I love taking astro-images with my telescope, especially when I'm out in the field with fellow amateur astronomers - but sometimes I prefer to leave my telescope in its box and just shoot with my camera instead.
It's fun, it's simple and with a little experience you can achieve some interesting astro-photography. So, if you want to explore the night sky but you don't want the expense of buying a telescope, this article can be a guide for starting off in basic astro-photography without a telescope.
If I were to sum up the requirements for achieving worthwhile astro-images without a telescope in just one sentence, it would be:
"You need a dark sky; reliable photographic equipment which you understand how to use; and a good working knowledge of the celestial sky."
There are no short-cuts to this formula.
ESSENTIAL #1: S SOLID TRIPOD MOUNT.
DSLR camera with wide angle lens on tripod. Note the suspended stabilising centre weight
- To avoid wiggles and blurs, a tripod is absolutely essential for astro-imaging.
- It is common knowledge that hand-holding a camera during exposures longer than about 1/20th of a second will result in the image becoming blurred squiggles.
- Astro-imaging of faint night-sky objects requires even longer exposures, typically up to about thirty seconds.
- An expensive camera will only produce the desired results if it is kept perfectly still during a long exposure.
- The tripod should be solidly built and extendable to about 1.5 metres.
- It should have smooth pan and tilt processes.
- Hang a weight below the tripod to keep it stable, reduce vibrations and provide low centre of gravity.
My Canon 60D DSLR with a 135 mm lens attached.
The basic camera requirements are:
- Tripod attachment socket.
- Remote control socket.
- Long exposure capability (up to thirty seconds and/or bulb)
- Manual focus capability because auto-focusing was not designed for shooting the night sky and will almost certainly not work.
- Manual settings for shutter speed, f-stop and ISO adjustment.
- A lens hood helps to keep out local stray light and reduces susceptibility to dew.
- 'Live view' to frame the desired field and assist with manual focusing.
- A swivel monitor to achieve this without straining your neck.
The bottom line of all the above is that you will almost certainly require a digital SLR camera, or similar.
ESSENTIAL #3: WIDE FIELD LENS
- Wide angle (low magnification) lens between about 10 mm and 50mm.
- Wide aperture lens (such as f/1.4 or f/2).
ESSENTIAL #4: REMOTE SHUTTER RELEASE
My old Canon 300D with remote shutter control intervalometer attached (right). The large grey button is the shoot button.
- To avoid vibration and movement, a hand held cord shutter controller is necessary.
- To take multiple images of the same object, use an intervalometer
- Using a celestial atlas, planisphere or computer software, determine which objects are visible at the time you wish to view.
- Set up and ensure the camera is firmly attached to the tripod.
- Attach the remote cord shutter release.
- Set the desired exposure time, aperture size and ISO.
- Frame the image
- Set the camera to manual focus and focus very accurately, using electronic zoom on 'live view'.
- Experiment with your exposures until a satisfactory setting is achieved.
- The longer your exposure, the more light you let into the camera but the more likely you are to get trailing.
- The wider the aperture, the more light you let into the camera.
- The higher the ISO, the more light you let into the camera but the more grainy it might appear.
- There is a limit to how much light you can let into the camera if the sky is not dark, due to either light pollution, moonlight or early evening glow.
- Experiment to find a happy medium between the three variables.
- Amateur astronomers learn very quickly that wide field/low magnification yields the best views.
- Over-magnification will rarely produce worthwhile results.
- Stars will trail faster across the image during exposure.
- The ever-present atmospheric turbulence is magnified by powerful lenses and will turn pin-point stars into disappointing blurry blobs.
- Using a high magnification, narrow field lens will generally result in blurred stars.
THE 500 RULE
- Earth rotates approximately 360° every 24 hours
- Celestial objects will appear to move across the field of view at the rate of 15° per hour - or 15 arc-minutes every 60 seconds.
- Because your camera is static, celestial objects will "trail" in long exposures.
- The higher the magnification, the quicker any object will move across the field of view.
- To avoid the trailing effect, select a shutter exposure time which is less than 500 divided by the lens focal length in mm.
- In practice, this limits non-tracking astro-imaging to less than one minute and probably considerably less, depending on the focal length.
- 500/10 mm lens = 50 sec max.
- 500/25 mm lens = 20 sec max.
- 500/50mm lens = 10 sec max.
- 500/200 mm lens = 2.5 sec max.
CHOICE OF TARGET
- Planetary conjunctions.
- Jupiter's four largest moons.
- Milky Way.
- Magellanic clouds.
- Open star clusters.
- Globular clusters.
- Messier 31.
- International Space Station.
WHAT PRODUCES DISAPPOINTING ASTRO-IMAGES?
- Hand-holding the camera instead of using a tripod.
- Using the camera shoot button instead of a cord shutter release.
- Unsteady or non-secure mount.
- Knocking the mount during exposure.
- Using auto-focus.
- Insufficient attention to manual focusing
- Excessive magnification
- High f-stop lens.
- Indiscriminate or random astro-targeting.
- Suburban light pollution.
- Street light or house light glare.
- Poor choice of exposure, ISO, f-stop settings.
- Over exposure or under exposure.
- Ignoring the effect of celestial rotation.
- Exceeding the 500 rule.
- Attempting to image faint objects in light polluted skies.
- Shooting JPG images instead of using RAW.
- Dew on the lens.
- Dirt, dust or smears on the lens.
- Excessive cropping of the object in post-processing, resulting in individual pixels enlargement.
- Lack of understanding of the relationship between shutter speed, focal ratio and ISO settings.
- Point and shoot cameras.
- Inadequate post-image processing.
- Over-enhancement in post-processing.
- The belief that you are shooting aliens in flying saucers.
CLOUDS & FOREGROUND OBJECTS
- Clouds, Earth satellites, aircraft or the Moon will often wreck astro-photography.
- However, with wide angle astro-imaging you can sometimes make these work to your advantage.
- Many of the best astro-mages include foreground objects.
">WHAT'S IN MY IMAGE?
- This is where a basic understanding of astro-photography is essential but in short you will see stars and planets with possible trails from aircraft and satellites.
- It is best to know which object you are shooting before you shoot it.
- The appearance of any object which you cannot identify is not evidence for aliens in their flying saucers or government conspiracies.
- It just means you either don't understand the celestial sky or you don't understand your camera.
- The key to successful astro-imaging without a telescope is a solid mount, precise manual focusing and correct exposure combined with knowledge of the night sky and choice of target.
- Use wide-field lenses with low magnification.
- Develop your own techniques.
- Join your local astronomical society for experienced tips and integrate them into your routine.
- Purchase a motorised sidereal tracking mount from your local astronomy shop.
- Learn how to enhance your images using software.
- If you fancy working with higher magnification, then to yield worthwhile results, invest in a decent astronomical telescope.
- However, even with a large 'scope, maximising the magnification is still not the key to success.
All images © R.Powell