Overview
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: | |
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| 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. |
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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 22" f/4.75 - first light May 2008. |
| Slit Test Process - Foucault in 15 minutes (5 for imaging, 10 to process, repeatability +/- 0.001") | ||
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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. |
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Using free ImageJ |
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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. |
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| Import Center of Masses (ImageJ's Results.xls) to SlitTest.xls and compute Foucault. | |
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Mask Holes |
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| The mask holes need to be accurately located. The mask material I use is 040 ABS (0.040" thick) 5 inches wide. The 3/4" holes were cut with a plunge router, with a threaded rod controlling the positioning - Figure 7. Molding glued to the bottom stiffens the mask. Cardboard is then used to covers the rest of the mirror. | |
| Figure 7. ABS Mask Making Jig |
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Note: Accuracy of the hole positions is vary important. The hole positions are adjusted to compensate for parallax i.e., rays are not at right angels to the mask. This is done in 'router mask' of SlitTest.xls. A scanning laser which would start pulsing on detection of the edge, would eliminate the need for a mask. |
Pixels/Inch |
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| To determine the pixel per inch, 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. | |
- Computations