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One-Axis Rectangular Optic Mount

  • 50 mm One-Axis Translation for Rectangular Optic
  • Compatible with 1/2" (12.7 mm) to 3" (76.2 mm) Wide Rectangular Optics
  • Vernier Scale with 100 µm Resolution
  • Multiple Mounting Orientations


Rectangular Optic Mount with 50 mm Translation

Application Idea

XF50 Mount Used to Translate an NDL-10C-2 Variable ND Filter

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Test Target Holder
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A channel allows for translation of the lockable support arms for accommodating optics with different widths.


  • Compatible with 1/2" (12.7 mm) to 3" (76.2 mm) Wide Rectangular Optics
  • Maximum Optic Thickness of 0.16" (4 mm)
  • 50 mm (1.97") Travel via Knurled Tumbscrew with 5/64" (2.0 mm) Hex
  • Vernier Scale Provides 100 µm Resolution
  • 8-32 (M4) Tapped Holes Support Several Post-Mounted Orientations
  • Four Nylon-Tipped Setscrews Secure the Optic Between Two Mounting Arms

Thorlabs' XF50(/M) One-Axis Rectangular Optic Mount accepts 1/2" (12.7 mm) to 3" (76.2 mm) wide rectangular optics up to 0.16" (4 mm) thick. It is designed for use with our selection of resolution, distortion, slant edge, and calibration test targets. In addition, it is also compatible with our rectangular microscope slides, filters, dichroic mirrors, variable ND filters, and fluorescence imaging filters.

The actuator on the side of the mount enables 50 mm (1.97") of travel along a single axis. The actuator can be adjusted by hand or with a 5/64" (2.0 mm) hex key (not included). A vernier scale that provides a resolution of 100 µm allows for repeatable positioning of the mounted optic. The actuator can be locked by a side-located 5/64" (2.0 mm) hex setscrew. Seven 8-32 (M4) tapped holes support several possible mounting orientations when used with our Ø1/2" posts. The three tapped holes along the front face of the mount are through holes allowing a post to be mounted on either side. Three of the mounting faces feature tapped holes spaced so that two Ø1/2" posts in post holders can be used next to each other for added stability.

Optics are secured with two support arms that each contain two 5/64" (2.0 mm) hex, nylon-tipped setscrews. Each support arm can slide along a channel in the mount to accommodate different optic sizes; a 5/64” (2.0 mm) hex setscrew is used to lock each support arm in place. This mounting mechanism can be seen in the photo to the upper right. By loosening the top locking screws, the arms can be removed from the channel and interchanged, as shown in the image to the bottom left. When reattaching the arms, simply align each locking screw with one of the 2-56 captive nuts in the channel and tighten. This feature is useful in applications that require the target to be positioned close to a lens or other optical element in a system. 

Please note that the support arms overlap the optic by 4.4 mm on each side.

Test Target Mount
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The support arms can be removed from the channel and interchanged by loosening the locking screws. 
Test Target Mount
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The XF50 Mount has seven 8-32 (M4) taps, allowing for several post-mountable orientations (see images to the right).

The XF50 Mount can be used to position a rectangular optic horizontally, vertically, or perpendicular to the post axis.

Figure 1: Vernier scale measuring 76.0
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Figure 1: An example of how to read a vernier scale. The red arrow indicates what is known as the pointer. Since the tick mark labeled 10 on the vernier scale aligns with one of the tick marks on the main scale, this vernier scale is reading 75.60 (in whatever units the tool measures).

Reading a Vernier Scale

Vernier scales are typically used to add precision to standard, evenly divided scales (such as the scale on Thorlabs’ rotation mounts). A vernier scale has found common use in many precision measurement tools, the most common being calipers and micrometers. The direct vernier scale uses two scales side-by-side: the main scale and the vernier scale. The vernier scale has a slightly smaller spacing between its tick marks (10% smaller than the main). Hence, the lines on the main scale will not line up with all the lines on the vernier scale. Only one line from the vernier scale will match well with one line of the main scale, and that is the trick to reading a vernier scale.

Figures 1 through 3 show a vernier scale system for three different situations. In each case, the scale on the left is the main scale, while the small scale on the right is the vernier scale. When reading a vernier scale, the main scale is used for the gross number, and the vernier scale gives the precision value. In this manner, a standard ruler or micrometer can become a precision tool.

The 0 on the vernier scale is the “pointer” (marked by a red arrow in Figs. 1 – 3) and will indicate the main scale reading. In Figure 1 we see the pointer is lined up directly with the 75.6 line. Notice that the only other vernier scale tick mark that lines up well with the main scale is 10. Since the vernier 0 lines up with the main scale’s 75.6, the reading from Figure 1 is 75.60 (in whatever units the tool measures in).

That is essentially all there is to reading a vernier scale. It's a very straightforward way of increasing the precision of a measurement tool. To expound, let’s look at Figure 2. Here we see that the pointer is no longer aligned with a scale line, instead it is slightly above 75.6, but below 75.7; thus the gross measurement is 75.6. The first vernier line that coincides with a main scale line is the 5, shown with a blue arrow. The vernier scale gives the final digit of precision; since the 5 is aligned to the main scale, the precision measurement for Figure 2 is 75.65.

Since the vernier scale is 10% smaller than the main scale, moving 1/10 of the main scale will align the next vernier marking. This asks the obvious question: what if the measurement is within the 1/10 precision of the vernier scale? Figure 3 shows just this. Again, the pointer line is in between 75.6 and 75.7, yielding the gross measurement of 75.6. If we look closely, we see that the vernier 7 (marked with a blue arrow) is very closely aligned to the main scale, giving a precision measurement of 75.67. However, the vernier 7 is very slightly above the main scale mark, and we can see that the vernier 8 (directly above 7) is slightly below its corresponding main scale mark. Hence, the scale on Figure 3 could be read as 75.673 ± 0.002. A reading error of about 0.002 would be appropriate for this tool.

As we've seen here, vernier sclaes add precision to a standard scale measurement. While it takes a bit of getting used to, with a little practice, reading these scales is fairly straightforward. All vernier scales, direct or retrograde, are read in the same fashion.

Figure 1: Vernier scale measuring 76.0
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Figure 2: An Example of a vernier scale. The red arrow indicates the pointer and the blue arrow indicates the vernier line that matches the main scale. This scale reads 75.65.
Figure 1: Vernier scale measuring 76.0
Click to Enlarge

Figure 3: An Example of a vernier scale. The red arrow indicates the pointer and the blue arrow indicates the vernier line that matches the main scale. This scale reads 75.67, but can be accurately read as 75.673 ± 0.002.

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