|Lateral Image Test
|Baldwin On LIT
The Lateral Image (CCD or film) Test (LIT) combines the best of the Caustic & LWT and avoids their difficulties:
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.
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).
|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
|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.
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.
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