these wavelengths. Although sodium lamps are slowly being replaced with more efficient LEDs, they are still one of the most common types of outdoor lighting and light pollution.
Telescope users can also use other types of filters, such as H filter in combination with an energy rejecting filter (ERF) Different types of Nebula filters including Broadband and narrow band for light pollution, Oxygen III for diffuse and planetary nebular, and H-Beta for the Horsehead. California and other faint nebular.
thousandoaksoptical.com/solar.html seymoursolar.com 17
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Long Exposure & ISO Noise Reduction
How to improve Signal-to-Noise Ratio
Dark noise
Noise in the digital sensor increases with the length of the exposure and also with the ISO setting. Some modern cameras have methods to minimize their effects Long Exposure Noise Reduction (NR) (in the custom menu settings)
Combining a series of images can improve the signal-to-noise (SNR) ratio and make the stars, star trails, etc. more visible from the background/sensor noise. How many images? • Light images – standard images that contain the images of stars, galaxies, nebula • Dark images – removes the sensor generated a dark signal (noise) that depends of
Camera automatically takes second “blanked” exposure at slow shutter speeds (i.e., longer than 1 s) then subtracts the exact dark current map (thermally-induced current that the sensor produces even when it isn’t struck by light) from the image data.
High ISO Noise Reduction (NR) – Canon in-camera or Photoshop/Lightroom
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Luminance Noise - gray or black-colored noise or “grain” you often see in a magnified view of an image Chrominance Noise - colored, speckled noise you sometimes see in mid-tone or shadow areas
Remember to cover or close the shutter on the viewfinder to prevent stray light in. In-camera noise reduction should be ON when taking long (>1s) exposures of stars. Noise reduction should be OFF during taking images of star trails, else small breaks in the trails will be evident due to the blanked exposure time. However, NR can be applied in post production by taking several dark frames after the primary Breaks in the star trail image, as discussed later. when NR is ON
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the exposure time, temperature and ISO speed Take images in the dark (hence the name) by covering the lens. Must have the same exposure time, temperature and ISO speed of the light frames Bias (or offset) images - removes the sensor readout signal from the light frames which is created by the electronics just by reading its contents. Take the shortest possible exposure (it may be 1/4000s or 1/8000s depending on your camera) in the dark by covering the lens Dark Flat images corrects the vignetting and uneven field illumination created by dust or smudges in your optics Take images of a uniformly-illuminated clean white card, etc. it is very important to not move your camera (including not changing the focus). Should have the same ISO speed of the light frames. The temperature is not important.
Between 10 and 20 dark, bias and dark flat (each) images are usually sufficient Use RAW not jpg images Source: deepskystacker.free.fr/english/theory.htm#CalibrationProcess 19
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Deepskytracker + Photoshop
Calibration processes for (all or some) light, dark, and bias/offset frames
Output image
Post processing is required to bring out the stars
Deepskytracker may be used to combine these different types of frames into a final image. See manual and hyperlink below for details markwalkerscreenwriting.wordpress.com/deep-sky-stackertutorial/
60 light (20s), 20 dark (20s), 20 bias (1/8000s) frames, D500, 16 mm, f/4, ISO 125, cropped
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© Michael K. Miller, 2016
After post processing the TIFF file, in particular repeated dark and mid level adjustments in Photoshop, many more stars and other celestial objects become visible.
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Using DSLR cameras on a telescope Amateur telescopes are available with longer focal length lenses (up to ~4000 mm) than DSLR lenses and have better light collection. This enables more deep space objects to be viewed. The camera body is attached to the telescope with a T-mount adapter that is specific to the camera mount and the eyepiece holder. Many modern telescopes use an external 42 mm thread (not the same as the Pentax 42 mm thread). Alternatively, short adapters are available with this thread. Weight • As the weight of the camera is considerably heavier than the eyepiece, the telescope has to be rebalanced. This is not always possible with all makes of telescopes! Focus • It is not always possible to focus the telescope even with short extension tubes. • As the lens’s electrical connections cannot be used, manual focusing is the only option. The simplest method is to use Live View either on the LCD screen or on a tethered laptop computer. The latter has the advantage of not moving the telescope so easily, but there is a cable that can be tripped over in the dark. Tracking • Even driven equatorial mounts have limitations to keeping the object in view. • The main issues are the accuracy of the polar alignment and Periodic Error Corrections required to overcome variations in the worm and wheel drive mechanism. Check compatibility of camera and telescope before purchase.
