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
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For a digital SLR (lens removed), you need to know: |
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CCD cell size.
Lens mounting to CCD spacing.
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Fortunately, this information was available on the WEB for the Olympus E-300, which I use [2, 3].
For film, all you need know is the DPI of the scanner. The slit image whether film or CCD
is processed exactly the same.
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| Figure 1. Setup |
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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.
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| Photoshop: |
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To facilitate measuring: |
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The image is rotated using the Measuring Tool and Rotate Arbitrary.
Using Selection and Levels the width of the slits are reduced to the brightest pixels - Figure 4.
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The pixel coordinate (Ypix) of the slits are measured using the Measuring Tool - Figure 2 and 5.
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| Sanity Checks: |
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E300 Specs: |
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The E-300 was replaced with a fixture positioning film at the CCD image plane - Figure 6.
The processed images confirmed the E-300's lens mount to CCD spacing, and cell size
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Star Tests |
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Two mirrors (22" f/4.7 and a 12.5" f/4.5) were figured using SIT, which were then star test validated.
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| Tester Alignment: |
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As is the case with the Caustic and LWT, the testers longitudinal axis needs to be
aligned to the mirrors optical axis, and the film plane needs to be perpendicular to that
axis. All of which is quite doable, and is described at http://yubagold.com/tests/alignment.php.
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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.
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| Figure 7. ABS Mask Making Jig |
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Note: Accuracy of the hole positions is vary important. To that end, the mask is flatbed scanned
(direction of scan is most accurate). Then 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 Spreadsheet.xls.
In the past, had local machine shop make the mask for about $90 - still scanned them.
A scanning laser which would start pulsing on detection of the edge, would eliminate the need for a mask.
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- Computations