Astrophotography Playing with your digital SLR camera in the dark
Lots of objects to photograph in the night sky Moon - Bright, pretty big, lots of detail, not much color Planets - Fairly bright, very small, some have interesting detail, some color Stars - Some bright some not, very small but lots of them in interesting groupings, some color Deep Sky Objects: Galaxies - Not very bright, most very small but some large, very interesting detail, some color Nebula Most very very faint, many sizes from small to huge, lots of very interesting detail, lots of color
Astrophotography Myths Celestial objects are very faint, I need a big telescope to photograph them. Celestial objects are small, I need a big telescope to photograph them. Celestial objects are far away, I need a big telescope to photograph them.
The Truth: You don t need a big telescope, or even a small telescope to create beautiful astrophotography images. **** Modified Nikon D700 with Nikon 105mm Macro f/2 lens *** but a big telescope doesn t hurt
Some celestial objects are LARGE! Veil Nebula 1400 light- years from Earth
Veil size relative to the full moon
Creating beautiful images is fun but challenging - Most objects are very faint (except the moon) - Many objects emit light your camera might have a hard time seeing - Everything in the sky is moving!
How faint is faint? So faint that it takes long exposures from over 1 minute to up to an hour to see anything at all! - You will need some way to steady you camera for the full length of the exposure (hand held astrophotography is a no-no) - You will need a device that lets you use the bulb setting on your camera to take long exposures (on most DSLR cameras anything longer than 30 seconds requires some external shutter control device) - You will need techniques to separate what you want in the image from what you don t want in the image.
Long Exposures and Faint Objects Finding the needle in the haystack -During long exposures your DSLR camera generates lots of noise (color speckles and weird artifacts in the image) that end up in your image - Noise comes from the electronics in your camera and is a function of heat and time, the longer exposure and higher the temp, the more noise -During long exposures you will capture stray light from unwanted sources - Sky glow from city lights - Changes in sky conditions, clouds, haze, etc. - The moon (unless the moon is your target) - Cosmic rays (yep, they will show up in your image) - Airplanes and satellites Getting rid of stray light and noise is an essential part of getting a good astro image
All DSLRs Generate Noise Nikon D700 1 minute exposure no lens, lens cover on, viewfinder shutter closed. 75 degrees F (extreme stretch in Photoshop) Nikon D700 5 minute exposure no lens, lens cover on, viewfinder shutter closed. 75 degrees F (extreme stretch in Photoshop)
Getting Rid of Noise Noise can be eliminated from your image by subtracting a noise frame from the image frame. There are a couple of ways to do this 1. Use your camera s long exposure noise reduction setting. What happens: The camera takes your image with the shutter open, and immediately after takes a second image of the same length with the shutter closed. It then subtracts the two images and reads out the difference as the clean image. 2. If you don t have a long exposure noise reduction setting; make your own noise frame to subtract from the original image. - Immediately after you take the image close the shutter, (put a lens cap on) and take an exposure of exactly the same length as the original image. - In PhotoShop; layer the noise frame on the original image and use the difference blend mode.
Stray Light Stuff that you don t want in your image Satellite trail
Getting Rid of Stray Light Getting rid of stray light is primarily done in processing of your images, but there are a number of things you need to do in acquiring the image data to get the best results.. 1. Try to image on nights when there is little or no moon (unless the moon is your target) 2. Try to find as dark a location as you can, away from street lights and other sources of stray light. 3. Shield your lens with a hood of some kind to block as much stray light as possible. AND 4. Take multiple duplicate images (same exposure, same ISO settings, same every thing) of the target object and stack them.