Wide-Field Astronomy including Time-Lapse Astronomy
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© Michael K. Miller, 2016
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Star Trails
Star Trails
A long static exposure or, more effectively, a series of exposures may be taken and combined can be used to generate star trails. Must NOT move the camera, so a sturdy tripod is required. The Celestial North (or South) Pole (CNP/CSP) must be in the field of view to show the point of rotation of the arcspossibly positioned at one of the power points of the rule of thirds. © Lee Smalley
Typical parameters – adjust as necessary - cameras with tilt LCD screens make focusing easier • A clear, cloudless, and moonless night well after Astronomical Twilight • Mount camera with a remote cable release/intervalometer on a sturdy tripod • Wide angle lens appropriate for the field-of-view desired: e.g., 16 - 50 mm Horizontal angle of view: 16 mm 97; 18 mm 90; 20 mm 84; 24 mm 74; 35 mm 54; 50 mm 50.
• • • •
M (or A) exposure mode In-camera noise reduction (NR) OFF to prevent breaks in the star trails Manually focus at infinity (in magnified live view); no lens filters, use lens hoods Adjust ISO and aperture as necessary for correct exposure of stars and to not over expose the background – test exposures are simplest way to optimize.
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Shutter speed and number of exposures are used to define the length of arc
– ISO: 200 - 1600 depending on camera (lower better); Aperture: f/1.4 to f/4 Length of star trail arc in degrees = 360 * exposure time (min) / time of 1 Earth rotation (24*60 min) e.g., 60 min. (120 x 30s) exposure = 15 rotation
• •
More artistic images often include a foreground object, such as a tree, or ground. For proper exposure of this object either light paint with a powerful flashlight, or take a separate exposure and combine in post production. This series of exposures method also works for meteorites.
Although the length of all the star trail arcs will subtend the same angle, they will increase in length with distance from the Pole to the Equator. Typical total exposure times: 30 min to 3 h (7.5 to 45 rotation – –
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use BULB, TIME, or LIVECOMP (Olympus) setting, 20 s exposures battery life is the limiting factor for some cameras (p.16)
Use fresh batteries due to the long exposures required.
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Star Trails – single or multiple exposures?
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Intervalometer/Remote shutter release An intervalometer attaches to the shutter release connector for longer exposure times or taking series of multiple exposures.
Why take a series of exposures rather than a single long exposure?
Alternatively, a remote shutter release with camera in continuous shooting CL mode + 20 s exposures
Single 1200s exposure @ f/5, ISO 200, 17 mm
5 parameters are required: Intervalometer Delay: the time before the shutter is opened for the 1st frame [e.g., 1 or 2 s] Each parameter may be set to Exposure time (LONG): the up to 99 h 59 min. and 59 s time that the shutter is Use the multifunction dial to set parameters open. [e.g., 20 s] Left and right arrows to select the function indicated Interval (INTVL): The time by the bar under the function between frames – this should be the Exposure Then press set button., then left or right time + the time needed to arrows to select hour, min, or sec , up write the file to the or down arrows to change value; left memory card. [e.g., 22 s]. and right arrows to move to next value, Must be longer than LONG press set to finish. Number of shots: the number of exposures to be To use after setting: Select Focus mode M; Exposure mode M; taken [-- = unlimited] Single frame S; shutter speed BULB; Compose and focus; Beep: toggles ON or OFF Press TIMER START/STOP to start and again to abort
60 x 20s exposures (1200 s) @ f/4, ISO 200, 16 mm
The long static exposure continuously integrates the photons from the stars but also the ambient light in the background. Therefore, the background is much lighter (i.e., overexposed) and detracts from the stars (poor signal-to-noise ratio). The combined extended series of exposures has a darker background and showed the colours of the star trails better. Combining multiple exposures can also bring out fainter stars. [Film suffers a reciprocity failure at long exposures] 27
There is no ON/OFF switch so they are on as long as the batteries are inserted. Use fresh batteries.
