Machining metre-sized gratings with nanometre accuracy

David Buscher + optical instrumentation group

The echelle spectrograph is the workhorse of optical/infrared astronomy

The size of the diffraction grating is constrained by fundamental physics

Uncertainty principle:

 

Conservation of étendue:

 

 

 

For a seeing-limited spectrograph:

\Delta \nu\Delta x = \text{constant}
\Delta \Omega \Delta S = \text{constant}
\text{grating size}\propto R \times D

For E-ELT HIRES the grating may need to be as big as 1.6m

The error in the groove positions need to be as small as 1nm

Conventional grating ruling engines are not scaleable to large sizes

Mosaiced gratings are difficult to make stable

A promising new grating technology is silicon wet-etching

This produces nanometre-smooth structures

Lithography on large scales is challenging

Interferometric delay lines achieve nanometre precision over large distances

The MROI delay line

Overall concept

"Lithography engine" concept

Measuring grating performance

Current project (2021-2024)

  • £900k grant from EPSRC for precision manufacturing
  • Develop "lithography engine" for patterning a 150mm wafer
  • Measure performance
  • Make and test gratings based on single wafers