Stellar Observations Network Group (SONG): SONG in New Mexico

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====== 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 2x2 binning (pixel size 3.76 um)
  - Spectral format shall be accommodated over a full frame sensor area (24x36 mm)
  - Slit height shall be compatible with 2 fibers
  - Free gaps between adjacent spectra shall be larger than 3 pixel under 3x3 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), 150x130 mm trapezoidal aperture 
  * Slit width: 50 um; slit height: 300 um
  *Echelle grating: R4 31.6 l/mm, ruled area 90x350 mm
  *Cross-disperser: SF6 prism, 45 apex angle, 120x110 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  =====
{{::spec:optical_design.png?direct&400 |}}

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 [[song_spectrograph#off-axis parabolas|off-axis parabolic]] (OAP) mirror (CM1) before reaching the [[song_spectrograph#echelle grating|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 {{ :spec:spec:files:song3_spectrograph_-_optical_design_report_-_reva.pdf |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 [[https://www.gratinglab.com/Products/Product_Tables/Efficiency/Efficiency.aspx?catalog=53-*-174E| 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 {{ :spec:spec:files:grating_efficiency_curves.pdf |here}}.
  * The Zygot measurements are {{ :spec:spec:files:grating_zygot.pdf |here}}.

==== 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.
{{gallery> :spec:spec:oap1?0&crop&lightbox }}

Related documents
  * Technical design drawings and specs {{ :docs:oc23040-prp-00001_signedos.pdf |file}}.
  * Detailed engineering drawing of mirror {{ :docs:song3_oap_370_rev2.pdf |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 {{ :docs:song_apo_oap.pdf |report}}.
==== Flat mirrors ====

==== Prism ====

The prism blank was manufactured by Schott and polished by Vertex. It is made of [[https://www.schott.com/shop/advanced-optics/en/Optical-Glass/SF6/c/glass-SF6#:~:text=A%20classic%20glass%20type%20with,component%20for%20outstanding%20optical%20properties.| SF6]] material.

The polishing of the prism was carried out by Vertex Optics.

The first polish resulted in a cracked prism.
{{ :spec:spec:prism:sidewall_2.jpg?direct&200| (Sad) }}

* Prism specifications {{ :spec:spec:files:nmsu_prism_blank_specs.pdf |document}}

==== Optical camera ====

* Optical camera design {{ :spec:spec:files:song_000_optical_camera.pdf |document}}
==== Detector ====

We have chosen to use the [[https://www.qhyccd.com/scientific-camera-qhy600pro-imx455/|QHY600 Pro]] detector with the liquid cooling option.

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