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Star Trails
Combining Series of Exposures 1. 2.
3.
4. 5.
© Michael K. Miller, 2016
Nikon D500, 16-35mm @ 16mm, f/4, ISO 200, 60 x 20s images
In order not the overexpose the background, a series of shorter exposure images are recorded with an intervalometer rather than one long exposure. This stack of separate images then have to be combined in post production in Photoshop, etc.
© Michael K. Miller, 2016
Each star produces a coloured arc or star trail
Nikon D800, 14-24 mm lens @ 24 mm, ISO 800, f/4, 90 min. with an intervalometer 180 images were combined in Photoshop
Star Trails: Olympus LIVECOMP
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Star Trails
60 min = 15arc
1. Suitable wide-angle Olympus (MFT) lenses 7-14 mm f/2.8 mm, 12-40 mm f/2.8, 8 mm f/1.8 fisheye, or equivalents. Mount lens on camera, fully charged batteries in camera 2. Setup and level tripod with a ball head. Mount camera on tripod. Attach a remote shutter release. Select any focal length between 7 and ~15 mm (wider is generally better). 3. Set lens in manual mode (pull focus ring toward the body of the camera or press AF button on the body and turn rear dial to MF), remove lens cap. 4. Set Mode dial to Manual (M) and view the LCD screen to focus the lens on the stars. If necessary, temporarily increase ISO (switch position 2, front dial) to increase brightness. 5. Compose your shot to ensure that it is positioned to show off as much sky as possible, the horizon (if present) is level, there is some foreground object. To include the center of the star trail circles, aim the camera North and Polaris (CNP) must be in the frame. 6. Set camera settings for night photography. ISO = 400 and aperture = f/4 7. Turn the shutter speed dial all the way clockwise to LIVECOMP (LCD will dim). A message will appear "Press shutter button once to prepare for composite shooting." 8. NEXT, select "Menu". The "Composite Settings" menu appears. Now set your base exposure between 1 and 60 s (start with 30 s) then press OK. This set how long each composite image will be exposed for and how frequently the LCD is updated. 9. Press the Shutter Release (1st): This first press will expose the base exposure for the time selected in the previous step. A message “Ready for composite shooting” will appear. 10. Press the Shutter Release a 2nd time to start superimposing additional images. Watch the LCD screen to see the star trails increase. An exposure histogram will appear bottom left and an increasing frame counter with exposure time will appear bottom right. [3h max] 11. When ready or to abort, press the Shutter Release a 3rd time to stop capture. File is then stored on the SD card. [To repeat with same settings go to 9, otherwise go to 4] Suitable cameras: OMD-E-M1, E-M5 II, E-M10, Pen E-PL7 and Tough TG-4; instructions for OMD-E-M1 v4.0 firmware
6. 7.
Open Photoshop CC Load files into layers - File> Scripts> Load files into Script... Browse for the files and add them ( click to select multiple files) File> Scripts> Load Files into Stack... Browse for the files and add them ( click to select multiple files). Selected files will have a lighter grey box around them. Do NOT Auto-align layers Change NORMAL to LIGHTEN This blending mode replaces dark pixels of the background with lighter ones of the stars Layer> Merge visible
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© Michael K. Miller, 2016
Reducing exposure in LR/PS can bring out the colours of the star trails Olympus OMD-M-E1, 12-50 mm @12 x2 mm, ISO 400, f/2.8, LIVECOMP – 1½ h (270 x 20 s exposures)
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plane
Star Trails
Images of Stars, the Milky Way, etc.
© Michael K. Miller, 2016
Very slightly out of focus can also bring out the colours of star trails Olympus OMD-M-E1, 12-50 mm @19 x2 mm, ISO 1000, f/4, LIVECOMP – 1 h (120 x 30 s exposures)
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Meteorites and Satellites
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Clear Aperture and Light Gathering The clear aperture (CA) refers to the full diameter of the lens and is a measure of the light gathering ability of the lens. clear aperture = focal length of lens / the f-number where the f-number = the focal length / the diameter of the entrance pupil.
