Astrophotography Without a Telescope - 38

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Dec 19: Astrophotography Without a Telescope - 38

Camera & tripod
What are the basic requirements for astro-photography with a camera?

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: 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.
ESSENTIAL #2: A RELIABLE DSLR CAMERA


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 

GETTING STARTED

  • 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.

MAGNIFICATION

  • 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


Milky Way star clouds & nebulosity around Rho Ophiucus. - Click to very enlargement in a new tab

  • 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


Comet Panstarrs - Click to very enlargement in a new tab

  • Moon.
  • Planetary conjunctions.
  • Jupiter's four largest moons.
  • Milky Way.
  • Magellanic clouds.
  • Open star clusters.
  • Globular clusters.
  • Nebulae.
  • Messier 31.
  • Comets.
  • 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.
  • Wind.
  • Using auto-focus.
  • Insufficient attention to manual focusing
  • Excessive magnification
  • High f-stop lens.
  • Indiscriminate or random astro-targeting.
  • Suburban light pollution.
  • Moonlight.
  • 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


Star vista -  Click to enlargeme in a new tab

  • 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

  • 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.

NEXT STEP

  • 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


Posted by roger in Camera & tripod

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