Star Struck

How Do They Make Those Telescopes So Big?

Telescopes gather light, and that light conveys information about distant and dim objects.  The more light the instrument gathers, the more information is conveyed.  And the bigger the telescope, the more light you can gather.

If you are of a certain age and have any awareness of astronomy, you probably remember that for most of your lifetime, the largest telescope in the world was the 200-inch (5-meter) reflector at Mount Palomar in California.  The casting, grinding, and polishing of this mirror was an impressive technological feat, unsurpassed until the Soviet installation of a 6-meter scope in 1975.  Today 8-meter mirrors are almost routine, and construction has begun on instruments with apertures of almost 40 meters!  Here is a diagram of the world’s largest telescopes, including some that are not yet actually built.

How do they make them so big?

The Hale telescope on Mount Palomar represents the limit of a particular mirror-creation technology, not of ultimate mirror size.  It was first cast in 1934 at the Corning Glass Works as a giant cylinder of the then new Pyrex glass, and that first casting was unsuccessful.  The cracked disc is on display at the Corning Museum of Glass in upstate New York.

The original cylinder blank was ground to produce a curved surface that would focus the light falling on it, then polished to make the surface as smooth as possible.  A thin coating of reflective aluminum was then vapor-deposited on that curved surface to make a mirror.

This cylinder was thick and had a huge thermal mass.  What that means is that the glass takes a long time to cool off or warm up.  This is not good when you need to eliminate air currents created by the mirror being at a different temperature from the air around it.  While the Hale telescope is able to compensate for this, the Soviet 6-meter instrument has been plagued by poor seeing that is at least partially due to its huge thermal mass.  This visualization of the convection currents created by a candle can give you some idea of the degraded view afforded by a mirror that can’t easily reach thermal equilibrium with its environment.

The mirrors had to be thick to maintain their shape against gravity or wind.  But beginning in the 1980s, thin mirrors were supported on their back sides by actuators that could maintain the proper curvature.  A thinner mirror not only has less thermal mass, it requires a less massive support structure too.  Another innovation that greatly reduces the time needed to create the curved mirror surface is spin casting, as explained in this video.

And finally, the really large mirrors are segmented—they are actually made up of hexagonal pieces whose overall curvature creates a single focal point.

Recently a mountaintop in Chile was leveled to provide a site for the European Extremely Large Telescope.  This image shows the size of the segmented mirror.

And this is an artist’s conception of the final installation scheduled for completion in 2022.

Posted in Observatory and Telescopes

Hold the Nobels For Now

Oops. Maybe.

In March, a team of astronomers working with the BICEP2 radio telescope at the South Pole announced an exciting discovery, claiming to have discovered patterns in the cosmic microwave background that would exist if the universe underwent an enormous expansion almost immediately after the Big Bang. This “inflation” is a well-established feature of modern cosmology, and seems to be consistent with all the observational data, but direct evidence for its validity had not yet been seen. If the BICEP2 team’s claims were true, their results provided that direct evidence. But I was relieved to find that in my original post, I had included the obligatory scientist’s word of caution that the claim needed to be confirmed and reviewed. It looks like we’ll need to wait for results from other experiments before we can say that we have that evidence for inflation.

This doesn’t mean that inflation is wrong. It just means that the BICEP2 team’s evidence for it is flawed.

I’m going to leave out an enormous amount of detail and scientific jargon to give the version with which you can impress your friends, neighbors, and casual acquaintances at your next social gathering. The claimed detection was of particular polarization patterns in the cosmic microwave background. (For explanations of these terms, go again to that March Star Struck post.) The problem is that there are other sources of polarization. The BICEP2 team used some preliminary data from another team’s work that hadn’t yet determined how much of the polarization seen came from each of two well-known sources. One of those sources is our own galaxy, whose dust polarizes microwave radiation as well.

The bottom line? We don’t know how much of the polarization announced in March is from the cosmic background radiation (CMB), and how much from our own Milky Way galaxy. The announced CMB polarization may (and may not) be overstated.

There are other teams working in this area, and we may have more definitive results by the end of the year. In the meantime, I hope no one from the BICEP2 team has bought non-refundable tickets to Stockholm.

Tagged with: , ,
Posted in Cosmology

July/August 2014 Sky Watcher’s Guide

This will be an every-two-months guide to what you can see in the night sky, geared to mid-latitude northern hemisphere observers. The original post with more detailed guidance can be found here. What can you expect to see in July and August 2014?

JULY, 2014

Moon Phases: Full on July 12th; new on July 26th. On the 26th, the Moon passes almost five degrees south of the Sun, which is about as far away from the Sun as it can get. It does this because the orbit of the Moon around the Earth does not precisely align with the orbit of the Earth around the Sun. They are actually tilted about five degrees (of course) away from each other. The Earth’s orbit defines the plane of the ecliptic.

Read more ›

Posted in Sky Phenomena

Dusty Mars–Bad. Windy Mars–Good.

My favorite writer about planetary science is, hands down, Emily Lakdawalla. She writes an occasional article for Sky & Telescope magazine, whose editor informed me on a recent astronomy-themed tour that “Emily is the consummate professional”. Where you can really keep up with her interests, however, is her blog at the Planetary Society website.

Here is a link to a post that gives you some idea of the sorts of things you can find here. The Mars rovers accumulate dust on their surfaces over time. For the Opportunity rover, still rolling after 10 years on Mars, this can be a real problem. It is powered by solar cells, and accumulated dust blocks sunlight and reduces the available power. The Curiosity rover that landed in August 2012 is powered by a radioisotope thermal generator (a slug of radioactive plutonium gives off heat which is converted to electrical energy), so power is not a problem. