Wednesday, 3 August 2016

The Jupiter Time Capsule

Originally posted by @LeighFletcher on the Leicester to Jupiter blog.

Given that we don’t yet know whether a planetary core exists within Jupiter, much of our understanding of giant planet formation comes from a different line of investigation:  the bulk composition of the planet.

The composition of the atmosphere that we observe today results from a combination of many processes – chemistry initiated by the Sun’s UV rays, condensation of gases to form cloud decks, and dynamic weather causing the mixing and transport of materials from place to place.  Nevertheless, Jupiter’s immense gravity means that all this chemistry is determined by the original balance of chemical elements and isotopes.  The materials that the young Jupiter captured from the protosolar nebula were never allowed to escape.  Jupiter can then be thought of as an ideal time capsule, revealing the processes that formed this gargantuan world billions of years ago.

Juno will explore the bulk composition of Jupiter’s atmosphere to constrain its origins. Credit: NASA

Why is this important?  Well, different theories of giant planet formation predict a different balance of elements and isotopes.  By creating a precise inventory of Jupiter’s composition, it might help us distinguish between those competing ideas.  For example, if Jupiter’s formation didn’t require a core, and simply started from a collapse of the nebula gas that surrounded our young Sun, then we might expect Jupiter to have the same sort of inventory as the Sun itself.  Conversely, if Jupiter did require a core for the formation to begin (core accretion theory), then that core material would have helped to enrich the planet in elements, over and above what we measure in the Sun.

A Job Left Undone

There are a couple of ways to determine this compositional inventory.  Remote sensing, using spectroscopic measurements of the soup of gaseous species from ground-based telescopes or visiting spacecraft, is one potential method.  Each gas has a different spectral signature, so provided the gas is present above the cloud tops, we can determine the composition by modelling the spectra.  This has been done for all four giant planets (although it’s really hard to do on the ice giants), and the general picture that has emerged is that the elements and isotopes are enriched with respect to conditions in the Sun.  Place a tick in the box for core accretion theory.

However, the upper atmosphere represents only a tiny fraction of Jupiter’s enormous bulk and might not be representative of the rest of the planet.  Indeed, some species like ammonia and water condense out at the cold temperatures of Jupiter’s upper atmosphere, forming the primary cloud decks.  The main repositories for these condensable species are therefore hidden from view, down below the cloud decks.  So lots of the species we’re interested in are inaccessible to us.

A better way of measuring the composition below the clouds is by firing a probe into the planet itself, with sophisticated instruments designed to sniff out the gases and aerosols present at levels hidden from view.  That’s precisely what we did in 1995 with the Galileo probe, which descended under a parachute for 60 minutes, reaching down to the 20 bar level, deep below the expected cloud layers.

But there was a problem. The cloud decks weren’t where they were supposed to be, and, crucially, the amount of water measured was much lower than expected (about 490 ppm, or 30% of the amount found in the Sun).  With only a single probe measurement, we fell into the trap of a Jovian hotspot, a dry, desiccated region of air just north of the equator where all the gases appeared to be depleted by powerful downwelling motions.  It was like trying to infer the abundance of water on Earth if we only had one measurement from the Sahara desert.  The Galileo probe did, however, measure enrichments in most gasesous species that were above the solar values.  So, another tick for core accretion theory.  However, the abundance of water remained a mystery.

The enrichment of elements in Jupiter is consistent with the formation of a protoplanetary core, except for water…

Juno and the Microwaves

Step in Juno, with its microwave radiometer on board, the first every flown to a giant planet.  At microwave wavelengths, we sense light emitted from extremely high pressures down to 100 bar or so, well below the levels sampled by the Galileo probe, and well below the expected cloud decks.  Here, water should be well mixed, allowing Juno to definitively constrain the water abundance in Jupiter and hence the amount of oxygen mixed into the giant planet as it was forming.

Oxygen is key, as core accretion theory would expect it to be similarly enhanced over the solar abundances, maybe by 400-1000%, depending on the details of the formation model. Indeed, one theory involves water ice cages trapping gases (like methane, ammonia, nitrogen, etc.) in the early solar system and delivering them to Jupiter.  If that’s the case, then we should find more oxygen (from those original water ice cages) than the other gaseous species.  The precise water abundance has the potential to rewrite the textbooks on how Jupiter first formed, and to complete the task that was left over from Galileo.

