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focusing a telescope

Causes of a blurred image:

  • excessive camera movement when shutter is opened:
    • this may be a problem due to the vibrations caused by a mirror in an SLR opening
      • Solution is to set the mirror to up position at least 5-10secs prior to exposure
    • may also be caused by the shutter mechanism itself, eg. the Canon EOS cameras are notorious for this
      • Solution is to try the hat trick - you control the exposure manually by placing an obstruction in front of the lens or telescope.
  • excessive movement of the telescope during the exposure:
    • this may be due to residual vibrations from when you last touched the system
      • solutions include using vibration pads on feet of tripod to shorten vibration duration and use a self timer or remote trigger to start the exposure
    • this may be due to wind:
      • solution is to avoid wind
  • poor tracking or guiding of the telescope:
  • if high magnification, then poor seeing due to atmospheric or telescope conditions:
  • telescope aberrations:
    • if the centre is in focus but not the outer areas, this means that the optics do not have a “flat field”
      • solution is to but a field flattener or get a different telescope design
    • other aberrations may be due to optic design or poor collimation.
  • camera cannot get to infinity focus:
    • this may be a problem with some digital cameras with lens set to manual focus
      • solution is to use auto-focus on an object near infinity then lock the focus.
    • this is more likely to be a problem when you attach a SLR to a telescope, as the telescope may not have sufficient “back focus” to allow the camera to get close enough to focus at infinity. 
      • Solution is to buy a different focuser for the telescope which allows camera to get closer physically.
  • focus is innacurate:
    • this is a most common cause as it is very difficult to achieve accurate focus manually, especially through a digital SLR
      • Solutions are outlined below.


Focusing a camera on a telescope:

  • focusing a digital camera can be very difficult when trying to get accurate focus of faint objects.
  • autofocus:
    • with any AF lens that is f2.8 or slower, autofocus with the center spot cannot be bettered by any manual means, thus use AF with focal lengths > 100mm
      • to lock in AF: After focusing on a bright star you just turn off Autofocus on the lens. The Canon lens focuser ring is very stable even if you change position of the lens.
    • focal lengths < 100mm, AF may not lock on a star, thus either focus at infinity in daylight and lock focus in, or use a laptop to nudge-focus
  • manual focus mode on camera set to infinity then use the magnified image in viewfinder or on LCD screen to attempt focus of the telescope, but this by itself may be impossible to see much, so here are some options
  • live view camera LCD screen with 7-10x magnification is THE EASIEST method:
    • eg. most modern dSLRs - I wouldn't buy one without Live Preview now.
    • most can focus on a bright star (eg. magnitude 3 or brighter) but often have trouble on really bright objects such as the moon unless it substantially fills the screen.
    • Olympus models have the additional benefit of Live Boost mode which allows use of fainter stars.
  • magnifying viewfinder for camera:
    • eg. Canon Angle Finder C or Stiletto Series IV focuser but these may make image darker to see
    • eg. adapted Olympus OM Varimagni finder for use on Canon dSLRs
    • Angle Finder C + special viewscreen “I” (clear center spot + crosshairs) works fine at night but only available for certain dSLRs.
  • interchangeable focusing screens in the camera:
    • Canon 10D has a interchangeable focusing screen whereas the DRebel/300D/Kiss does not and you are stuck with a fresnel screen not very suited to astrophotography
  • diffraction mask on the telescope:
  • laptop-based:
    • manual method:
      • If you do have a laptop, but not the focusing software, you could take a shot in low resolution…download it using the Canon Utility…look at it… adjust focus…and repeat until you get it right.  I know, it sounds like a pain, but one can get perfect focus in 5 to 10 minutes using this method.  BTW, this is the same principle used with the software products…it's just automated. 
      • with the new Live Preview dSLRs, some can transmit live images to the laptop which can speed this process up significantly BUT the prolonged use of Live Preview may increase noise in subsequent images due to heating of the sensor.
    • automated method using dedicated software:
    • attach the external monitor to the camera, take an image and display it on the monitor; no real time feedback. The monitor is much larger than the camera's LCD, so it just aids in manual focusing with no feedback other than your own “eyeballing.”

