Slit Image Test
Overview Computations Setup
Baldwin On SIT Contact


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 (3) 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 (4).
    • 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 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.

    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".

        To compute the mirrors Foucalts, input ImageJ's Slit Center of Masses into the SIT Conic spreadsheet.

    To download the SIT Conic spreadsheet click here


    SIT ATM'ers
       Contact Jeff Baldwin
    Contact Bill Thomas

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

    Note:The hole positions are adjusted to compensate for parallax i.e., rays are not at right angels to the mask. This is done by the SIT Conic spreadsheet.

    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

    Karine and Jean-MarcLecleire, A Manual for Amateur Telescope Making.
      (2)  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
      (3)  LWT

    J. Francis, "Improved Test Methods for Elliptical and Spherical TCT Mirrors,", ATM J. 14, 34-39, (ISSN 1074-2697).

    Unfortunately, references to the LWT on the Internet have mostly faded away. Jim Burrows author of Sixtests.exe does mention the LWT in passing at LWT was a variant of the moving source Caustic where instead of trying to determine a Caustic zone holes focus laterally and longitudinally precisely, the LWT measurements were done manually in the same lateral plane. Being that for the LWT the longitudinal value was irrelevant, didn't require a mask and little judgment was required to determine laterally where a zone ray crossed the lateral plane, the LWT was an improvement over the standard Caustic. However, the required lateral accuracy required was very tight. Given small room temperature and the time to take the measurements, I could not get repeatable LWT results.

    Lens mount to CCD spacing
       Recommend saving this webpage to your computer, since it has vanished in the past.

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