Wednesday, 20 July 2016

Birth of Giants

Originally posted by @LeighFletcher on the Leicester to Jupiter blog.

The presence of Jupiter has had a profound influence on the architecture of our solar system, shaping the conditions that have led to the stable, habitable environment that exist here on Earth.  But it didn’t have to be this way.  Maybe the young Jupiter could have wandered in too far to the inner solar system, scattering all the young terrestrial worlds so that Earth never formed properly.  Or maybe Jupiter’s interaction with other giants could have ejected it completely, to be a free floating planet.  This might have meant that Jupiter never shaped the population of icy comets and watery asteroids that delivered key species – water, and maybe even the ingredients for life – to our forming home planet.  And maybe, without Jupiter’s stabilising presence, cataclysmic impacts could be going on to this day, never allowing intelligent life to develop on Earth?

Probing the unknown interior of Jupiter with the Juno spacecraft, and the possible presence of a core. Credit: NASA

It’s likely that all of these scenarios have played out somewhere in our universe.  But Jupiter’s formation and influence on the other planets is key to the question of how our Solar System came to be.  And yet, there remains a surprisingly large gap in our understanding of how Jupiter first formed and how it evolved.  Three key questions remain unanswered – did Jupiter form with a massive protoplanetary core, is that core still present today (or are the heavy materials mixed throughout the planetary interior), and did accretion of icy materials massively enrich the water content of Jupiter?  Indeed, Jupiter can be thought of as a time capsule, its massive gravity being so powerful that the proto-planetary material from the birth of our solar system never managed to escape.  Resolve these mysteries and we’ll have a much better idea of how giant planets form, both in our Solar System and around other stars.

Two prevailing theories exist for the formation of Jupiter.  The favoured model is known as the core accretion theory, where a massive rocky core (a protoplanet, or planetary embryo) is formed first in the young solar system, and when it attains a high enough mass (somewhere around 10 Earth masses) it begins to suck in all the surrounding hydrogen and helium gas from the forming solar system, growing to become the giant we see today.  If that’s true, then the presence of the original core material will serve to enrich Jupiter’s chemical abundances over and above that found in the nebula.  The composition of our own Sun is a good measure of the composition of the original solar nebula, so we have a good point of comparison.  On the other hand, if no planetary core was required, then the planet could begin to form by gravitational collapse (just like a star). In this case, there’d be no significant enrichment in the planetary chemical abundances.  So, was a core required to form Jupiter, and is the planet’s chemical inventory enriched relative to the Sun?

Gravity Mapping

NASA’s Juno mission is tantalisingly close to providing those answers via two techniques – precision mapping of the gravitational field, and microwave mapping to determine the composition of this time capsule far below the clouds.  We’ll look at the second point in a later blog post, but for now let’s look at Juno’s gravity mapping.

Measuring the internal density structure of Jupiter via gravitational mapping, using slight perturbations in Juno’s orbit. Credit: NASA

Juno’s close-in orbit is specifically designed to map the gravity field, as small changes in the interior distribution of densities will pull and tweak Juno’s orbit around Jupiter over its 37 orbits.    By monitoring the Doppler shift in Juno’s radio signal, Juno will be able to map those perturbations to assemble a 3D map of the insides of Jupiter.  If there are discrete layers, then the gravity mapping should be able to reveal their depth and the density changes.  For example, if a rock/metal core exists – the remnant of the protoplanet that initiated Jupiter’s formation – then maybe Juno will be able to detect its gravitational signal.  At the very least, Juno will probe the exotic transitions in Jupiter’s hydrogen-helium mixture as it is compressed by crushing pressures (up to 30-50 million bar) and temperatures (tens of thousands of degrees) at the planet’s centre.

The main expected phase transition is to a bizarre state called metallic hydrogen – at high pressures, hydrogen’s single electron can be detached from its proton and allows it to become electrically conducting.  Droplets of helium and neon might rain out in this strange fluid.  The properties of metallic hydrogen are poorly understood, given that it might only be produced for fleeting instances between diamond anvils in labs on Earth (and this is highly contested), so this could all add a lot of complexities to interpreting Jupiter’s gravity field.  We think the metallic hydrogen layer might start about 25% of the way down, where temperatures exceed 10000 degrees and pressures exceed 2 million bar, but this is highly uncertain.  To put that in perspective, the pressure of the Earth’s core is around 3-4 million bar.  Furthermore, convective motions in this fluid might have served to erode any original protoplanetary core away, redistributing its materials over the aeons since the planet first formed.  Nevertheless, if Juno detects the presence of a core it would be a smoking gun for the core accretion theory of planetary formation.

