Fabrication of the Cryostat Vacuum Chamber

In order to preserve a cold, stable environment for the HPF optics, the entire instrument must be kept in a giant vacuum chamber, called a cryostat.  The cryostat must seal out the ambient atmosphere for years at a time, while simultaneously maintaining a rigid shape against the enormous forces of the external air pressure working to crush it like a soda can.  Needless to say, this is quite a challenge both to design and build!  Despite minor delays with regard to the machining of individual parts, the three primary assemblies for the cryostat shell are progressing very nicely, and are extremely close to completion.  Let’s take a closer look at the cryostat and its component parts.

The vacuum boundary is comprised of three assemblies. The box frame, the upper hood, and the lower hood.

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Here we see the box frame assembly. The ½” thick stainless steel perimeter seen on the top face of the upper box frame members will soon be machined to receive the large o-ring, responsible for keeping out as many air particles as possible.

When two components are welded together, they tend to distort relative to one another. Even with high capacity restraints, this fact is inevitable and unavoidable. The only thing one can do is to acknowledge that this distortion will take place, and plan accordingly. In this case, when the ½” plates are welded to the top of the box frame, and in fact even when the two sections of box members are welded together, they warp slightly. This is an issue, because in order to achieve uniform compression of the o-ring, this face must be as close to planar as possible. An o-ring is quite simply a rubber material, with a spherical cross section. When the o-ring is compressed between two plates, it fills any tiny imperfections in the surface of the plates. If done properly, an o-ring can be extremely good at keeping air out, allowing the HPF team to keep an extremely stable vacuum pressure within the cryostat. So in order to overcome the distortion induced by welding, and to achieve the flatness on this o-ring face that is required, the entire assembly you see in the photo above will be placed on a planar mill. The assembly will be restrained on the table, and the mill will then take several cutting passes, removing the upper layer of material, and leaving a surface that is flat to within 5 thousands of an inch (about the thickness of a dollar bill!). This same process will be repeated for both the upper and lower hoods, ensuring that any mating faces are as flat as possible.

The construction of the cryostat is truly an impressive process, and really must be seen to be appreciated.  Below, please enjoy some photos from our recent trip to the shop!

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Tack welds secure the o-ring plate during the welding process. The entire assembly is tack welded together, followed by stitch welds (connecting sets of tacks), or continuous welds where necessary.

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Here is a set of vacuum feed-through locations. A metal plate with an o-ring will bolt onto these faces, which you can see have been milled flat. By mounting hermetically sealed electrical connections on the metal plate, we are able to run wires and optical fibers into the instrument, as well as all necessary plumbing accessories.

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Checking the flatness of the top surface. The end caps were still not tack welded in place.

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Beautiful work

 

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Part of the HPF team discussing the welding progress on the upper hood. You can see the large clamps spaced around the perimeter, holding components in place during welding. Notice the aluminum foil wrapped around the clamps… always trying to keep things as clean as possible!

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This is the lower hood. It is nearly identical to the upper hood (seen in the background), except that it has 4 symmetrically placed circular penetrations. These will accommodate brackets designed to restrain the bench during shipment, keeping it from slamming into the cryostat body. Once the instrument is ready for operation, plates with gaskets will seal these 4 penetrations.

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Close-up of the end cap and support webs. When vacuum is drawn, these caps tend to deform into the cryostat volume. These three webs act as I-beams, keeping the ends from collapsing inwards. You can clearly see where the plate at the bottom of the photo was tack welded to the support beams, keeping it in place during the assembly process.

 

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The Color of Sunspots: Studying Solar Activity

In this blog, we have highlighted how stellar activity can hinder and even masquerade as planetary signals (see Gliese 581 and Gliese 667C). The stars discussed in those posts were important, not only because they may harbor habitable zone planets, but also because they are M Dwarfs. HPF is explicitly being built to discover and confirm planets just like the ones discussed around these stars. There are definitely challenges to overcome, including being able to separate the stellar signals from the planetary signals, however HPF’s use of the near-infrared should help with these problems.

False color image of a solar prominence as seen by NASA’s SDO mission. They have many more images and a fantastic youtube page.

The NIR has a number of advantages when it comes to stellar noise. One of the biggest is that because sunspots are cooler, their emission peak (think of this as a “temperature color.”) is located in or close to the NIR. This reduces the contrast between the normal surface (the photosphere) and the spot areas. We ‘see’ less of a difference between the two areas. Additionally, the effects of high energy events such as solar flares and coronal mass ejections is lessened as they emit much if not most of their light in the visible, ultraviolet, and X-rays.

