Rasnik Study


Research Goal:

    To verify the accuracy of the LWDAQ Rasnik Instrument when it is analyzing large-angle rotation.


Research Content:

1. How Rasnik Instrument works and why this experiment is needed.

Rasnik Instrument can take an image of a chessboard pattern and determines the point in the chessboard projected onto a reference point in the image sensor. When the rasnik mask is rotated for a small angle less than 3 degree, the Rasnik Instrument is able to tell the rotation directly.


However, if the rasnik mask is rotated for a large angle, then the Rasnik Instrument will not be able to read out the rotation directly and we need to introduce a ‘pre-rotation’. The ‘pre-rotation’ is a tool that rotates the analysis area before the Instrument analyzes the rasnik mask based on our estimation of what the actual rotation will be.



This pre-rotation method enables our Rasnik Instrument to measure large-angle rotation of the rasnik mask, and my research is aimed to find out how accurate this measurement is.

2. Apparatus

In this experiment, I clamped a camera head onto an aluminum plate and stabilized the plate with a block of metal. The camera is connected to the LWDAQ driver so that I can use the Rasnik Instrument through it and the rasnik sits on a rotation stage.


3. Experiment Procedure

  • The precision of the stand

I started with the stage at 0 degree and takes an image using the Rasnik Instrument to get the initial angle of the rasnik mask. Then I rotated the stage to a random degree and rotate it back to 0 degree and read out the angle of the rasnik mask again. I did this for ten times and the standard deviation of the rasnik mask’s angle is 0.6mrad.

  • Error caused by Pre-rotation

In this part, I fixed the rasnik mask at 45 degree while I changed the pre-rotation input and read out the rasnik mask measurement. In this way, I can see how the choice of pre-rotation influence the measurement. 

  • Large-angle rotation

In this part, I rotate the rasnik mask from 0 to 50 degree and then rotated back to 0 degree and used the Rasnik Instrument to read out the measurement every 5 degree. Then I used the measured rotation from Rasnik Instrument and the actual rotation read from the rotation stage to do a linear fit and calculated the residual.

4. Results

Based on this study, there appears to be a linear error for large-angle rotation and the Electrical Engineer started looking into this problem. At the same time, the data for the error caused by the pre-rotation is also used to improve the Rasnik Instrument.