The Echelle Grating

As fate would have it, the start of the HPF blog coincides with the arrival of the heart of the spectrograph itself: the echelle grating.  Since the grating is so important to the workings of HPF, let’s take a moment to go over its purpose and function.

Inspection of the grating (center) to ensure it was undamaged during shipping.

Suvrath Mahadevan (HPF PI) inspecting the echelle grating to ensure it was undamaged during shipping.

What is a grating?

A spectrograph operates by separating incoming light (in our case, starlight) into its constituent wavelengths in a process called diffraction.  Diffraction is observed most commonly with a prism; the rainbow of colors that emerges from the prism is just the incoming light separated into individual wavelengths.

While some spectrographs do use prisms, HPF will use a grating instead.  A grating works in a similar manner, but instead of splitting wavelengths by using glass to change the index of refraction, a grating uses finely-ruled or spaced surfaces to create interference between light waves.

Diffraction via grating (1) and prism (2).  (Image courtesy Cmglee via Wikimedia Commons)

Diffraction via grating (1) and refraction via prism (2). (Image courtesy Cmglee via Wikimedia Commons)

Our grating works slightly differently from the image shown above, as it will reflect (rather than transmit) light.  In addition, our echelle grating is blazed so that the the light is efficiently dispersed into certain high orders to enable high dispersion and high efficiency.

What is it made of ?

The 125mm thick grating is made on a single block of Zerodur.  Zerodur has a very low coefficient of thermal expansion and helps to keep the grating length changes minimal, even at 180 K.  The top layer of the grating is coated with pure unprotected gold to enable high reflectivity in the near-infrared wavelengths.

How will HPF use the grating?

As described on our “What is HPF?” page, we will search for planets by looking for the Doppler motion of a star’s absorption lines as an orbiting planet’s gravity causes the star to “wobble.”  The grating is what allows us to observe the stellar lines in the first place.  When we observe a stellar spectrum (the rainbow from our prism example), there are dark bands where atoms in the star’s atmosphere have absorbed light at that particular wavelength.  These are the lines that we use to measure the stellar motion, or radial velocity.  Along with the telescope, the grating is the key to the whole project!

Of course, as a facility instrument at the Hobby Eberly Telescope, HPF will be used for all sorts of interesting science projects.  If you can think of an astronomical experiment that requires a high-resolution infrared spectrum, there’s a good chance that HPF will be used for it!

What makes this grating special?

Our grating is a specially replicated mosaic grating manufactured for us by Newport Richardson Grating Labs. The high blaze angle, size, stability and goal coating are all optimized for the particular application of searching for planets around low-mass stars.

The large mirror diameter of the HET (nearly 11 meters!) means the grating must also be physically very large.  Manufacturing something so delicate and precise on such a large scale is quite costly, making the grating one of the most expensive individual components of HPF.  Its 125 pound weight also makes handling difficult!

Suvrath, Eric and Ryan lift the grating onto the table for inspection.  Not an easy lift!

Eric Levi, Ryan Terrien, and Suvrath lift the grating onto the table for inspection. Not an easy lift!

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