Tuesday, 12 July 2016

When it Rains....

Originally posted by @LeighFletcher on the Leicester to Jupiter blog.

Water, water, everywhere.  

Dr. Leigh Fletcher appeared on this month’s episode of BBC Sky at Night to discuss Juno’s goals at Jupiter, and describes the importance of Jupiter’s water in this new post.

If our ideas about the formation of giant planets stand up to the observational tests of the Juno spacecraft, then Jupiter’s extensive atmosphere should be moist, humid and drenched in water. Moist air contains energy, released as gaseous water condenses to liquid droplets.   That energy may be powering the fascinating meteorology that shapes the face of the giant planet, and driving lighting storms that flicker and flash in Jupiter’s belts.  In turn, the distribution of water may help to explain the contrasts in storm activity and colouration between the white zones and brown belts that criss-cross the face of Jupiter.  Understanding Jupiter’s water might be the key to understanding its churning weather.

But if the distribution and availability of water is key to explaining the meteorology of Jupiter, then why don’t we already have a better handle on this question?  The answer lies in Jupiter’s cold atmospheric temperatures.  The topmost clouds that we can see through our telescopes are composed of crystals of ammonia ice, mixed with various chemical contaminants that cause the different cloud colours.  These condense at a lower temperature (roughly -100 degree Celsius) than water vapour, so the clouds of ammonia ice sit higher up than the water clouds.  In fact, this cloud layer almost completely hides the deeper layers from view, and we have only glimpsed water ice in very limited locales under very special circumstances, when a powerful storm dredges the water ices upwards to the levels where we can see it, before precipitation (as rain or snow) causes it to sink back down again.

Jupiter in infrared light – spectra of the brightest regions show some signs of the presence of water, but cannot map the deep, drenched interior. Credit: ESO/L.N. Fletcher 

Certain wavelengths of light can start to probe down through these topmost cloud layers, like removing the skin of an onion to see what lies beneath.  For example, Earth-based telescopes can use observations of Jupiter’s infrared glow at 5 microns to peer through gaps in the ammonia ice clouds, such as those recently released by the Very Large Telescope in Chile.  Previous spacecraft, such as Galileo and Cassini, have had instruments that can observe Jupiter in this spectral `window’, and allowed us to place lower limits on the amount of water present (around 0.04% by volume).  However, they never sense down deep enough to see the amount of water in the deeper atmosphere, which is expected to be in the 0.1-1.0% range, depending on the different models used.  Juno carries a similar infrared instrument called JIRAM, provided by the Italian Space Agency, which will perform similar measurements.  But the deep water abundance remains out of reach for these infrared mappers.

A much better solution, then, is to actually send a probe into Jupiter itself.  And that’s precisely what we did in 1995 with the Galileo probe, which carried all the sensors required to sniff out the gases and clouds in Jupiter’s cloudy layers.  However, with only one probe there’s always the risk that you’ll find a region that isn’t representative of the full planet.  Imagine trying to understand the amount of water on Earth if you only sampled the Sahara desert.  The probe descended into a location known as a ‘hotspot’ just north of the equator, where powerful downwelling regions dried the atmosphere out and removed almost all traces of water.  The probe transmission ended at 20 bar of pressure, below the expected altitude of the water cloud (around 5-7 bar), but still without finding the deep abundance.  The maximum was 490 ppm near 20 bar (0.05% by volume), around 30% of the solar abundance.  Based on Galileo’s measurements of other species, like methane and ammonia, we’d expect that value to be more like 400-1000% of the solar abundance.

Mapping of Jupiter’s deep water and ammonia, key condensible species, far below the obscuring clouds. Credit: NASA 

It’s this mystery that’s driving Juno, and its microwave radiometer experiment.  Microwave light is another way of peering below the topmost clouds, and this should give us access down to 100 bar of pressure to see the distribution of water down at great depths.  We’ll be going deeper into the churning, convective weather layer than ever before to understand not only the bulk abundance of water, but also its distribution.  Maybe there’s more water available beneath the belts to power the moist convection and lightning storms that we see there?  But it’s a balancing act, and too much water actually stabilises the atmosphere and prevents convection – maybe that’s what’s happening beneath the white zones, where we don’t see as much convection?  Or is water really depleted throughout the cloud forming region, as the Galileo probe results suggested?  Juno is about to provide the answer.