Upcoming showers October 7, 2016 Draconids October 20-21, 2016 Orionids November 4-5, 2016 South Taurids November 11-12, 2016 North Taurids November 16-17, 2016 Leonids December 13-14, 2016 Geminids
f/2.8 lens
Clear aperture, millimeters
Light gathering ability compared to a 50 mm lens
18 mm
6.4
0.36 times
24 mm
8.5
0.48 times
50 mm
17.7
1 (reference value)
200 mm
70.7
4.0 times
(actually 2.828)
© Michael K. Miller, 2014
Meteors are pieces of comet debris that heat up as they enter the atmosphere and burn up in a bright burst of light, streaking a vivid path across the sky as they travel at 37 miles (59 km) per second. Blink and you will miss a single meteor, so take many consecutive exposures with an intervalometer to catch them.
The Milky Way - Kalahari Desert, Botswana, D800, 26mm, f/3.2, ISO 3200, 30 s
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Therefore, longer focal lenses of the same aperture, should be able to detect more stars at the expense of field-of-view, assuming that the atmosphere and other sources of noise are not limiting factors. This is why astronomers would like larger diameter telescopes, as they have better light gathering abilities, and can therefore see more stars, etc.
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“500 (or 600 rule)” for shutter speed
Untracked* Aperture Rating of Lenses One measure of the suitability of a lens for astrophotography is the untracked* aperture rating (higher numbers are better)
Due to the rotation of the earth with respect to the universe, the longest shutter speed to retain stars as circular dots of light rather than streaks can be empirically expressed as maximum shutter speed = 500/focal length of lens. For example, for a 24 mm lens for wide-field astrophotography, the maximum exposure time would be 21 s.
Untracked aperture rating = aperture area * angular area of lens* exposure time focal Horizontal Aperture, 500 rule Clear Aperture Angular Untracked length of Angle of f-number shutter aperture, area, Area, aperture rating 2 2 lens, mm view, degrees speed, s mm mm radians (higher better) f/2.8 35.7 5.0 19 2.5 1773 14 81.2 f/2.8 27.8 6.4 32 1.8 1633 18 67.4 f/1.8 25.0 11.1 97 1.6 3840 20 61.9 f/2.8 25.0 7.1 39 1.6 1556 20 61.9 f/1.4 20.8 17.0 226 1.6 5626 24 53.1 f/1.8 20.8 13.3 140 1.6 3472 24 53.1 f/2.8 20.8 8.5 57 1.2 1406 24 53.1 f/1.8 10.0 27.8 606 0.3 1973 50 27.0 f/2.8 10.0 17.7 246 0.3 799 50 27.0
This empirical formula was developed for film cameras, whereas modern DSLR cameras have finer (pixel) resolution and so the 500 value has to be reduced, as this 20 s exposure shows streaks.
Untracked aperture rating – Increases with larger apertures, but the cost of lens and/or imperfections increases – Increases with shorter focal lengths (i.e., wider angle lenses )
However, chromatic aberration (or coma) from imperfections in the lens at the widest apertures can produce distortions or tails in off-axis point sources, such as stars. *Untracked means that the stars are not synchronously tracked with a motorised equatorial mount.
focal 500 rule D800 4.88 mm time for Clear Light gathering length of shutter pixel crossing acceptable aperture, ability compared lens, mm speed, s time, s sharpness, s mm to a 50 mm lens 35.7 4.8 9.6 5.0 0.28 14 31.3 4.3 8.4 5.7 0.32 16 29.4 3.9 7.9 6.1 0.34 17 27.8 3.7 7.5 6.4 0.36 18 25.0 3.4 6.7 7.1 0.40 20 20.8 2.8 5.6 8.6 0.48 24 16.7 2.2 4.5 10.7 0.60 30 14.3 1.9 3.8 12.5 0.70 35 10.0 1.3 2.7 17.9 1.00 50
A star will cross a single pixel in the plate scale/15 (with both measured in arc-seconds) and the maximum shutter speed in seconds is this value times the number of pixels for acceptable sharpness of the star. A perfectly-focused modern DSLR is able to resolve 2 pixels Max shutter speed, s
20 mm lens
microns
Across 2 pixels
Pixels crossed
x00
24.3
5.98
8.2
6.1
164
36.3
4.88
6.7
7.5
134
50.6
4.13
5.7
8.8
114
“500” rule equivalent
For modern high pixel count DSLR cameras, the “500” rule should be replaced by “100-150” to get approximately the same 2 pixel crossing.
