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photo:ast_photodigital1

astrophotography with digital cameras

Choosing a dSLR for deep sky astrophotography:

  • see also: 
  • in the days of film, the legendary Olympus OM-1n was king of astrophotography as it was light, had mirror lockup, bulb time exposure with cable release, worked without batteries and you could buy focussing screens that enabled easier focus when used on long focal length lenses. But today digital is king, although battery life can be an issue, particularly if you want to do those star trails over several hours!
  • most beginners should aim to take multiple 60sec sub-exposures at ISO 800-1600 and at f/2.8 in dark skies with focal lengths no longer than 300mm in 35mm terms using a tracking mount (unless you have a really good mount +/- autoguiding gear).
    • you can use longer focal lengths (eg. prime focus with camera attached through a telescope) for bright subjects such as the moon and Jupiter, but for other subjects you will need a VERY GOOD mount probably with auto-guiding system to get good results.
  • ideally the camera should have:
    • low noise at high ISO (most currently use ISO 800-1600)
      • Canon wins here but the latest Olympus models are OK at ISO 800
      • for dark sky sites, using a camera with low noise at high ISO like the latest Canon dSLR's, it seems that using 1600ISO to give you shorter sub-exposures (60sec at f/2.8) and thus less guiding error is the way to go. 
      • in light polluted sites, sub-exposures are limited by the light pollution anyway so lower ISO's are better (eg. 400-800). 
      • see http://www.pbase.com/terrylovejoy/optimal_isos for details on the best ISO to use for DSO's
      • see Roger Clark's many articles on sensor performance in low light situations
    • true RAW mode that doesn't obliterate faint stars 
      • ie. not Nikon unless you want to use Mode 3 controllers
        • NB. there are concerns that the RAW file is not a true RAW file in Nikon dSLRs as Nikon applies a blurring noise reduction in-camera process even to the RAW files which removes faint stars, thus they need to be used in “Mode 3” which is a bit of a pain.
          • see here for more details.
    • its IR filter replaced with UV/IR Ha-enhanced filter to give good nebula images via improved Ha infrared response
      • modern dSLRs block the H-alpha region allowing only ~27% transmission, removing the IR blocking filter increases transmission to nearly 98%, thereby giving 4x more sensitivity or 2EV for H-alpha nebulae.
      • ie. Canon & Nikon dSLRs or Olympus E410
    • Live Preview to accurately manual focus
      • most new dSLRs, although in addition, Olympus models have Live Boost for seeing even fainter stars
    • preferably an articulating LCD so you don't get a sore back
    • high quality prime lens (not a zoom lens) with good performance at aperture f/2.0-f.2.8 at 35mm effective focal length 200-300mm, preferably without optical IS which degrades star images.
      • lenses wider than 100mm in 35mm terms have limited utility as light pollution gradients are problematic and the lens preclude the use of interference type filters such as most narrow band light pollution filters, while the only objects for a wide angle are the Milky Way, aurorae and perhaps meteor showers, but if you do wish to try them out, be aware that you will need to stop them down for good star images: eg: 50mm lens at f/2.8-4, 28mm lens at f/5.6.
      • for Canon:
        • most get the Canon EF 200mm f/2.8 L non-IS lens with the tripod mount ring (buy separately as not supplied), better star shapes at f/3.2
        • Canon 100mm f/2.8 USM macro gives excellent results at f/2.8 (don't use the older non-USM one) as does the 100mm f/2.0 when used at f/2.5.
        • one might also consider the Canon 135mm f/2.0L although there seems to be little experience on the web forums.
          • with 1.4x TC gives 189mm f/2.8 but perhaps not quite the image quality of the 200mm f/2.8 but at least you can use it at f/2.0 when you need more aperture and focal length not so important, eg. comet tails.
      • for Olympus:
        • the expensive but very nice ZD 150mm f/2.0
        • the cheaper but still high quality ZD 50mm f/2.0 macro lens +/- 1.4x teleconverter to give an effective 140mm f/2.8 lens although lacking in telephoto reach.
        • legacy manual focus 135mm f/2.8 lenses such as Nikkor, Leica R or Carl Zeiss lens may give good results at effective 270mm f/2.8
    • be light, compact with good battery for cool conditions
    • mirror lockup:
      • this is to avoid vibrations of the camera mirror blurring image - this is essential for exposures from 5sec to 1/250th sec
      • Olympus - mirror lockup can be achieved through setting a timer in the menu under “AntiShock” which will put mirror up for that length of time prior to shutter being opened.
      • Canon - seems there is a problem that you cannot set mirror lockup when using the Timer Remote Controller!
    • timed 60sec exposures as this is the most commonly used sub-exposure duration
      • Olympus has this but you need a TC-80N3 Timer Remote Controller for Canon dSLRs (but not available on the Rebel dSLRs)
      • the latest Nikon or Fuji dSLRs can use the Nikon MC-36 Multi-Function Remote Cord
      • You can't remote release many Nikons (eg. D40) using Camera Control Pro when the camera is in Bulb mode, you need to buy 3rd party tools like Shoestring Astronomy's DSUSB-IR and download DSLR Shutter from Stark Labs. 
    • time lapse facility so you can do 20-100 sub-exposures easily
      • for Canon, use the TC-80N3 Timer Remote Controller (with N3-capable dSLRs)
      • Nikon D300 has a built-in intervalometer