At the same time, we’ll be amassing data from Earth that reveals the temperature, cloud and compositional structure above the clouds to see how this relates back to the distribution of water far below.  Will the deep atmospheric dynamics be different to what we’re used to above Jupiter’s clouds?  We hope to finally have a handle on the ‘Jovian water cycle’ that powers the weather on this gas giant world.

Thursday, 7 July 2016

Juno Tour of the BBC

The last 48 hours have been incredible, emotional, exhausting, and inspirational!  I was really lucky to be involved in some of the BBC's coverage of the NASA Juno mission's arrival at Jupiter.  I'll try to write a post on all the excitement at some point soon, but here are a collection of video clips from my day!

An interview with myself and Glenn Orton on BBC Breakfast at 07.40am, July 5th:

An interview on BBC Breakfast at 06:15am, July 5th:

An interview on the BBC News Channel at 5:20pm on July 4th, before arrival:

Wednesday, 29 June 2016

Glowing Jupiter awaits Juno

This article was released by the European Southern Observatory, Royal Astronomical Society and University of Leicester press office on Monday June 27th to coincide with the UK National Astronomy Meeting.

Stunning new images and the highest-resolution maps to date of Jupiter at thermal infrared wavelengths give a glowing view of Juno’s target, a week ahead of the NASA mission’s arrival at the giant planet. The maps reveal the present-day temperatures, composition and cloud coverage within Jupiter’s dynamic atmosphere, and show how giant storms, vortices and wave patterns shape the appearance of the giant planet. The observations will be presented on Monday 27 June at the National Astronomy Meeting in Nottingham by Dr Leigh Fletcher of the University of Leicester.

In preparation for the imminent arrival of NASA’s Juno spacecraft in July 2016, astronomers used ESO’s Very Large Telescope to obtain spectacular new infrared images of Jupiter using the VISIR instrument. They are part of a campaign to create high-resolution maps of the giant planet to inform the work to be undertaken by Juno over the following months, helping astronomers to better understand the gas giant. This false-colour image was created by selecting and combining the best images obtained from many short VISIR exposures at a wavelength of 5 micrometres. Credit: ESO/L. Fletcher

The high-resolution maps and images were created from observations with the European Southern Observatory’s Very Large Telescope (VLT) in Chile, using a newly-upgraded thermal imager called VISIR. The observations were taken between February and June 2016 to characterise Jupiter’s atmosphere ahead of Juno’s arrival.

“We used a technique called ‘lucky imaging’, whereby individual sharp frames are extracted from short movies of Jupiter to ‘freeze’ the turbulent motions of our own atmosphere, to create a stunning new image of Jupiter’s cloud layers,” explained Dr Fletcher. “At this wavelength, Jupiter’s clouds appear in silhouette against the deep internal glows of the planet. Images of this quality will provide the global context for Juno’s close-up views of the planet at the same wavelength.”

Dr Fletcher and his team have also used the TEXES spectrograph on NASA’s Infrared Telescope Facility (IRTF) in Hawaii regularly to map Jupiter’s changing appearance. The team made observations at many different wavelengths, optimised for different features and cloud layers in Jupiter’s atmosphere, to create the first global spectral maps of Jupiter taken from Earth.

These maps were created by slicing Jupiter's atmosphere using spectroscopy from the IRTF/TEXES instrument, and include a comparison to a visible light map from amateur observers. The 8 micrometre wavelength senses stratospheric temperatures near 1 mbar, showing wave activity in the northern hemisphere and heating associated with Jupiter’s powerful auroras. The 8.6 and 10.4 micrometre wavelengths sense tropospheric temperatures, ammonia humidity and cloud coverage. Adapted from Fletcher et al. (2016). Credit: NASA/Infrared Telescope Facility/M. Vedovato/JUPOS Team/Fletcher et al.

“These maps will help set the scene for what Juno will witness in the coming months. We have seen new weather phenomena that have been active on Jupiter throughout 2016.

