Slit Image Test
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The Slit Image (CCD or film) Test (SIT) combines the best of the Caustic & LWT and avoids their difficulties:

  • The Caustic requires determining the Center of Curvature (COC) within 0.001". A difficult task [1 - page 223].
  • With the LWT [6] each zone needs to be measured mechanically to better than 0.0005". A difficult task unless the room temperature is closely controlled i.e., distance from the mirror to tester changes with temperature while the readings are being taken.

SIT employees a Caustic type mask, each mask hole returning an images of the testers slit, which by the way is visible in an eyepiece. But unlike the Caustic, all the mask holes are open allowing the capture of all the LWT measurements with one exposure.

Film Verses CCD
  For a digital SLR (lens removed), you need to know:
  • Pixels per inch.
  • Lens mounting to CCD spacing [3].
    • Olympus E300 Evolt
         Lens mount to CCD: 38.8 mm
         Pixels/inch: 4771
    • Canon T2i EOS Rebel
         Lens mount to CCD: 44 mm
         Pixels/inch: 5905
  •   For Film (posted 2005) all you need to know is the DPI of the scanner
      The slit image whether film or CCD is processed exactly the same.

          Summary 1

    NOTE: The Slit or CCD or both can be in front or aft of ROC. LWT experts recommend both in front, avoiding the confusion of which ray belongs to which hole (behind ROC the rays cross the optical axis). My setup has both aft of ROC because of the way the camera is attached to the tester. That forces both to be behind where the outer hole ray becomes the outer slit in the image.

    Examples are that of a 12.5" f/4.6 (first light May 2005) and 22" f/4.75 (first light May 2008).

    Align tester to optical axis using the pre-aligned laser.
    Position tester to fill CCD with the Slit image.
    Upload the Slit image to the computer.
    Slit Test Process - Foucault in 15 minutes (5 for imaging, 10 to process, repeatability +/- 0.001")
        Using free ImageJ
    Narrow Slits by increasing contrast (Image->Adjust->Brightness/Contrast).
    Analyze->Set Measurements-> [Check Center of Mass]
    Analyze->Set Scale-> [Click to Remove Scale]
    Compute Center of Mass. Select Slit, then control m

    However for ImageJ to accurately compute the Center of Mass, the image must to be properly exposed. The proper exposure of the image can be verified by examining the image's histogram either in the camera or in ImageJ. Analyze->Histogram in ImageJ will display the Histogram plot. Clicking "List" will generate a list of brightness levels and the corresponding pixels at that level, with 255 being the Max brightest level. The exposure should be such that there are no pixels at that Max 255 level. However, there should be a number of pixels close to that Max 255 level. With digital cameras, pixels which are at the Max are displayed as flashing bright spots, which alerts to over exposure and the loss of information i.e., "clipping".
        Import Center of Masses (ImageJ's Results.xls) to SlitConic.xlsm and compute Foucault.

    SIT ATM'ers
    Contact Lonnie Robinson
    Contact Jeff Baldwin
    Contact Bill Thomas

    Mask Holes
       The mask holes need to be accurately located. The mask material used is 060 ABS.

    ATM'er Lonnie Robinson developed the "Digital Read Out" - DRO Mask Making System shown below in Figure 7 which provides the required accuracy.

        Figure 7.
    Lonnie Robinson's
    DRO Mask Making System


    Note:The hole positions are adjusted to compensate for parallax i.e., rays are not at right angels to the mask. This is done in 'mask' of SlitTest.xlsm.

    A scanning laser which would start pulsing on detection of the edge, would eliminate the need for a mask.

    To determine the pixel per inch of the CCD, cover all holes of the mask except the center. Using a fixed slit light source, position the camera to capture the slit image at the left edge of the CCD. Then move the camera laterally while measuring so as capture the image at the right edge of the CCD. Divide the image pixel difference by the camera displacement.

    • Olympus E300 Evolt
         Lens mount to CCD: 38.8 mm
         Pixels/inch: 4771
    • Canon T2i EOS Rebel
         Lens mount to CCD: 44 mm
         Pixels/inch: 5905

      [1]   Karine and Jean-MarcLecleire, A Manual for Amateur Telescope Making.
      [2]   Kodak Spec KAF-8300CE
      [3] mount to CCD spacing
       Recommend saving this webpage to your computer, since it has vanished in the past.
      [4]   Jim Burrows Sixtests.exe
      [5]   Foucault Test by Leon Foucault 1858
      [6]  Conics
    Optical Shop Testing Edited by Daniel Malacara, Wiley and Sons, 1978, Appendix 1 page 479, 482
    Conic approximation
    Conic equations
    Conic constant - Schwarzschild constant

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    © 2005 - 2017 Bill Thomas
    Contact Bill Thomas