other accessories to consider:

  • Hutech front filter LPS-V3, $US299
  • a good quality portable tracking mount:
    • Kenko SkyMemo around $US1000 - this allows for quick set-ups, travelling and wide-field imaging
  • TC-80N3 timer for Canon 1D, or modified for the 450D

Using a digital camera at higher magnifications:

  • for bright planets and the moon:
    • a SCT telescope is probably best for this high magnification work although a Newtonian will do but need a good, big mount.
      • APO and prime lenses do not usually have sufficient aperture for the required magnification
    • via prime focus on a telescope:
      • removable lens to allow  direct attachment to the telescope eyepiece mount ie. must be a DSLR
      • generally a webcam with eyepiece (see next) is a better choice than a dSLR as can take many photos in 1 minute and then stack them.
      • for optimum resolution & brightness then aim for a f/ratio = 4 x pixel size of your sensor
    • via eyepiece on telescope (afocal method):
      • ability to be attached to the eyepiece via an adapter
      • small lens where its diaphragm is near the front of the lens to avoid vignetting
      • not too heavy camera
      • many use a webcam or the Nikon 4300 for this
      • I have used a Canon S30 point and shoot with my 10“ Newtonian via an eyepiece - see astrophotography
  • when purchasing a camera to use, consider:
    • can it be easily mounted to the telescope eyepiece
      • cameras with a thread on the front of the lens can usually be easily adapted to fit eyepieces via adaptors
        • options include:
          • all-in-1 DigiT kit specific to a camera style: $A179-209
          • DigiT adapter (Pentax T screw mount) clamps onto eyepiece ($A60) + specific camera T adapter ($A130-170)
          • MaxView 40 eyepiece with inbuilt T thread ($A279) + specific camera T adapter ($A130-170)
            • better for wide aperture lenses to minimise vignetting
      • cameras with removal lenses (dSLRs) can also use either:
        • prime focus if a T-mount adapter is available
        • eyepiece projection method using an eyepiece and an adapter but no camera lens:
        • afocal method using an eyepiece and an adapter and a camera lens
        • with “fast” fixed focal length lenses or telephoto lenses for better piggy-back pics
          • photos of constellations, aurorae, or meteor showers can be done with almost any lens
          • comet photos ideally need a 200-500mm lens, preferably with aperture f/3.5-5.6, but will need to be piggy-back mounted on a telescope which is guided on the comet for good results as exposures at 400ASA need to be 15sec - 2min on digital cameras, to get image of the fainter tail, it is important to have a dark sky away from light pollution otherwise the whole image will be light & obscure the tail.
      • other cameras can by mounted with a EZ-PIX universal camera adapter ($A109) although this require some time to align properly as there is no direct connection to the eyepiece axis.
    • issues with non-dSLR digital cameras using afocal method:
      • weight may be an issue as heavy cameras may:
        • create balance issues with telescope mount
        • digital camera lens attachments may stress the lens filter or the lens itself, permanently damaging the camera - may need to use an attachment system that clamps onto camera body or via the tripod mount.
      • the larger zoom lenses usually have a very proximal iris diaphragm which is impossible to get sufficiently close to the telescope eyepiece to avoid vignetting making the afocal method impossible
        • may need to use 2” eyepieces &/or eyepieces with long eye relief
      • cameras with larger lenses are best suited to piggy-back photography only. 
photo/ast_photodigital1.txt · Last modified: 2013/02/07 14:36 by gary1