The standard SPS-300 has a 50mm translating stage which can obtain resolutions of 0.119cm^-1. This high resolution upgrade option extends the translating stage to 300mm, allowing the SPS-300 to reach a resolution of 0.020cm^-1. Due to the increased stage length, an extension piece to the vacuum proof body is required to accommodate the extended stage travel.
Stage Positional Accuracy
The 300mm stage has a built-in encoder which provides the absolute position of the translating roof mirror to the SPS-300 host computer. This encoder has been individually calibrated at the factory using a laser interferometer for accurate positioning. The calibrated positions are stored in a look-up table and then downloaded to the stage software for referencing in real-time operation.
Resolution Determination
The published resolution of the SPS-300 is not the theoretical resolution as determined by the length of the roof mirror translating stage as published by some competitors. The reason is that this theoretical resolution is not practically achievable. Instead, Sciencetech includes an apodization factor which degrades the resolution to a value that is more practically achievable. Although there are several apodization factors that can be used, Sciencetech choose the Bessel function, which degrades the resolution by 1.904 from the theoretical resolution.
The resolution formula is as follows:

• Resolution = A/(2*2*0.80)*L [cm^-1]
• L = stage length [cm]
• A = Apodization factor
• 0.80 is the stage location factor being only 20% from end of stage

Apodization is the act of multiplying the interferogram by a function that smoothly goes to zero at the ends. It is required to minimize the oscillations produced in the Fourier transformed spectrum from the abrupt stopping of the data collection at the ends of the interferogram. This process, however, tends to degrade the resolution. Four different apodization functions can be selected on the SPS300 software. Each one has its own factor (A) on the resolution:

• Boxcar: A = 1.207
• Triangular: A = 1.772
• Bessel: A = 1.904
• Cosine: A = 2.000

The recommended apodization for the very far infrared and THz domains is the Bessel function and hence is what we used as the published resolution.