streaks
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The light gathering ability and shutter speed are also determined by the focal length of the lens. For different focal length lens with a constant aperture of f/2.8
Plate scale = the number of arc seconds in a radian (206265) * pixel size of camera’s sensor in mm / focal length of lens (small angle approximation)
Sensor pitch
circular
The length of star trails increases with distance away from the Poles. Therefore, check the entire image and reduce exposure time if necessary.
Plate scale – Focal length of lens
A better approach to determining the maximum exposure time to have circular stars is the plate scale method together with the observation that stars on the celestial equator move 15 arc-seconds per (time) second
Megapixels
3:1 mag
Each sensor pixel is magnified to 3 pixels (3:1 magnifiction)
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Plate scale - Megapixels
Slight elongation to stars = too long
Horizontal AOV, degrees 104.3 96.7 93.3 90.0 84.0 73.7 61.9 54.4 39.6
For longer focal length lenses – The shutter speed to prevent star trails is shorter – The clear aperture and light gathering ability is higher for the same aperture – The field-of-view is smaller 39
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Advanced
The night sky
SkyTracker – motorized equatorial mount
Longer exposures of stars require a motorized equatorial mount, such as the iOptron SkyTracker, Vixen Polarie Star Tracker Mount, etc. to synchronize the movement of the earth/camera to that of the stars and prevent star trails.
1. Level tripod then mount Skytracker on tripod’s 3/8 in thread 2. Set latitude to 0 then rotate the azimuthal base on Skytracker until the front face is perpendicular to N on compass, then lock Azimuth screw. Keep camera away or it may interfere with compass. 3. Mount the camera mounting block to a ball head then attach to Skytracker 4. Mount the camera/lens on the ball head. 5. Focus lens at infinity. Set ISO and f-number. Set and attach intervalometer. 6. Insert the Polar Scope and lock in place. 7. Tilt Skytracker till it reads your Latitude (Oak Ridge is 36 N). You should be able to see Polaris through the Polar sight hole. 8. Fine tune the position with tilt and azimuth controls until Polaris is in the correct position on the Polar scope for the current time (see later) . 9. Lock tilt and azimuth controls. 10. Using ONLY the movements on the ball head, point camera at desired location in the sky. 11. Turn on Skytracker and activate intervalometer on camera. 41
Polaris can be difficult to find – first use compass to find North
Polaris α Ursae Minoris
The night sky Hubble
Perkab
Kochab
Dubhe Merak
Adding the imaginary constellation lines help find Polaris in Ursa Minor
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Polaris is only visible in the Northern hemisphere
Polaris α Ursae Minoris (α UMi), “of/near the (north) pole” is also known as the “Pole Star” or the “North Star”. Polaris can be located by drawing a line from Merak through Dubhe in Ursa Major (Big Dipper, Plough), and go ~5 times the Merak/Dubhe distance to Polaris. It also is the final star in the handle of Ursa Minor (Little Dipper). Polaris climbs higher as you travel North, is directly overhead at the North Pole, and on the horizon at the equator. Stars rotate anti-clockwise in the Northern hemisphere will have Polaris near their center of rotation. 44
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Polaris - close to the north celestial pole
South Celestial Pole
When aligning cameras (or telescopes) for long time exposures, it is important to point to the Celestial North Pole rather than Polaris. The position of Polaris in the sky is slowly moving towards the Celestial North Pole. At noon on Jan 1st 2000 (J2000.0) Polaris was located at 89° 15 50.8. According to Stellarium, it is now located at 89° 19 23. Today, it rotates each day around the Celestial Pole at a radius of approximately 40 (arc minutes).
