The SONG spectrograph

The SONG spectrograph

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

Requirements

Main requirements include:

The spectrograph shall be fiber fed by a 1-m aperture telescope (PlaneWave PW1000 F/6)
Minimum design resolving power shall be 80000
All wavelengths from 430 nm to 700 nm shall be covered with no gaps
Minimum pixel sampling shall be 3 pixels, even with 2×2 binning (pixel size 3.76 um)
Spectral format shall be accommodated over a full frame sensor area (24×36 mm)
Slit height shall be compatible with 2 fibers
Free gaps between adjacent spectra shall be larger than 3 pixel under 3×3 binning mode
Coating design on optics shall aim to maximize efficiency in the 480-630 nm range

Some other overall characteristics are:

Collimated beam diameter: 82 mm
Off-axis parabolic collimator focal length: 450 mm (F/5.5), 150×130 mm trapezoidal aperture
Slit width: 50 um; slit height: 300 um
Echelle grating: R4 31.6 l/mm, ruled area 90×350 mm
Cross-disperser: SF6 prism, 45 apex angle, 120×110 mm rectangular aperture or Ø120 mm circular aperture
Design resolving power: rectangular slit = 72000, circular fiber >82’000
Camera focal length: 364 mm (F/4.4)
Pixel sampling: 10.7 pixel over 50 um slit width

Optical Design

The basic optical design of the SONG spectrograph is pictured here. The light from the telescope exits a fiber that is positioned vertically. The light is focused onto a 50 micron slit. It then hits FM1 at a 45 degree angle to the vertical. Then it 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.

SONG spectrograph technical design report (7/2024)

Echelle grating

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

Off-axis parabolas

The OAPs are the collimating mirrors. We need 2 in the spectrograph but have all 4 from the parent mirror. They have been provided by Officina Stellare.

During the aspherization process of the parent parabola, an undesired edge was given by the entry tool effect of the polishing CNC machine. This edge brought the surface error of the OAPs out of specification. However, excluding a bit of the edge and considering an aperture of 348 mm instead of 356nm, all 90 mm sub-apertures have a SFE around around 25 nm RMSi, with PVr well below specification.

Here are surface error maps of some sampled 90 mm sub-apertures, both of the full 356 mm aperture and of the reduced 348 mm one, on the edge and in the middle part of the parabola. A report on the effect on the PSF is down below in one of the documents.

The coating of the 4 mirrors 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 some photos of the mirrors after coating.