The spectrographs among all sites are effectively the same, and have been designed by Paolo Spano.

The basic optical design of the SONG spectrograph is pictured here. The light from the telescope exits a fiber and is focused on a slit. It then gets collimated by an off-axis parabolic (OAP) mirror (CM1) before reaching the diffraction grating. After recollimation at the second OAP (CM2) it gets further dispersed by a prism before forming an image that falls on the detector after reimaging by an optical camera. The overall design is extremely compact.

The grating is manufactured by MKS | Newport Richardson. The item number is 53A33ZD06-174E, with serial # MR263-2-3-1. Some of the commercial information can be found online here.

Grooves/mm: 31.6
Coating: AgMF2
Curvature: plano
Blaze angle: 76 deg.
Ruled area: 96 mm
Blank size: 100x340x50 mm

The efficiency curves from Richardson Gratings testing are here.

The Zygot measurements are here.

These are the collimators. We have 4. They have been provided by Officina Stellare.

The coating in July/August 2024 resulted in some irregularities. The issue is mostly cosmetic and are seen as “pits” less than 50 microns in size. Below are about a dozen photos of the mirrors after coating.

Related documents

• Technical design drawings and specs file.
• Detailed engineering drawing of mirror file
• When the parent mirror was polished, some edge effects were produced by the CNC machine.The remedy was to reduce the aperture 356 mm to 348 mm. The details are in this report.

We have chosen to use the QHY600 Pro detector with the liquid cooling option.