Star trails in the Southern hemisphere will have the South Celestial Pole at their center of rotation.
The location of the South Celestial Pole can be found 1. from the pointers stars in the Southern Cross and the Southern Pointers near Sigma Octantis. 2. The third corner of the equilateral triangle formed by Canopus (the second brightest star in the sky) and Achernar. 3. Two faint ‘Magellanic clouds” or gallaxies in the southern sky. 4. From the line where Canopus is halfway between Sirius and the pole.
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Note the spotting scope inverts the image which is corrected for in the iOptron app.
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Moon
rise and set times aa.usno.navy.mil/data/docs/RS_OneYear.php
As the moon is illuminated by relatively low intensity, reflected light from the sun, no special viewing precautions are required. Full Moon
What is the best time in its cycle to photograph the moon? New Moon is essentially invisible. Waning Gibbous Full Moon gives largest image but the light is head-on to the surface, so there are no shadows and little 29.5 day cycle detail. Moon rises at sunset and with respect to Sun First Quarter Last Quarter sweeps through the night sky all Synodic month night long till sunrise. New Crescent Old Crescent Between the quarters and the Gibbous moons gives better shadows to see the craters. moonconnection.com/moon_phases_calendar.phtml New Moon Shutter speed = N * N/ (ISO * 2Q), Typical parameters for a clear, cloudless night where N is the f-number M (or A) mode– auto (or manual) focus on moon Q is the brightness exponent Base ISO: 100 - 200 depending on camera ISO is ISO Aperture: f/8 for maximum resolution or f/11 Q = 8 for a full moon – no eclipse Shutter speed: ~1/400 to 1/200 s or use spot meter Bracket exposures. Use a tripod 48 29.5 day cycle Synodic month Waxing Gibbous
Solar System
NASA
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Moon – size of image
Size and Orbit of Moon
100% crop
Day 7 of Lunar Cycle
The orbit of the moon makes around the Earth is slightly elliptical.
Olympus OMD-EM1 with 300 mm and 1.4x TC, effective 840 mm ISO 200, f/5.6, 1/320s, Aperture mode Spot meter and Spot autofocus near the terminus
Day 8 of Lunar Cycle Nikon D500 with 500 mm and 1.4x TC, effective 1050 mm ISO 400, f/5.6, 1/1000s, Aperture mode Spot meter and Spot autofocus near the terminus
100% crop
© Michael K. Miller, 2016
Semi-major axis Mean distance Inverse sine parallax Perigee (supermoon) (i.e., min. distance from Earth) Apogee (i.e., max. distance from Earth)
384748 km 385000 km 384400 km 362600 km (avg.) (356400–370400 km) 405400 km (avg.) (404000–406700 km)
Mean radius
1737.1 km
Equatorial radius
1738.1 km
Polar radius
1736.0 km
200 mm 300 mm 400 mm 500 mm 600 mm 1000 mm
Therefore, the angle-of-view varies by 12-14° between the perigee and apogee between 0.49° and 0.56° Average: 0.518°
~8% of FF camera frame ~11% of FF camera frame ~15% of FF camera frame ~19% of FF camera frame ~23% of FF camera frame ~38% of FFcamera frame
© Michael K. Miller, 2016
Teleconverters and crop sensor cameras extend focal length of lens The moon moves relatively quickly through frame at long focal lengths lenses, so realign after each exposure, and use short exposure times (1/500 – 1/2000s) to prevent blurring. Shorter focal length lenses with a moon as part of the screen or for recording multiexposures of an eclipse from start to end (up to almost 2h) are also good approaches. 49
Brightness of total lunar eclipses
Photographing the Sun
French astronomer André-Louis Danjon proposed a five point scale for evaluating the visual appearance and brightness of the Moon during total lunar eclipses. However, the scale is more useful for evaluating the transparency of Earth's atmosphere. Lunar eclipses vary greatly in their brightness from L = 0, almost invisible at totality (~330 s exposure at f/8, 400 ISO) L = 1, dark gray or brown (~80 s exposure) L = 2, deep red or rust coloured with a dark central shadow and bright umbral edge (~20 s exposure) L = 3, brick red with a bright or yellow rim to the umbral shadow (~5 s exposure) L = 4, very bright copper-red or orange with a very bright blueish rim to the umbral shadow (~1.5 s exposure)
WARNING – Viewing or photographing the sun can be extremely dangerous to your health (and your camera) and many people have lost their sight doing so! Do NOT look directly at the sun! Do NOT look through the viewfinder at the sun! Do NOT use smoked glass! Do NOT use sunglasses! Do NOT use standard Neutral Density filters! Do NOT use exposed film of any type!
