Today marked the arrival of both Guanacos and our new team member Leah Albrow from MIT!
*Disclaimer: The Guanacos may have already arrived but I have yet to see one. For the time being, I must rely on third-party sightings of these mythical beings.*
Parker is attracting the attention of all of the animals on the mountain–just call him the Pied Piper of guanacos at this point.
Tonight also marked an important milestone in MagAO-X’s history. We conducted a whole night of observations with an unmodulated pyramid wavefront sensor using Rico’s new neural network!
The pyramid wavefront sensor exhibits a non-linear response, limiting its ability to accurately measure wavefronts. To mitigate this issue, AO groups will often modulate their pyramid wavefront sensors by spinning a little pyramid prism round and round (like a dreidel or a ballerina or me in my middle school computer lab with the little rolly chairs). But you can only spin a pyramid (or a Josh) so fast. The modulated pyramid is also limited in its sensitivity (the Josh, not so much). But with Rico’s non-linear reconstructor with a convolutional neural net (or NNRCNN for a fun acronym that rolls off the tongue), we don’t need to modulate our pyramid. We can run our AO loop fast and furiously, sensing wavefronts with the greatest of ease.
To celebrate, I have provided a list of things that are modulated and unmodulated.
Modulated:
My sleep schedule (some nights I go to bed early and wake up around noon, other nights, I dream about permanently residing in Florida and things don’t end well):
Florida.
Parker’s facial expressions:
Themanyfaces of Parker.
The number of group members on our observing run:
While we witnessed the departures of Matthijs, Tiffany, and Laird over the last few days, we also gained new LCO first-timers being Rico, and most recently, Leah!
Leah and co. enjoying the Dupont telescope.
Unmodulated:
Empanada volume on Sundays:
No explanation needed.Is this legal?
The wavefront sensor of the hour:
Unmodulated pyramid WFS (left) vs modulated pyramid WFS (right)The modulated pyramid image kinda reminds me of this 1988 classic.
Our excitement in getting to spend another 5 days at the best telescope in the world!
We’re all here to figure out puzzles like “Do rocky exoplanets have the same composition as Earth?”, “Do all protoplanets produce Halpha emission as they accrete?”, and “Can our AO system reach the contrast necessary to image low mass planets in reflected light?”
It turns out, we also have a lot of puzzles to solve that we didn’t realize we needed PhDs for, such as, “Why is the attempt to upgrade the AO computer cursed?” and “Why does Joseph no longer think computers are deterministic?”
The saga of the long-legged PI and the Cursed Computer (photo credit: Joseph)
Then there are the questions that make us think we should have gone into philosophy rather than science, such as, “Why do we find clouds so pretty even though we hate them?”
Sunset yesterday
The hardest questions are the deep mathematical ones like, “Why is the seeing when I’m on the telescope always worse than the median?”
Worse than median seeing
And then there are the idle puzzles one wonders about at 2 AM such as, “What is the collective noun for a bunch of graduate students? [my vote is for ‘pile’ as in the P in PhD]”, and “How many snacks does it take to satisfy said pile?”
Perhaps a better question is, “Can we get Trader Joe’s to sponsor MagAO-X?”
I’m optimistic that all of these questions will be answered in my lifetime.
But when the seeing gets bad or the sky clouds over, the puzzles we really like are the NYT crossword type (see 2024Ab Day 10). I’ve never made a crossword puzzle before. Tonight I experimented with two free grid-creation applications, and my main takeaway is that the folks edited by Will Schortz are quite impressive. Nevertheless, here for your enjoyment is the AO Puzzle:
Fact of the Day: A record 889 contestants participated in the 2025 American Crossword Puzzle tournament, which is held every year in Connecticut.
Today’s blog post is going to highlight my raison d’être for working with MagAO-X: its new polarimeter!
Polarimetric differential imaging
The MagAO-X polarimeter was initially installed in spring of 2025, consisting of a rotating half-wave plate (HWP) and polarizing beamsplitter cube (PBS). The combination of these two optics enables MagAO-X to measure the angle of linear polarization of light (i.e., the orientation the electric field is oscillating). Detecting polarized light is useful for imaging scattered light in astronomical scenes, in particular light reflecting off of circumstellar disks!
