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photo:ast_photography_intro

introduction to astrophotography

Astrophotography without a telescope:

  • a digital camera can be used to take images of:
    • the sun:
      • you will need a solar filter t ensure your camera and your eyes do not melt!
      • you will need a focal length of at least 200mm to get detail such as sunspots, venus transit or the solar eclipse
      • eg. hand held Olympus C8080 8 megapixel with 1.4x telephoto with solar filter hand held in front of lens was used to take photos of the 2004 venus transit at 1/1000th sec f/4.5 at 100 ISO although the resolution would not be sufficient to see detail such as the black drop effect
    • the moon:
      • a tripod-mounted digital camera can take short exposures (up to 15-30sec) with minimal star trails evident depending on focal length used
        • on a Canon 10D using a 200mm lens, earth's rotation will mean that a star will move one more than one pixel if exposure is longer than 0.5sec and thus start to look elongated.
      • stars and planets will appear as dots or curved lines depending on focal length and duration of exposure
      • for basic deep sky images of stars without trails using only a tripod, dSLR at 1600ISO and a prime lens at f/2.8, see:
      • if you want those beautiful images of long star trails which are exposures of some hours, at present you will have to still use a film camera as digital cameras tend to develop too much noise as they heat up in very long exposures, even if you have a power supply to stop the battery running out.
      • alternative methods of star trails using sequences of short exposures (30sec - 4min each at ISO 100-400):

Astrophotography with camera mounted on motor driven equatorial telescope mount:

  • this allows you to get into the beautiful realm of deep sky objects such as star fields, comets and nebulae
  • don't even think about buying a telescope from a camera shop or department store - these are usually crap - you need a good motor driven mount!
  • this will allow much, much better images of stars, planets, comets and meteors and even deep sky objects such as nebulae, globular clusters, galaxies if the focal length is more than 400mm
  • the motor drive allows considerable reduction in the star trails from the earth's rotation and this can be further improved by either auto-guiding or manual guiding to correct any deviations by tracking an object through the telescope if one is attached
  • it will allow much easier photography of any celestial object as the camera will always be fixed on the object and so you will not need to adjust the tripod to follow the object every minute or so otherwise
  • for photographing dim objects such as nebulae and other deep sky objects, the mount is perhaps THE MOST IMPORTANT part!
  • as a rule of thumb, the maximum duration of exposure on a motor-driven mount without autoguiding is inversely proportion to the effective focal length of your lens.
    • thus, if your mount allows 1 minute exposures with a 100mm lens, you can only do 30sec exposures with a 200mm lens.
  • the quality of the mount will determine how long you can expose for a given focal length and thus the maximum usable focal length, and thus the size of the objects you wish to image:
    • a cheap ($A1000) motor driven equatorial mount may allow 1min at 100mm focal length, thus an ideal lens to use would be f/2 or f/2.8 lens with a focal length in the 100 to 200mm range.
    • an expensive mount ($A15,000) may allow 2min at 2400mm focal length, thus one could use a 1200mm f/5.6 telescope for 4min exposures, or a 600mm f/8 telescope (eg. 80ED type refractors) for 8min exposures.
    • obviously there will be a range of mounts between these extremes, and these mid-priced mounts may suit a Tak 180ED astrograph for example which is a 500mm f/2.8 for 1min exposures.
    • moral of the story, to avoid the hastles and bulk of autoguiding, you need to match your lens to how good the mount will track, the better the mount, the cheaper the lens or telescope you can use for the same focal length as you can get away with longer exposures and slower apertures.
    • remember, for adequate signal:noise capture in dark sites with Canon dSLRs you need minimum sub-exposures of about 1min if using f/2.8 at 1600ISO, thus 2min if f/4 & 4min if f/5.6 which means a more expensive mount.
  • a really good outfit for the beginner with a bit of money:
    • a Hutech-modified Canon 450D or 40D digital SLR, type 1a filter
    • Canon EF 200f2.8L II lens - seems to be the most popular and most useful and gives good results at f/2.8, but better at f/3.2 - you should really use a lens tripod mount with this lens to minimise strain on the camera lens mount.
    • Hutech front filter LPS-V3, $US299
    • a good quality tracking mount (Kenko SkyMemo around $US1000 - this allows for quick set-ups, travelling and wide-field imaging) 
    • TC-80N3 timer, modified for the 450D
  • for those who can afford more:
    • a very good dual-axis German equatorial mount which allows dual axis correction such as
      • NB. weight limit 14-15lb instead of 20-22lb for visual astronomy
      • SkyviewPro mount
        • eg. with 80ED-APO refractor is $US870 - best bang for bucks - allows 2-4 minute unguided tracking
      • Meade LX-55
        • eg. with Meade 5“ refractor is $A2000
      • Celestron CG-5GT - has GOTO controls
      • Meade LX-200 on equatorial wedge:
        • this is a fork mount, and requires the equatorial wedge to improve tracking accuracy
      • Losmandy
        • a better mount which does not suffer from periodic correction which effects tracking of cheaper mounts and limits their exposure to 3-5minutes
    • a high quality refractor lens attached to the camera:
      • 80mm aperture ED/APO refractor attached to a digital SLR camera, or even better, an astronomical CCD camera
      • some use the highest quality camera lenses such as:
        • 200mm f/2.8 L Canon but even these tend to show aberrations when dealing with point light sources and thus best results may be with the lens stopped down by 1/3rd to 1/2 a stop.
        • Canon 400f5.6L - less useful than the 200mm as requires longer sub-exposures and thus a much better mount.
        • Canon 400mm f/2.8 - very expensive but shorter sub-exposures so mount not needed to be as good as a 400mm f/5.6
    • a method to guide the mount that will minimise inaccuracies in its tracking esp. if exposure duration > 30secs:
      • manually via a telescope on the same mount - preferably need an illuminated reticle eyepiece to improve your visual accuracy
      • some astronomical CCD cameras have auto-guiders incorporated
      • off-axis auto-guiding system