What to photograph Sunset and sunrises Leave to the professionals Sun spots and coronal ejections Solar eclipses (rare) Mercury traverses of sun (rare)
For direct viewing ONLY, use special SOLAR GLASSES available at Amazon, etc. or Black Polymer solar viewing film and cards from Thousand Oaks Optical Make sure there are no pinholes by looking through the filter at a lightbulb
For cameras ONLY, use special SOLAR FILTERS - a strong 0.002" thick polymer that is a MINIMUM of ND 5 (ND100000) filter which blocks 99.999% of the suns rays.
Best done by the naked eye just after the beginning and before the end of totality when the moon is near the edge of the shadow to provide an opportunity to assign an 'L' value to the outer umbra These long exposures require a tracking mount to accommodate the rotation of the moon about the earth. see timeanddate.com/eclipse/list.html for list of eclipses for next 10 years
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thousandoaksoptical.com or seymoursolar.com(~$52 for a 52 mm, ~$70 for a 77 mm)
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Use special SOLAR TELESCOPES Use a pin-hole camera (i.e., a black card with a small 0.25 mm (0.01 in) hole) and look or photography the projected image on white paper - mrpinhole.com/calcpinh.php
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Things to leave to the Solar Dynamics Observatory/NASA JPL Loops
Corona
Solar Eclipse – Aug. 21, 2017 & Apr. 8, 2024 Total solar eclipse passes directly over Nashville
Lasts for 2 min 40.2 s
2012 Mercury transit This meteorological phenomenon is caused by the reflection, refraction and dispersion of light in water droplets resulting in an arc of the colours of the spectrum. Rainbows caused by sunlight always appear in the region of sky directly opposite the sun.
Sun spots
© Michael K. Miller, 2008
visitmusiccity.com/visitors/eclipse Solar glasses are mandatory – do not look directly at sun see timeanddate.com/eclipse/list.html for list of eclipses for next 10 years
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Photographing Solar Eclipses
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Solar eclipse – exposure times Corona
An aura of plasma that surrounds the sun and other stars. The corona is approximately one solar radius around the sun's disk
Chromosphere A reddish gaseous layer immediately above the photosphere Baily's beads The row of brilliant points of sunlight shining through valleys on the edge of the moon that are seen for a few seconds just before and after the central phase in an eclipse of the sun Prominences A tongue-like cloud of luminous gas rising from the sun's surface
During totality, the Earth obscures the sun so that the corona can be observed. This increases the size of the coronal image to be recorded by a large factor – 2 to 5 times.
Eclipse feature
Partial with 4 ND Partial with 5 ND Baily's beads Chromosphere Prominences Corona - 0.1 Rs Corona - 0.2 Rs Corona - 0.5 Rs Corona - 1.0 Rs Corona - 2.0 Rs Corona - 4.0 Rs Corona - 8.0 Rs
Focal Lengths in BLUE are for 1.5 Crop frame DSLRs
Due to the corona, the focal length of the lens required for photographing the sun is shorter than that for the moon even though they have almost the same angle-of-views. An effective focal length of more than 500 mm on a high megapixel camera (i.e., 300 mm on a cropped sensor camera) on a sturdy tripod 55
Brightness exponent, Q 11 8 11 10 9 7 5 3 1 0 -1 -3
Shutter speed, s (best to bracket exposures) 0.000313 0.002500 0.000313 0.000625 0.00125 0.005 0.02 0.08 0.32 0.64 1.28 5.12
s s s s s s s s s s s s
or or or or or or or or or or
1/ 3200 1/ 400 1/ 3200 1/ 1600 1/ 800 1/ 200 1/ 50 1/ 12.5 1/ 3.1 1/ 1.6
For 100 ISO and f/8 Shutter speed = N * N/ (ISO * 2Q), where N is the f-number and Q the brightness exponent
ND4 ND10000 neutral density filter ND5 (ND100000) MINIMUM Baily's beads change rapidly
Rs is the Solar Radius
Diamond Ring effect
Use bracketed exposures due to the large variation in brightness of different features
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Photographing Sunsets and Sunrises
Other Meteorological Phenomena
Sunsets and sunrises can be taken with any focal length lens. The best times are often during the magic hours before and after the actual rising and setting of the sun to catch the colours of the reflected light off the bottom of the clouds.