Stacking multiple images Stacking is a digital process where computer software looks at each pixel in an image and compares it to the same pixel in all the other images. It determines what the most likely value is for that pixel and uses that value for the final image.*** Stacking helps get rid of stray light that might affect a single image but probably not all images.. - An airplane or satellite - Occasional car headlight - Cosmic rays and other random noise ***There are a variety of software programs on the market that support stacking
Stacking also has other benefits Multiple short exposures stacked can produce a better quality image than a single long exposure..here s how it s done First get a collection of images: Multiple shorter exposures are made moving the camera very slightly between each exposure. (This is called dithering) Special image processing software then. 1. Aligns the images so that the target object and all stars are in the same location in each image. 2. Examines and compares each pixel in an image to the corresponding pixels in all of the other images in the stack and rejects pixels that are not common to all images within a specified range of value. 3. Creates a new image that uses an average or mean value for all corresponding pixels in the stack of images
Stacking Has Benefits
Stacking has big benefits
Light from the Night Sky Celestial objects emit light across the entire spectrum, but we can only see the a small portion. Most nebula only emit light in very narrow bands of the visual spectrum OII SII The moon, stars, planets, and galaxies emit light across most of the visual spectrum Hydrogen is the most abundant element in Nebula and emits most of its light in the very narrow Ha band of the red spectrum
Narrowband light Spectrum in Red light Spectrum in Red light minus Ha
What your camera sees DSLR cameras are optimized to see daylight and miss much of the light from nebula Cameras designed specifically for astrophotography see more of the spectrum * DLSRs can be modified to see more of the spectrum as well
Modified DSLR Light Response Unmodified Canon EOS sample exposure 10 minutes @ ISO 800 Modified Canon EOS astrocamera sample exposure 6 minutes @ ISO 800
Oh yeah, everything is MOVING! 30 second exposure 105mm f/6.3 ISO 800 5 minute exposure 105mm f/6.3 ISO 800
Everything in the sky rotates around the North star (pole) 10 minute exposure 105mm f/6.3 ISO 800 North Star
You have two choices to deal with the moving sky 1. Take short exposures OK for moon shots, marginal for wide field shots 2. of stars, very poor for planets or deep sky objects. Very high ISO setting 3. and short focal lengths help. 2. Use some form of polar aligned tracking device lots of options from a few hundred to thousands of dollars. Can be manually or automatically guided to follow the object across the sky during exposures.
Camera lens vs. telescopes -Camera lens work just fine especially for wide field imaging - 50mm to 500mm is typically a wide field - wide field is good for large nebula and star fields (Milky Way) - faster (f/2.8 or f/4) is better, but many lens limitations will show up at faster settings -Most amateur telescopes today offer good support for astrophotography - Many amateur telescopes come with a mount that can track and guide for long exposures - Most telescope vendors provide adapters to attach your DSLR: 1. Prime focus mode- using the telescope as the lens (most common for shooting deep sky objects) 2. Afocal mode Using the telescope with an eyepiece in front of camera and lens (most common for shooting planets) 3. Piggyback mode mounting you camera and lens on the telescope to just use the telescope mount to track and guide - Telescopes come in many focal lengths typically from 500mm to 4000mm
Choosing the right camera for astrophotography You can use whatever camera you have, but some features really help -Raw mode If your camera supports raw, use it at the highest bit depth supported to get widest dynamic range possible -High ISO settings Typically you will want to use ISO 800 or higher if possible -Live View A great tool to aid in focusing. - PC or laptop control software- Sometimes is much easier to work with the camera tethered for set-up, focusing, and review of images.
Other useful things to have on an astrophotography outing.. A good star chart It s hard to photography something if you don t know where it is. Remember you probably can t see it with your naked eye. A redlight flashlight Red light doesn t destroy you night vision. Warm clothes The best nights are clear nights and the best imaging times are usually very late at night and early morning. Even in the summer it gets cold. Comfortable chair You end up doing a lot of sitting waiting for those long exposures to finish.
Canon XT (modified), 400mm at f/2.8, ISO 800, 10 x 2min stacked exposures
SBIG ST-11000 camera, mosaic of 4000mm f/8 images, over 90 hours of exposures
Canon 20D, 200mm EF lens at f/4.5, ISO 800, 10 x 2min exposures stacked.
Canon 350D (modified), 250mm at f/4.7, ISO 800, 5 x 1min plus 5 x10sec exposures
SBIG ST-11000 camera, 2200mm at f/6, over 30 hours of exposures
Canon 30D, 500mm at f/5, ISO 800, 10 x10min exposures stacked
Nikon D70 (modified), 460mm at f/3/8, ISO 1600, 8 x 10 min exposures
Canon 20D, 2700mm at f/10, ISO 800, 184 x 4min exposures
SBIG ST-11000, 3000mm at f/9, 6 hours of exposures
Hubble Space Telescope