Schematic of polarized light generated by a linear polarizer. Design of the HWP and PBS for MagAO-X (credit: T. G. B. de Souza)
The major benefit of polarimetry is that light reflecting off disks, for example, is polarized, while the on-axis starlight is unpolarized. The PBS directs half the unpolarized starlight to each camera along with the small percentage of the fainter disk light which is polarized in orthogonal directions. By subtracting the two camera images the unpolarized starlight is almost completely cancelled out, leaving only the polarized circumstellar signal. This technique is powerful for attenuating the starlight, orders of magnitude better than using angular or spectral differential imaging, alone (although these methods can be combined with polarimetry for even better attenuation!).
Examples of polarimetric disk images from VLT/SPHERE (credit: C. Ginski and DESTINYS team)
Calibrating the MagAO-X polarimeter
The fundamental reason disks have polarimetric signal is due to the reflection of starlight towards the observer–reflections polarize light! One may wonder, what about all the reflections off the mirrors inside our instrument, won’t those also polarize the incoming light? Yes, indeed, every reflection will polarize the light, inducing instrumental polarization (IP), which cannot be distinguished from circumstellar signal. In addition, certain optical elements will rotate the angle of linear polarization of the incoming light, and can even turn linear polarization into circular polarization (crosstalk). Because the PBS only filters linear polarization states, any light which becomes circularly polarized effectively disappears, leading to a net loss in polarimetric throughput.
The effects of instrumental polarization are mitigated to first order by the HWP itself–by placing the HWP as early as possible in the instrumental beam path, we can cancel out any IP induced downstream of the HWP through another layer of differential subtraction by rotating the HWP and taking multiple images. This will not correct for any crosstalk though, which requires more sophisticated methods. Modern polarimeters overcome both IP and crosstalk by directly modeling the polarimetric response of the instrument and using the solution for correcting astronomical observations. This modeling requires calibration sequences injecting light with known polarization states into the instrument and measuring the response.
MagAO-X does not have the capability of injecting polarized light on the same path as the on-sky beam, so I designed a polarization generator which can be aligned on the exterior of the instrument precisely for measuring the calibrated polarimetric response. I got it designed and built in Tucson before shipping here to the telescope, where it will remain in the clean room for future polarimetric calibrations!
The polarization generator aligned in front of the HWP, ready to inject a flat-field of polarized light.
Initial results from the polarimetric calibration
After a week of Joseph and Jared working tirelessly on getting my calibration data in the right format with the right metadata, I was able to reduce it and begin the measurements of the polarimetric response of MagAO-X!
Normalized single difference flux at different HWP and IMR angles.
These images show the direct polarimetric response of the image on the two cameras at the three filters we can use for polarimetry (r, i, and z). Each data point is the fractional polarized flux measured between the two cameras: +1 means 100% of the input vertically polarized light was measured, -1 means 100% of the input vertically polarized light has now become horizontally polarized, and 0 means 0% of the input flux was measured. An ideal curve goes from +1 at HWP=0° to -1 at HWP=45°. Shifts in the locations of the curves indicate retardance in the instrument, and when the curves are squished less than ±1 it indicates a loss in polarimetric efficiency due to crosstalk.
From these curves we can see immediately by eye a lack of sensitivity at r-band, which can be attributed to the PBS–it has non-ideal performance below 650nm, which is half of the r bandpass. We can also see that the image rotator (IMR) has clear retardance and crosstalk effects as the different colored curves shift and squish at different IMR angles.
The overall polarimetric efficiency can be estimated from the maxima and minima of the normalized single-difference curves. A curve that goes from +1 to -1 has 100% efficiency, while a curve that goes from +0.4 to -1 only has 70% efficiency. The above plot shows the estimated polarimetric efficiency for each filter at different image rotator angles. The highlighted region indicates the typical image rotator angles used on-sky by MagAO-X.