In addition to its primary mission of searching for habitable planets around M stars, the reduction of stellar noise in the NIR could potentially enable a second exciting exoplanet study with HPF. Today, astronomers are searching for Earth 2.0. We describe this ideal planet as an Earth-sized planet, orbiting a Sun-like star, within the habitable zone. The challenges in finding our twin are immense. Currently we are only sensitive to large planets and planets at shorter periods. To put some numbers to it, the best instruments in the world can currently measure a Doppler shift of ~1 m/s (walking speed). Just as a side, remember, we are doing this across the galaxy! But back to Earth 2.0. If you were an alien observing the Sun, the largest signal you would see is from our most massive planet, Jupiter. Jupiter tugs on the Sun, making it move at about 12 m/s (the top speed of Usain Bolt – the world’s fastest man). The Earth induces a mere 0.1 m/s on the Sun’s velocity (about the speed of a turtle walking on land). This is a TINY number and it is hidden in the signal of other planets and the stellar noise previously discussed!

Since we expect a reduction in the noise around M-Dwarfs when using HPF, the question arose: “could HPF also observe solar type stars and confirm low-mass planets due to a reduction in stellar noise?” Or in other words, could it help discover Earth 2.0? This is actually a complicated question as the mechanism that creates the magnetic activity in M-Dwarfs can be much different from solar activity.

In an attempt to provide an answer, members of the HPF team–led by Penn State graduate student Robert Marchwinski–conducted a study of radial velocity (RV) variations of the Sun as a function of wavelength.  In reality, this is a very complicated measurement to make. We are actually too close to the Sun to just point a telescope at it! To do this correctly, we need the Sun to appear unresolved (like a point source). Some ambitious astronomers have tried to do this using reflected light from asteroids in the solar system. However, this introduces many additional factors you have to correct for: like the speed of the asteroid, how it is rotating, its orientation, and more. Thankfully, we have another way.

The F/F’ Technique

Astronomers have long been aware of the problem of stellar activity and much of the research has focused on modeling how activity works and how it would affect measurements. In 2012, a new approach was developed. Dr. Suzanne Aigrain and her collaborators came up with a method to use the brightness variations of a star to estimate the radial velocity variations induced by magnetic activity. This technique is called the F/F’ method. At its core, this method assumes that we have one spot or spotted region that is carried along as the star rotates. Spots emit less light and thus you lose some of the light you would normally see from the region where the spot inhabits. This missing light has an intrinsic Doppler shift thanks to the rotation of the star and the convection of the star’s surface, all of which lost. Thus you can get a slight change in your radial velocity. (See below).

A rotating starspot and its effect on the shape of a stellar absorption line.

A rotating starspot and its effect on the shape of a stellar absorption line. (Animation credit: Svetlana Berdyugina)

Using this model and combining it with high quality, well-sampled photometry (brightness measurements), you can estimate the Doppler variations you would see coming from a star. We call it the F/F’ method because it uses the flux measurements, F, and the change or derivative of the flux with time, F’ (pronounced “F-prime”).

With this method, some well-educated assumptions, and solar data, we set out to measure these variations as a function of wavelength.

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Obtaining spectra of stars is normally a time intensive process and you need to have lots of data for this method to work. Thankfully, the Sun is the most well-studied star in the Universe. We use solar measurements to educate us on how stars work, how they are powered, and the effects they have on planetary companions.

SORCE Satellite in a clean room. Courtesy of LASP.

Enter SORCE. The SOlar Radiation and Climate Experiment was launched by NASA in 2003 and has been observing the Sun almost continuously. The main goal of SORCE is to measure how the variations in solar output affect the climate of the Earth. To do this, it includes an instrument that takes photometric measurements of the Sun, at different wavelengths, everyday. Exactly what we needed! We also have gotten around the issue of needing to see the Sun as a point source. Photometry is much easier to combine and get a disk totaled value then adding spectra together.

With this data in hand, we could then use the F/F’ method to estimate how the RV variations from the Sun change as you look in different wavelength regions.

Results

We found that by looking at solar type stars in the NIR (where HPF will look), you could see a reduction in the stellar magnetic noise by up to a factor of 4! This particular result came from the analysis of solar data over the entire SORCE mission. If you focus on smaller chunks of time, you consistently see that the NIR has lower variability than other regions of the spectrum. While HPF will not have the sensitivity to detect eta-Earth, there is a very good chance that HPF could help detect other low-mass planets and could be extremely useful in identifying signals that are stellar in origin.