Ronchi focuser (from Yahoo forum posts by Bill Keicher):

  • Stellar Technologies International (STI), Series IV Stiletto focuser
    • can be used with several different Ronchi rulings or a knife edge, they are interchangeable, but the Ronchi rulings are very fragile
    • The focuser uses a Ronchi grating of 180 lines/inch to achieve critical focus
    • The principle of operation is the same for knife edge or Ronchi. The Ronchi ruling is a periodic bar pattern that is positioned near the primary focus of the telescope. The periodic bar pattern interupts the diffraction pattern of a single star and thus acts as a spatial filter when it is exactly at focus (Ronchi pattern in the focal plane of the telescope). One can imagine the ruling bar blocking the star's central Airy disk at focus. The Stiletto optical system includes a re-imaging lens and a Ploessl eyepiece which image the pupil plane of the telescope when in focus. The telescope central obscuration is present in the imaged pupil. 
      When first set up, you see a bar pattern that changes spatial frequency as you attempt to focus. Increasing spatial frequency indicates movement away from focus. Simply adjust the focus to 
      reduce the bar pattern to a single “bar” that covers the entire pupil. At this point, you are in focus and the bar is either lighter or darker depending very critically on focus position. Focus is achieved quickly. The Stiletto is then removed and the camera is attached. The assumption is that the distance to camera focal plane is the same as the distance to the Ronchi ruling.
    • focus sensitivity will be a function of the Ronchi ruling spatial frequency, that is, the 300 line/inch ruling will produce a tighter focus than the 180 line/inch ruling. This would especially be important for faster optics with a smaller depth of field and smaller Airy disk. Compare the size of the bar to the Airy disk. Image brightness and contrast may work in the opposite direction, with the 180 line/inch image being brighter than the 300 line/inch image.
    • The knife edge is the ultimate focus aid, since it would be more sensitive to focus error than any Ronchi ruling. It blocks one half of the star's diffraction pattern. The claim is that it is more difficult to use, since it is easy to move right through focus without noticing it. Use of the Ronchi  is more forgiving. 
    • They are both spatial filters since they are at the Fourier plane of the telescope. The size of the Airy disk is linearly proportional to the focal ratio.
      • Airy disk = 2.44*(wavelength)*F#, thus for f/10, = 2.44*(0.5 micrometers)*10 = 12.2 micrometers
    • A 180 line/inch Ronchi ruling has a bar size of 141 micrometers, about twelve times larger than the Airy disk. So a single Ronchi bar may block the central order of the diffraction pattern (Airy disk) as well as quite a bit more of the circularly symmetric star diffraction pattern when it is in the focal plane. This is why, when looking into the Stiletto, the pupil image is either dark or light at focus - 
      dark if a bar is centered or near centered on the Airy disk, light if the space is centered or near centered on the Airy disk. Only a “single bar” is left when at focus.  
    • For a telescope focused at infinity, i.e., a star, the Fourier plane and the image plane are the same. The magnitude squared of the Fourier transform of the amplitude and phase distribution across the 
      circular aperture of a telescope is the diffraction pattern seen in the focal plane. The image of a star is the resulting diffraction pattern. Again, the image of a star is the magnitude squared of the 
      Fourier transform of the aperture function. If the telescope has phase distortions or a central obscuration, the diffraction pattern represents the magnitude squared of Fourier transform of those phase and amplitude distortions. 
    • Focus error, coma, astigmatism, etc. can all be viewed as phase errors. A special set (normal set) of mathematical functions called Zernike polynomials can be used to describe these and higher order phase distortions.
    • Using the knife edge is a good way to view higher order phase aberrations in your telescope or in the atmosphere. 
    • As for references, try Fourier Optics by Joe Goodman to start and probably a Google search on Schlieren optics, Fourier optics, Foucault test, phase contrast microscope, spatial filtering, etc. may 
      be useful since all these terms all are related. Even analog communications theory with a vector representation of narrowband phase modulation is useful in gaining insight into this problem. 


photo/telescope_focus.txt · Last modified: 2013/02/08 01:16 by gary1

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