First off, we can see that at i- and z-band we can reach an efficiency of almost 100% without any additional polarimetric components in the instrument, which is great! However, this is reached at image rotator angles that we never use on-sky due to the fixed orientation of the instrument pupil offset compared to the telescope pupil–this is difficult to change because this angle is optimized to minimize the effects of dead actuators on the deformable mirror (DM) and all pupil stops in the instrument are co-aligned with the DM. Thankfully, even with the current pupil offset we’re only losing 50% of the polarized flux, at most.
Future work
Now that these data have been measured, I am going to begin working on fitting polarimetric models for the instrument. In a few nights, I will also take measurements of polarized and unpolarized standard stars, which allows me to measure the polarimetric response of the telescope’s tertiary mirror (which we know has IP and crosstalk). Hopefully I will also get some time on a circumstellar disk, too!
Before next observing run I am going to work on the design of a polarimetric compensator to optimize the polarimetric efficiency by inverting the effects of the instrumental retardance and crosstalk. This will require new optical components and motorized stages, which means I’ll be preparing a new proposal asking for money! If all goes well, we should have an optimal polarimeter in time for the 2026 observing runs.
Other news
Following the arrival of Rico, we’ve had to say goodbye to Matthijs, keeping our Dutch equivalency number constant
Hallo, Rico; tot ziens, Matthijs!A trio of knit-wearing AO scientists
As we progress further and further in the run, our attempts for consistent sleep are getting more and more exasperated, to the point of insanity for some
A Faraday-cage construction around one of the dorms
Observations are continually mostly well, although our luck with the weather is becoming more tenuous. Everyone keep your fingers crossed and knock on wood we can finish out the remaining 5 nights successfully!
Song of the day
twenty one pilots – Polarize
Fact of the day
Hawai’i is the extinct and endangered species capital of the world. Most native Hawaiian plants and animals are endemic, meaning they are found nowhere else in the world but Hawaiʻi. Once they are lost to extinction, they are gone forever, largely due to habitat loss and impacts of invasive species. Current estimates show 95 of 142 endemic bird species, 50% of land snails, and 134 species of plants have been extinguished from the Hawaiian islands.
Today people discovered I am just an average girl who loves astrology. No offense, but why having to choose between astronomy and astrology when – guess what – you can have both? Not only that, but I like to entertain myself in a variety of spiritual practices, for example, I am currently charging my tarot cards under the light of this gorgeous full Moon. My favorite childhood anime was Cardcaptor Sakura… What did you expect?
Since today marks the moment when everyone has arrived at LCO, and we are about mid-way, I have decided to proudly present the members of our group who joined the run this time, in Elena’s own special way.
The gorgeous full Moon that inspired today’s blog post. Photo credits to Miles Lucas
The girlies
Elena: Temperance The creator of this bizarre blog post, carefully mixing science and magic, continuously balancing opposites: Enthusiasm VS crippling anxiety, aligning PIAA lenses to highest precision possible to mitigate pineapples.
Eden: The Star Eden brings a delicate touch and kindness to anything she does. Carefully handling DM cables and the AO system, she is like the trump card of the group: Give her something do to and she will turn it to being successfully completed. Also, she gave us sparkles. Twinkle, twinkle.
Katie: 8 of pentacles Katie is not afraid to handle delicate optics. With patience, firm hands, and arms that can extend for multiple hours in a row, she can manage any practical task the big bosses throw at her. Recently crowned the new manager of MagAO-X’s uninstall, she is ready to surpass master Laird.
Tiffany: 3 of pentacles Tiffany is the new addition of the girlies group. She is carefully observing every move of the more experienced members to learn as much as possible about the delicate and complex system MagAO-X revealed to be.
American bois
Josh: The Moon Josh is fighting against Zemax just like the dogs in this card bark at the Moon. But hey, at the end of the day, isn’t the Moon almost like a big form of cheese in the sky? Also, he loves sticks.
Parker: Strength Parker befriended the donkeys and tamed the vibrations of the telescope. Hand-working and tireless, he is always down for a run up the hill or some benching. I only wish he could eat his food without mixing it all together in a disgusting mush.