The main results from our analysis showing the estimated RV variability of the Sun with wavelength. We have identified the regions where HPF and 2 other optical instrument's operate/will operate. The gray region is blocked out because the detector introduces noise in this region.

The main results from our analysis showing the estimated RV variability of the Sun with wavelength. We have identified the regions where HPF and 2 other optical instruments operate/will operate. The gray region is blocked out because the detector on SORCE introduces noise that we could not correct for.

An additional result was that while the NIR is quieter overall, there are times in the solar cycle where the optical is almost as quiet as the NIR. This happens during the minimum of the solar cycle, when the Sun is the most inactive and quiet. This suggests that if you are looking for eta-Earth and you have knowledge of the target you are looking at, you could optimize your search by looking only during the star’s stellar cycle minimum.

All of these results were extremely promising and we hope to continue building on these results and making the F/F’ method more accurate. For more information, the results will soon be published in the Astrophysical Journal in a paper submitted by team members Robert Marchwinski, Suvrath Mahadevan, Paul Robertson, Larry Ramsey, and collaborator Jerry Harder.

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MLI Blankets

In recent weeks much progress has been made in making Multi-Layer Insulation (MLI) blankets for the HPF spectrograph. Below follows a video of some of the fabrication steps in the HPF clean-room lab: but what are MLI-blankets, how do they work, and why are they needed for the HPF?

What are MLI blankets?

MLI blankets are blankets made out of multiple alternating thin sheets of a highly reflective material – in our case we use 6 micron aluminized Mylar (looks a lot like aluminum foil but more durable) – and a netted spacer material – Tulle in our case which is, yes, commonly used to make bridal veils. MLI primarily reduces heat loss by thermal radiation, but is ineffective at reducing thermal losses by heat conduction and convection. They are therefore widely used as thermal control elements for vacuum applications where radiation losses dominate. Satellites are a great example: MLI gives them the characteristic appearance of being covered with aluminum, or sometimes gold foil.

<b>The Hubble Space Telescope (HST)</b> - With MLI blankets (Image from Wikipedia.)

The Hubble Space Telescope (HST) – With MLI blankets (Image from Wikipedia.)

How do they work?

The main idea behind MLI blankets is the principle of radiation balance and the Stefan-Boltzmann law. Ideally, the perfectly insulating blanket would be a blanket that reflects 100% of the incident radiation. It is, however, very hard to fabricate a single-sheet blanket that accomplishes this, but by stacking many highly reflective layers on top of each other we can achieve higher and higher reflectivity, and reduce radiation losses further. The individual reflective layers can’t touch, as then they would transfer heat between them resulting in no added insulation benefit (that is, we short-circuit the heat transfer); we need a spacer material to space them apart.  A netted plastic material such as tulle (bridal veil) is great for this purpose. It is very thin and light allowing for very easy handling of the overall multi-layered blanket.

Why do we need them for HPF?

HPF is an infrared spectrograph which will operate at cryogenic temperatures of 180K, cooled with liquid nitrogen.These low temperatures are needed so we don’t saturate the infrared detector with background radiation. Moreover, the spectrograph operates under vacuum so radiative thermal losses will dominate, which makes MLI a great choice for thermal insulation.

<b>MLI blanket making</b> - The blankets are assembled in a clean-room to reduce particulates, which are unfavorable for vacuum pump-down.

MLI blanket making – The blankets are assembled in a clean room to reduce particulates, which are unfavorable for vacuum pump-down.

Traditionally, MLI blankets are sewn together; the multi-layered blanket being held together by stitches. However, any kind of hole that punches through the layers tends to degrade the overall thermal performance of the blanket. Another method, of using tag-pins – the small nylon “I” looking pins that are used to hook price tags to clothes in stores – to fix the layers in place, has been mentioned in the literature, see this paper by R. Hatakenaka. That way you don’t need to punch as many holes as when you are sewing, and tagging – a few inches between tags – is faster and less error-prone than sewing around the whole perimeter of the blanket. Moreover, the tag-pins allow you to fasten the layers together without compressing them, which reduces stress around the holes. Lastly, the blankets tend to contract in the direction of sewing, which might lead them to be too small if not over-sized properly.

The blankets need to be properly sized, aligned, and held together to cover the whole radiation shield, liquid nitrogen tank, and copper thermal straps. Strategically placed Velcro-pads are used to align and hold the blankets in place on the instrument. We sew the Velcro onto the blankets to strongly fix them to the blankets. This results in more holes punched on the area of the Velcro than the tag-only method, but sewing fixes them better to the blankets.

See the video at the top of this post for all the details on how we put everything together!

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