Slit Image Test | ||||
Overview | Computations | Setup | ||
Baldwin On SIT | Contact |
OverviewThe Slit Image (CCD or film) Test (SIT) combines the best of the Caustic & LWT and avoids their difficulties:
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. In summary, SIT computes Foucaults and is a Fixed Source Caustic Lateral Wire Test as is Sixtests.exe which computes the mirrors profile. Film Verses CCD
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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. |
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).
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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 |
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SIT ATM'ers |
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Contact Jeff Baldwin Contact Bill Thomas |
Mask Holes |
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The mask holes need to be accurately located. The mask material used is 060 ABS.
ATM'er Lonnie Robinson ( http://hourglass-telescope-mirrors.com ) 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 |
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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. |
Pixels/Inch
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References
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![]() 2005 - 2023 Bill Thomas This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. |