Astrophotography through a telescope:

  • for most people, taking photos through a telescope is best limited to the moon and the brighter planets (mars at opposition, jupiter and perhaps saturn) with all other astrophotography with the telescope is with a camera (with a lens up to 300mm or so focal length) attached to the telescope mount system so that the camera can be guided via the telescope's mount +/- the use of an auto-guider (you don't really need the telescope itself). The degree of difficulty in getting good shots of less bright objects through a telescope with focal length > 1000mm becomes very high and relatively expensive. 
  • optical aberrations of the telescope may be a problem especially in outer areas of field of view where vignetting, uneven brightness, and optical aberrations such as coma, spherical, and chromatic may be an issue
    • Newtonian telescopes have many of these problems
    • Schmidt-Newtonians have a little less
    • Schmidt-Cassegrain have even less but still problematic
    • high quality refractors have the least aberrations in general but suffer from chromatic aberration esp. if fast f/ratios, and thus need to either:
      • use an expensive ED or APO refractor
      • use an astronomical CCD camera with colour filter wheel so take images in only one colour at a time, and then these images are stacked later.
  • this can be achieved in several ways:
    • a digital SLR camera or an astronomical CCD camera without a lens attached either:
      • directly to the telescope without an eyepiece, usually via a T adapter - “prime focus method
      • directly to the telescope with an eyepiece via an eyepiece adapter - “eyepiece projection method
        • higher magnification but needs longer exposures and has more optical aberrations
    • a mirrorless digital camera or webcam with lens attached to telescope with eyepiece in place using some type of adapter or attachment mount - “afocal method
      • higher magnification but needs longer exposures and has even more optical aberrations with potential vignetting of image and uneven image brightness
      • webcam allows video capture mode which allows stacking of hundreds of image frames to improve the image.
  • without a guide correction:
    • restricted to bright objects to allow shutter speeds less than 1/30th sec such as:
      • the sun (with a solar filter over the front of the telescope)
      • the moon - allows details of even small craters
      • jupiter - see my photos jupiter
      • venus
      • the planet Mars, saturn at opposition, bright comets - but will need fast ISO eg. 400-3200
  • with guide correction:
    • restricted by quality of guide correction, light pollution
    • can take images of deep sky objects - best with a fast APO/flourite refractor on a high quality mount
photo/ast_photography_intro.txt · Last modified: 2013/02/07 15:10 by gary1