Lateral Image Test
Overview Computations Setup
Baldwin On LIT Contact


The Lateral Image (CCD or film) Test (LIT) 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.

LIT employees a Caustic type mask, each mask hole returning an images of the testers source, 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. That image is then processed by imageJ without human judgement. Those results are then the input to the LIT Spreadsheet, which computes Foucault's for a specified Conic Constant (not just a parabola). The LIT spreadsheet equations were verified using sixtests.exe in the Caustic, Fixed Source mode.
That is, the LIT is a Caustic, Fixed Source test. The Caustic was invented by R. Platzeck and E. Gaviola as reported in Journal of Optical Society of America, 20 (November 1939).

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
     In 2002, while figuring a 12.5” f/4.6, I was flunking Foucault 101, and that begain my use of film to figure that mirror. It worked and is documented here.
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.
PoLITion 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

Also of note, Lonnie Robinson ( ) is using a precision pinhole instead of the slit, and with imageJ Center of Mass function, getting repetable results to a thouands of a pixel.

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 LIT Conic spreadsheet.

To download the LIT Conic spreadsheet click here


   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 poLITions are adjusted to compensate for parallax i.e., rays are not at right angels to the mask. This is done by the LIT Conic spreadsheet.

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


As an example, using the Canon EOS Rebel T6 ( specs are as Follows:

Horizontal Sensor of 22.3 mm or 0.8779528 inches
Max horizontal resolution of 5184 pixels
Pixesls/inch 5905 = (5184/0.8779528)

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 did mention the LWT. 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 and didn't require a mask. 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.
  (5)  Dr. Cuffy's Hartmann

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