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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More on stellar activity: an investigation of Gliese 667C

A few posts back, we explored how even with a highly precise planet-hunting instrument such as HPF, radial velocity noise from magnetic activity on our target stars may hinder our ability to detect exoplanet signals.  In that post, we used Gliese 581 and its planetary system as an example of how stellar activity such as the spots we see on our own Sun can create signals that look deceptively like exoplanets.  The results of that study generated a lot of public and media interest, and one of the most frequent questions presented to our team was whether any of the other well-known exoplanet systems might be secretly harboring any activity-induced false positive planets.  It is a great question, and one we very much wanted to pursue!  HPF team members Paul Robertson and Suvrath Mahadevan published a new research paper on the arXiv preprint server today (it will appear in the Astrophysical Journal Letters shortly) discussing a stellar activity investigation of Gliese 667C–the “other” most-famous red dwarf planet system.  There is a lot of technical information in the research article, but one bit of good news jumps out immediately: Gliese 667Cc, widely believed to be a promising candidate for low-mass exoplanets in the habitable zone, looks to be real!  Let’s look briefly at the results in more detail.

About Gliese 667C

An artists' rendition of what sunset might look like on Gliese 667Cc.  The host star, Gliese 667C, orbits a distant stellar binary (Gliese 667AB), seen in the background.  Image credit: ESO/L. Calçada

An artist’s rendition of what sunset might look like on Gliese 667Cc. The host star, Gliese 667C, orbits a distant stellar binary (Gliese 667AB), seen in the background. Image credit: ESO/L. Calçada

Gliese 667C, or GJ 667C, is an M dwarf star that is part of a triple star system about 22 light-years away from Earth.  It is a distant companion to GJ 667AB, a binary pair of more massive stars.  In 2013, astronomers using the HARPS spectrograph discovered a pair of planets around this star–GJ 667Cb and GJ 667Cc.  Both of these planets have super-Earth masses, but planet c is especially interesting, since it has a minimum mass just 4 times that of Earth and it lies in the stellar habitable zone!  This makes Gliese 667Cc one of the best candidates yet for a potentially habitable planet.

To make things even more compelling, a separate group of researchers using different data reduction and frequency analysis techniques announced as many as five additional super-Earth planets around GJ 667C shortly thereafter.  This announcement left the system with up to three small planets in the star’s habitable zone!

But there is a catch.  It was clear from the original HARPS analysis that stellar activity is present in the data for GJ 667C.  Aside from planet b, essentially all the planet candidates have periods close to the star’s 105-day rotation period or its aliases.  If we are to be truly confident that Gliese 667C hosts any habitable zone planets, we must correct for this activity and see how many signals remain.

Our Results

We are fortunate that a large amount of data exists for this star, including a good number of stellar activity indicators.  For Gliese 667C, we found the most sensitive indicator was the width of the HARPS cross-correlation function, or FWHM for short.  This is a technically complicated measurement, but it can be understood simply as the average width of the star’s spectral absorption lines.  If magnetic features like starspots are altering the spectrum in a way that would produce false Doppler shifts, FWHM should change.

As for Gliese 581, we found that Doppler measurements made during certain active periods were contaminated by stellar activity signals, as revealed by correlations between the star’s velocity and FWHM.  Fortunately, we were again able to model and remove the stellar activity signals and re-model the star’s planetary system.

So, on to the obvious question: are the planets real?  The good news is that the signals of planets b and c were still present after we removed the stellar activity effects, suggesting those planets are real.  For the rest of the planet candidates, the outcome is not so exciting.  “Planet d,” which was believed to orbit near the outer edge of the habitable zone, is definitely a stellar activity signal created by the star’s rotation.  As for the rest of the planets, we cannot be so sure.  The signals associated with them are so small that they cannot be seen with “industry-standard” analysis techniques, regardless of whether we have corrected for activity.  However, considering how successful our activity correction has been at boosting the signals of real planets, the fact that we see no sign of any of these planet candidates after the activity correction leads us to strongly doubt their existence.

Posted in Exoplanet Science, HPF Science, Stellar Science | 3 Comments

HPF Subsystem Assembly: Heater Panels and Thermal Straps

Fabrication of various subsystems related to HPF is well under way, and some of them have reached their final assembly stage. Below you can see a video of the final assembly in one of our clean rooms by team members Eric and Guðmundur, of (1) the Aluminum Heater Panels, and (2) the Copper Thermal Straps.

Both of these subsystems have been described in more detail in former blog posts, see for example:

http://hpf.psu.edu/2014/06/17/hpf-keeping-it-cool/ and

http://hpf.psu.edu/2014/05/28/cryostat-design/

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