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All © Michael K. Miller 58
Rainbows
Aurora Borealis and Australis or Polar Lights
© Michael K. Miller, 2008
© Michael K. Miller, 2008
Auroras are produced in both polar regions when charged electrons and protons from both the solar wind and magnetospheric plasma are ionized in the Earth’s upper atmosphere producing waves of light of varying colours and complexity. 18-24 mm lens, ISO 400-800, f/2.8, 4 to 25 secs depending on speed of aurora, tripod, no filters
Do NOT look directly or through the viewfinder at the sun.
This meteorological phenomenon is caused by the reflection, refraction and dispersion of light in water droplets resulting in an arc of the colours of the spectrum. Rainbows caused by sunlight always appear in the region of sky directly opposite the sun. 59
70 mm focal length, ISO 200, 1/750 s at f/6.7 - matrix metering
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Halo, nimbus, icebow, or gloriole
Summary of Other Tips Wide-field astrophotography • Make sure you are totally familiar with the controls and functions of your camera • Check sensor for spots – professionally clean if necessary. • Make sure to set the camera settings as much as possible before going dark • Use fresh batteries and memory cards, as you don’t want to changes these in the dark. • Use a study tripod for the long exposures. • Always use a dim red flashlight or red headlight when outside to prevent losing your night vision • Have a compass or smartphone to find the Celestial North Pole (at the end of the handle of Ursa Minor – Little Dipper) • The Magnetic North pole is currently at 86.4° N 166.3° W • Polaris is currently located at 89° 19 23 N • Dress appropriately, as it can get very cold at night • Bring bug spray to repel insects, but KEEP it well away from cameras and lenses as it is nearly impossible to remove
© Michael K. Miller, 2006
This rare optical phenomenon is produced by ice crystals in the cirrostratus clouds in the upper atmosphere creating coloured or white arcs and spots in the sky near the sun or moon. Do not look at the sun directly or through the viewfinder, use live view 24 mm focal length, ISO 100, 1/250 s at f/8 - matrix metering off sky
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Appendix: Types of telescopes
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Appendix: Telescope mounts Telescope mounts have 2 functions 1. Provide a rigid support for the weight of the instrument 2. Control the movement to point and guide the instrument to the object Altazimuth Mount - The simplest mount with two motions, altitude (up and down/vertical) and azimuth (side to side/horizontal). Dobsonian Mount - A newer, modified version of the Altazimuth mount invented in the 1970's by John Dobson for larger and heavier Newtonian Reflectors. Equatorial Mounts • German Equatorial Mount - Both Newtonian Reflectors and Refractor telescopes normally use this type mount. A large counterweight (to balance the weight of the optical tube) extends on the opposite side of the optical tube. • Fork Mount - Most Catadioptric telescopes and other shorter optical tubes use this style mount, which is generally more convenient to use than the German mount, especially for astrophotography. State-of-the-art computer-controlled telescope allows fully automatic operation making it extremely easy and quick for the observer to locate objects.
1. Refracting telescope which uses lenses (dioptrics) to form an image. 2. Reflecting telescope which uses an arrangement of mirrors (catoptrics) to form an image. 3. Catadioptric telescope which uses mirrors and lenses to form an image. A telescope's light gathering power and ability to resolve small detail is directly related to the diameter (or aperture) of its objective (the primary lens or mirror that collects and focuses the light). The larger the objective, the more light the telescope collects and the finer detail it resolves.
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