Imagine Optic’s HASO™ wavefront sensors, based on patented Shack-Hartmann technology, are excellent tools for the precision metrology of optical systems. Easy to use and integrate, HASO’s wide dynamic range and exceptional precision allow users to characterise a wide range of dioptrical and catadioptrical systems.

This document explains how using a HASO 32 wavefront sensor with a compatible member of our LIP™ family of turnkey optical systems enabled us to precisely characterize the wide-aperture optical system found in a Schmidt-Cassegrain telescope. The data acquired provided the information necessary to asses the effects of the secondary mirror’s misalignment and to use that information to correct its alignment.

To the left, the testing setup followed by a Laser Diode (included with all LIP models) being attached to a LIP 32 with an f/10 objective fitted onto a HASO 32 wavefront sensor.

Configuring the test

The diagram below shows an easy to configure method for measuring the wavefront error (WFE) of a Schmidt-Cassegrain telescope using an Imagine Optic HASO-32 wavefront sensor equipped with a LIP32-10 optical system on a standard optical workbench. In this example, the source was adapted to the numerical aperture of the telescope by using a standard LIP f/10 objective.

Measuring the telescope’s WFE

The first step in measuring the telescope is choosing an adapted experimental configuration - in this case a vertical setup was chosen. In order to ensure maximum accuracy, and based on the telescope’s 10” aperture, an 8” ?/150 rms (Root Mean Square) flat mirror was employed for autocollimation. The mirror’s flatness is very important as it will directly impact the accuracy of the measurements obtained (several tests are available to verify your mirror’s flatness). Once the experimental set-up was in place, we performed the following:

1. Calibrated the HASO32/LIP32-10 to a perfect sphere and saved its WFE reference file. This was done by placing a spherical reference mirror in front of the LIP32-10. Although HASO wavefront sensors can measure in both absolute and relative (“referenced”) modes, we chose to work in referenced mode to ensure WFE calibration with the referenced optical elements. In this configuration, WFE measurement accuracy was better than ?/150 rms.
2. Aligned the HASO/LIP system in-line with the telescope’s aperture (rotation of the autocollimation mirror)
3. Measured the telescope’s WFE in referenced mode using the data file acquired in step 1.
   

Results

Before alignment - The wavefront display windows below shows us that there was a significant field aberration. When LIP was aligned to the telescope’s mechanical axis we discovered that the secondary mirror orientation had introduced coma aberrations into the optical system.

After alignment - HASO’s real-time measurement features enabled us to correct the telescope’s alignment and observe the effects of our adjustments as we worked. We were able to align the secondary mirror to the primary mirror, thereby reducing the optical system’s field aberrations to almost 0. Both the rms and PV (Peak to Valley) measurement images below allow us to appreciate the effects of the telescope mirrors’ proper alignment (WFE improvement from 226 nm rms to 19 nm rms).
Conclusion

This test, performed on a simple optical system, was designed to demonstrate LIP’s easy integration into a precision HASO metrology system. Systems of this kind from Imagine Optic are ideal for characterizing optical elements ranging from small-diameter mirrors through to large-aperture telescopes. For example, a customized LIP solution was used to characterize and align the 3m Herschel telescope’s optical system, and the CFHT uses HASO and LIP to perform regular maintenance.

Many of the world’s most advanced astronomical facilities depend on Imagine Optic’s metrology platforms to detect high-spatial frequency aberrations caused by polishing defects and to precisely position dependant devices including cameras and spectrometers. If you would like more information, please contact us by clicking here. To download this application note in PDF format, click here.