False colour images generated from VLT observations in February and March, showing two different faces of Jupiter. The bluer areas are cold and cloud-free, the orangey areas are warm and cloudy, more colourless bright regions are warm and cloud-free, and dark regions are cold and cloudy (such as the Great Red Spot and the prominent ovals). The wave pattern over the North Equatorial Band shows up in red. This view was created by combining VLT/VISIR infrared images from February 2016 (left) and March 2016 (right). The orange images were obtained at 10.7 micrometres wavelength and highlight the different temperatures and presence of ammonia. The blue images at 8.6 micrometres highlight variations in cloud opacity.

These include a widening of one of the brown belts just north of the equator, which has spawned wave patterns throughout the northern hemisphere, both in the cloud layers and high above in the planet’s stratosphere,” said Dr Fletcher from the University of Leicester’s Department of Physics and Astronomy. “Observations at different wavelengths across the infrared spectrum allow us to piece together a three dimensional picture of how energy and material are transported upwards through the atmosphere.”

Both sets of observations were made as part of a campaign using several telescopes in Hawaii and Chile, as well as contributions from amateur astronomers around the world, to understand Jupiter’s climate ahead of Juno’s arrival. The ground-based campaign in support of Juno is led by Dr Glenn Orton of NASA’s Jet Propulsion Laboratory. Once in orbit around Jupiter, Juno will skim just 5000 km above Jupiter’s clouds once a fortnight – too close to provide global coverage in a single image. The Earth-based observations supplement the suite of advanced instrumentation on the Juno spacecraft, filling in the gaps in Juno’s spectral coverage and providing the wider global and temporal context to Juno’s close-in observations.

“The combined efforts of an international team of amateur and professional astronomers have provided us with an incredibly rich dataset over the past eight months,” said Dr Orton. “Together with the new results from Juno, this dataset will allow researchers to characterise Jupiter’s global thermal structure, cloud cover and distribution of gaseous species. We can then hope to answer questions like what drives Jupiter’s atmospheric changes, and how the weather we see is connected to processes hidden deep within the planet.”

Further Information:

  • Coordinated observing campaign details available at
  • Fletcher et al., (2016), Mid-Infrared Mapping of Jupiter’s Temperatures, Aerosol Opacity and Chemical Distributions with IRTF/TEXES, Icarus, accepted (doi: 10.1016/j.icarus.2016.06.008).

Thursday, 19 May 2016

Harold C. Urey Prize

Needless to say, I'm completely overwhelmed and delighted by all the messages of support and congratulations over the past week.  Here's a press release from the University of Leicester about the Urey Prize from the American Astronomical Society's Division for Planetary Sciences.  The original prize announcement can be found here.  To all those colleagues, family and friends who have kept me sane over the past decade, thank you thank you thank you from the bottom of my heart!  It's incredible to be awarded for simply doing a job that I love, and I now have some high expectations to live up to!

International award goes to Leicester planetary scientistInternational award goes to Leicester planetary scientist

The American Astronomical Society has honoured a planetary scientist in our Department of Physics and Astronomy with one of its prestigious prizes.

The Division for Planetary Sciences (DPS) has awarded its Harold C. Urey Prize 2016 for outstanding achievement in planetary research by an early-career scientist to Dr Leigh Fletcher in recognition of his ground-breaking work in understanding physical and chemical processes in the atmospheres of the outer planets.

His research uses sophisticated interplanetary spacecraft and world-leading ground-based observatories to study the climate and environment on giant planets.  His work has resulted in insights into such phenomena as the distribution of temperatures, chemicals, and clouds in Jupiter's Great Red Spot; the chemical make-up of Saturn's atmosphere, which reveals clues about its origin; the planetary-scale changes to the banded appearance of Jupiter; the discovery of a major hot vortex in Saturn's stratosphere spawned by powerful storm activity; the implications of changes of Saturn's temperatures and gaseous constituents for seasonal variability in its dynamics; and the distribution of Neptune's stratospheric temperatures and minor constituents.

Dr Fletcher is currently a Royal Society University Research Fellow and Lecturer at the University of Leicester.  He arrived at the University in 2015, after a career moving from NASA’s Jet Propulsion Laboratory in California to research fellowships at the University of Oxford.  He received his PhD in planetary sciences from the University of Oxford in 2007, and is a co-investigator on the Cassini mission to Saturn and future NASA and ESA missions to Jupiter and its icy moons, Europa and Ganymede.

Dr Fletcher said: “I’m completely humbled and overwhelmed at receiving this award, made even more special as it comes from my peers within the planetary science community. I’m so very fortunate to be doing a job that I love, and grateful to all my friends and colleagues for supporting me along the way.