They say life goes on after obtaining a PhD
Joseph: The Magician Joseph is the wizard that installs the software on your computer. Knowledgeable and resourceful, he is full of tools and tricks to compromise the entire group and save the day approximately four times per day – one time for each element – just like an expert magician.
Miles: 2 of pentacles Miles simply enjoys taking cute pictures of animals and stars. Two polarization states, one difference: Guaranteed success! He can also juggle pretty well (when he is not taking pictures and he is not sleeping).
Matthijs: The Sun Matthijs was revitalized by vitamin D like never before in his entire life. Feeling the Sun on his Dutch face every day after waking up made him a new man. If only the cooks were not trying to poison him, he would have already solved every possible problem of MagAO-X.
Rico: Knight of wands Rico has just arrived at LCO and has already observed closely how to drive the instrument. Inspired by our science, he has already planned to do some daytime engineering tomorrow to make his neural network smarter. Always moving forward!
The final bosses
Sebastiaan: The Fool Sebastiaan is always in a good mood, even when he is extremely sleep deprived. He finds joy in the little things of life, like shaking the mouse and seeing the cursor becoming full screen size. He also loves to continuously start new adventures: He has endless hope and endless data to reduce, piled up since 2021.
Alycia: The Empress Alycia knows everyone around and no one wants to mess with her. If something is not quite right, just let her know and she will have it sorted out. She is basically the true owner of this observatory. She says the word, and Chile answers.
Laird: The Hanged Man Laird is the giant on whose shoulders we all stand. He has sacrificed himself again and again for the sake of MagAO-X: Positioning himself in the most bizarre ways – almost break-dancing – unscrewing countless nuts, and making peanuts disappear throughout the years.
Jared: Universal Judgement Jared has to manage all this circus. That’s quite the task, especially when there are so many things going on at the same time. But hey, remember? Having more things just means… Peeking from the sky with a huge trumpet (?) deciding when people can finally get a PhD.
Special
Vizzy: The Hermit Vast mountains, rocks, and silence, under the perfect night sky. This, the setting where vizzy spends all her time pondering about the meaning of life. The Universe made her a cute, little animal without a way to communicate with us humans, because it knew she would be too powerful otherwise.
Fun fact
Did you know that Black Jack’s mother, Judith Love Cohen, was an aerospace engineer that is credited to have helped save Apollo 13? Shooting for the Moon.
As we enter the final month of the year, I would like to share my Astronomy: Wrapped with you all.
You deployed six monitors with one viscacha background!
It takes real vizzion to do something like that.
This year you experienced five Empanada Sundays!
(That means up to 20 empanadas, at maximum order size!)
You hung out with Dr. Alycia Weinberger on three occasions!
(That’s once more than people who just went on the observing runs!)
You commited to git on the magao-x/MagAOX repository 137 times!
(Jared’s at 261 commits for the year, but don’t let that discourage you!)
And we still have seven nights to go.
Today marked the departure of Laird* and arrival of Alycia. Alycia is our local debris disk enthusiast, so it was good for her to meet Miles Lucas (our local polarimetry enthusiast).
*Attempted departure. The Holiday Inn club will be graced by his presence for a night, apparently.
For those of you following along at home, polarimetry and debris disks are best friends. It’s sort of like putting your polarized sunglasses on to cut through haze, only in reverse (to see all the dust).
The second half of the night was Eden’s to look at the beta Pictoris system again in hopes of getting it to reveal its secrets. We have a saying in this field: “It’s never a planet.” However, this is one of the rare exceptions. It’s actually pretty easy to find beta Pictoris b, especially if you already know it’s there.
Of course, as Katie put it, this wouldn’t be interesting unless we were seeing some things nobody had seen before. Time to analyze some data!
Song of the Day
This guy sounds like a didgeridoo, which is really something.
“Peace Somehow” by Avi Kaplan
Fun Fact of the Day
A lot of the Spanish words beginning in “a” (or especially “al”) are Arabic in origin:
