The HPF Astro-Comb

After beginning operations earlier this year, HPF continues to patiently watch for the subtle Doppler shifting of stellar spectra that hints at the back-and-forth tug of orbiting exoplanets. As described in previous blog posts, HPF records spectra on a detector array (similarly to how a digital camera records an image), and the shifts we seek amount to microscopic motions of the spectra back and forth by just a few nanometers!

To be able to detect this shifting, we have engineered HPF to be very stable – the optics and detector are built from special materials and mounted carefully so that they don’t cause any shifts of the spectra that could be misinterpreted as Doppler shifts. We’ve carefully thought about all the potential causes of “false” Doppler shifts, from slight thermal changes moving the diffraction grating to systematically weird ways that the light can be launched from the fiber into the spectrograph.

However, this precision engineering doesn’t eliminate the “false” Doppler shifts, which are often referred to as “drifts” of the spectrograph. Even with a perfectly still, unmoving stellar spectrum shining into the telescope, we would still observe the spectrum moving in position by a few nanometers on the detector. These drifts may be attributable to any number of physical causes, from mechanical relaxation of optical components to slight differences in the weight distribution of the optical bench when we fill up HPF with liquid nitrogen coolant every day.

No matter the cause, these drifts must be corrected if we hope to detect the true ~1 meter per second signals from habitable zone planets orbiting M dwarfs. We go about correcting the measurements by measuring the spectra of two sources at once: one being the star of interest, and the other being a calibration source that is known to stay as still as possible. Any measured drifts of the calibration spectrum should be entirely due to “false” Doppler shifts. We can then subtract these from the measured shifts of the stellar spectrum, which contains both the “true” Doppler shift of the stellar spectrum as well as the drift. This isolates the “true” Doppler shift of the star (or more precisely, the relative motion between the star and the Earth).

What types of spectra can we use for this calibration?

An ideal calibration spectrum should have spectral lines throughout the region we are measuring, and those lines should be as stable as possible. The lines should also be roughly uniform in brightness, because we don’t want the detector to be blinded by one spectral line that is much brighter than the others.

Historically, the main option available for such calibration has been atomic emission lamps. These lamps are filled with certain well-characterized element mixtures (such as Thorium + Argon), and when an electrical current is passed through this mixture, it emits light at wavelengths corresponding to the energy levels of the atoms. Unfortunately, the wavelengths of these lines can change over time as the lamps age (the composition or pressure within the lamp can change slightly), and the lines are neither regularly positioned nor of equivalent brightness (see bottom panel of figure below).

An example comparison of calibration spectra for astronomical spectrographs.

Far better is the new generation of calibration sources, called laser frequency combs (LFCs), or “astro-combs.” These sources produce spectra with emission lines that are evenly spaced, roughly of uniform brightness, and stable in their positions. These properties result from the physical mechanisms that generate the comb, and there are several different pathways for generating the comb.

The HPF Astro-Comb

The schematic of the HPF Astro-Comb.

HPF is outfitted with a laser frequency comb developed by our team members at NIST.  This one-of-a-kind comb was developed in parallel with the spectrograph itself, and is really an instrument unto itself!  As shown in today’s other post, the combination of the HPF spectrograph and astro-comb unlocks the full potential of the instrument.

The HPF comb uses the so-called “electro-optic” technique. It is based on a single continuous-wave laser (at one wavelength/frequency, in this case approximately 1064 nanometers or 282 terahertz). A set of modulators blink the laser on and off, and also shift the phase of the laser wave back and forth. The effect of this is that the calibration spectrum goes from being one wavelength to being a comb of many wavelengths (i.e. the modulation generates “sidebands”). The separation between these lines is equivalent to the rate of the modulation, and the main requirement on this is that it has to be sufficient that we can resolve the comb lines with HPF. This means that the HPF comb modulators have to operate at 30 GHz (they swing back and forth 30 billion times each second), an impressive feat!

After the laser modulation, the calibration spectrum is a comb, but it only spans about 10 nanometers in wavelength, while a full HPF spectrum covers almost 500 nanometers. Clearly, another step is needed in order to broaden the HPF comb spectrum. This is where a neat device called a nonlinear waveguide comes in to play. By shining the “narrow” comb into the nonlinear waveguide (which is made of silicon nitride), we are able to stack and shuffle the comb lines together (a process called four-wave mixing) to generate new comb lines and a much broader spectrum that spans the full HPF range. This process requires a lot of light, however, so first the comb has to be amplified to 2 watts – enough power to really burn your eyes! (So we have carefully built a safe system – the laser turns off automatically if you open its opaque box)

The full spectrum then spans the HPF range, and it is important to note that the whole system is connected to an atomic clock. This means that the various components are able to operate with knowledge of precisely how the best clocks in the world are ticking – so it knows and can control precisely the wavelengths (or frequencies) of all the comb lines. So not only do we have comb lines over the full HPF spectral range, but they are also completely stable!

The spectral profile of the HPF comb.

However, this comb spectrum is very bright near its central wavelength around 1064 nanometers, and much fainter towards 820 and 1300 nanometers. So the final step is that we need to make sure that the comb lines are all roughly the same brightness. We use a combination of a static filter and an active liquid-crystal-display-type filter (actually a mini-spectrograph!) to make the brightness of the comb lines as equal as possible.

As you can tell from this description, this system has many interlocking parts, and it is one of the most advanced astro-combs yet demonstrated.

Installation and performance of the HPF comb

Photos from the installation of the HPF astro comb.

The HPF comb was installed in February of 2018, and underwent some engineering upgrades in April and May of 2018. A few photos of the deployment are shown above. The comb has been running smoothly since May, and the impressive measurements it has enabled are detailed in an accompanying blog post (see for example the simultaneous stellar and astro-comb spectra from HPF below). Our recent paper also describes the performance of this system in much more detail.

Side-by-side spectra of a star and the HPF astro-comb.

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HPF Goes On Sky!

The HPF team poses in front of the instrument after a successful delivery to HET.

Introduction

Things have been quiet here on the HPF blog for the last few months, but that has certainly not been the case in our lab!  Our team has been working diligently, making the final push to deliver HPF to HET and commission the instrument.  In this post, we are excited to announce that HPF has been installed at HET, and is now acquiring spectra on sky!  Here are some highlights from delivery and commissioning.

Delivery and Installation

HPF was delivered from Penn State to McDonald Observatory on a temperature-controlled, air ride truck that was routed to avoid as many potholes and speed bumps as possible.  After a three-day trip, it was carefully offloaded at HET and placed in the telescope basement.  We made sure to capture all the action from multiple camera angles:

Once we had the instrument placed in the basement, we opened the vacuum chamber to verify that the optics had made the trip intact.  While we could only admit a few team members for this task, rest assured that everyone else was nervously pacing the floor just outside!


Once the instrument was placed safely in its permanent home, the team spent approximately three weeks connecting and testing the various subsystems.  These subsystems include the environment control system, the fiber optic feed, the calibration suite, and the near-infrared detector, which we have detailed in previous posts here.

Of course, the new wrinkle this time is that HPF is now attached to an actual telescope!  Ensuring the whole system operated properly required sending a few brave team members above the primary mirror on a lift.

Outfitted with the proper personal protective equipment, HPF team members Chad Bender, Suvrath Mahadevan, and Sam Halverson accompanied HET Mechanical Engineer Emily Mrozinski to the telescope’s prime focus structure.

To put a long story short, HPF is now happy and healthy in the HET basement, and it is now time to get some spectra.

First Light

On the night of November 29-30 2017, HPF acquired first light on the M dwarf GJ 3470, which hosts a transiting exoplanet that astronomers believe has a blue sky.  Here is what the star’s spectrum looks like to HPF:

HPF’s first-light spectrum of GJ 3470.  Each set of three lines is a spectral order–a small section of the full spectrum–with the star’s spectrum in the middle, the spectrum of the night sky above it, and our wavelength calibration source below.

The first light spectrum wouldn’t have happened without great support from all of the talented staff at the Hobby-Eberly Telescope:

During first light observations on the night of Nov 29th: telescope operator Emily Bevins and resident astronomer Steve Odewahn hard at work to ensure HPF records high-quality spectra !

The following day, a subsection of the HPF team celebrated first light at the HET control room, alongside HET telescope operator Justen Pautzke, resident astronomer Sergey Rostopchin, Professor Gary Hill—and senior research scientist Phillip MacQueen who gladly took the photo for us !

Celebrating first light at the Hobby-Eberly Telescope control room!

HPF’s science commissioning has just started, and we will provide more updates as we fully characterize the system’s performance.  For now, though, it is an exciting milestone to get starlight on the detector.  With tantalizing new exoplanets being discovered all the time, and TESS just around the corner, it is a good time to be on sky!

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The Camera of HPF

Introduction: Spectrometer Cameras

A spectrometer, as the name implies, records a ‘spectrum’ of an object. This spectrum, in its most basic form, is just a series of images of the instrument entrance aperture (whether it be a star, slit, optical fiber, or otherwise) that has been spatially separated based on the intrinsic color distribution of the source. The separation of these monochromatic (single-wavelength) images depends on the dispersion properties of the spectrometer.

Simulated star and wavelength calibration source spectra, as recorded by the HPF spectrometer. The calibration source spectrum (center channel), in this case a highly specialized laser source which only emits at discrete wavelengths, produces a spatially separated set of images of the spectrometer entrance slit. On the other hand, starlight (bottom channel), contains flux across a broad range of frequencies. This produces a continuous spectral image across each diffraction order.

The optic at the heart of the spectrometer, the high dispersion echelle grating, provides the chromatic dispersion required to separate the individual colors encoded in the incoming starlight, but it is the spectrometer camera that ultimately produces our final image of the stellar spectrum.

Cartoon overview of the HPF spectrometer optical train. Light from the telescope exits the fiber, gets dispersed by a pair of diffraction gratings (first the high dispersion Echelle grating, then the volume phase holographic grating), and is finally imaged onto our infrared detector array by the HPF camera. The camera assembly provides the needed focusing mechanism to deliver our final spectral image.

The spectral resolution of the spectrometer, which tells us how well we can separate individual colors, is entirely set by the properties of the entrance aperture and diffraction grating(s). However, the spectrometer camera must also have pristine image quality, or else our spectral resolution will suffer. Any blurring of the image of the spectrum due to uncorrected aberrations within the camera will degrade our ability to separate individual colors of the star.

Example of image quality degrading spectral resolution. On the left shows a near-diffraction-limited image of our spectrometer slit, while the right shows a highly aberrated image. The effective spectral resolution of the aberrated slit image (right) is nearly 5% lower.

The HPF Camera

Our spectrometer resolution directly affects how well we can measure minute stellar radial velocity shifts, so typical high resolution Doppler spectrometers require specially designed cameras that maintain excellent image quality across all of the diffraction orders. This high level of image quality is also needed to lower our sensitivity to illumination changes coming from our optical fiber delivery system.

Image quality of the HPF camera across the wavelengths that fall onto our infrared detector. The grey boxes represent individual detector pixels, magnified by a factor of 1000x. The colored points show the aberration distribution produced by the camera and upstream optics (a ‘perfect’ imaging system would have no geometric aberrations). As you can see, the expected aberrations in the system are, for the most part, far below what we’d be able to measure in single pixel. This means that the camera will produce a sharp, well-corrected image of our entire spectrum.

The HPF camera system, designed by team member Christian Schwab and fabricated by New England Optical Systems and Optimax Systems, was recently delivered to PSU and is now undergoing vacuum and cold testing.

The delivered HPF camera system! The camera is made of specialized lenses, which are coated with custom anti-reflective coatings. These complex coatings (appearing as green-yellow tints in the images) ensure that we capture every last photon from our targets. The mechanical assembly is unique in its own right; as the camera cools to the HPF operating temperature, the mounts compensate for thermal expansion differences between the various glasses and the aluminum housing. This allows us to test the camera both at room temperature and at the 180K operating temperature of HPF.

With the camera now delivered to Penn State, we’ve received almost all of the optical components for HPF. We’re now knee-deep, for the first time, in our alignment and testing of the entire optical system. Stay tuned!

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