“There’s an illustrious list of previous awardees of the Urey prize from all over the world, so I now have a lot to live up to!

“We have so many exciting projects coming up that will explore the Outer Solar System in new and innovative ways - space probes to Jupiter, infrared space telescopes, giant observatories on Earth - that it’ll keep our team in Leicester at the forefront of planetary research for years to come, and I’m proud that I can be a part of it.”

Professor Paul O’Brien, Head of the Department of Physics and Astronomy, said: “It's a wonderful surprise and a great tribute to Leigh. This prize highlights the excellent planetary research being carried out at the University.”

Professor Mark Lester, Head of the University’s Radio and Space Plasma Physics Group, said: “This is excellent news and demonstrates the standing in which Leigh is held in the planetary research community. It is thoroughly deserved.”

The 2016 DPS prizes will be presented at the joint 48th meeting of the Division for Planetary Sciences (DPS) and 11th European Planetary Science Congress (EPSC) in Pasadena, California, 16-21 October 2016.

Win(Mac)JUPOS - Installing WinJUPOS via Wineskin

One of the most incredible things about the amateur astronomy community is that they've attracted some pretty impressive software development to aid in their citizen science projects.  One piece of software has become rather mainstream - WinJUPOS, a program that allows you to map the locations on a planetary disc and use them for a whole host of tasks, including the production of maps.  Whilst we have software that also does this, it's often a handy quick-look tool for new data.  So imagine my endless frustration that it's only ever been designed for Windows machines!

Porting WinJUPOS to MacJUPOS

Step in Wineskin, a porting utility that allows you to run Windows executables on a Mac without setting up any virtual machines.  Here are the steps I followed:

1.  Download Wineskin Winery (v1.7) from here:

2.  As soon as Winery is installed, click on 'update' to update the wrapper - I'm using v2.6.2 for this installation.

3.  Install a Wineskin Engine (I'm using the latest, v1.9.9) by clicking on the '+' symbol in the window.  You now have almost everything required to install a Windows program.

4.  Create a Blank Wrapper - this will churn away for a few minutes, and prompt you to install a couple of windows components, Mono and Gecko - just say yes and it'll all work smoothly, prompting you to view the new Wrapper in the Finder window.  Name it something recognisable (i.e., the executable name).

5.  Next click on the Wrapper in the finder window, choose the option to 'Install software', and navigate to wherever you've downloaded the WinJUPOS application (  It'll churn away and complete the installation in the 'dummy' folders that it's generated, making it look like a Windows file system. Note that the first time you click the Wrapper it might fail to open - just click it again and all will be fine.

6.  Now, when you next click on the Wrapper it should open and run WinJUPOS without any issues.  There's only one peculiarity I found so far - when mapping images with pre-saved measurements (*ims files), I found that the 'finder' didn't see the file, and I had to type in the name manually to proceed.

Quick Start Guide with WinJUPOS

The software isn't entirely intuitive, but is OK when you get set up and running.

1.  Start with Recording > Image Measurement to load up your new image.
2.  Add the date, time, longitude and latitude of the observation to the 'Image Tab'
3.  Then go to the 'Adjust tab' to fit the limb of the planet - PgUp/Dn makes the limb larger or smaller; the arrow keys move it around, and P/N move the silhoette clockwise or anticlockwise.
3.  Click 'save' to save the image measurement file (*ims) associated with this image.

Then you can move onto the mapping:
4.  Go to Analysis > Map Computation, click on 'Edit' and add the *ims file (noting that this might have to be typed in manually on the Mac version, for some reason).
5.  Choose the parameters of the map you want to generate (planetographic latitudes and System III west longitudes with equirectangular cylindrical projections or stereographic polar projections for professional use).
6.  Compile the map and save as a JPEG.

This forms the basics of what I intend to do with the software, purely for presentation purposes, but there are tonnes of other tools within Win(Mac)JUPOS that can be helpful, all described in the documentation.

(PS.  I've also been able to use Wineskin Winery to install Autostakkert on the Mac, another excellent free tool for stacking lucky imaging, following essentially the same process as that above).

Note:  In the map computation window, you can change the output map quality by changing the map width. When you enter an image measurement file, an optimal map width is proposed. If you enter a smaller width, you will lose small details because of interpolation.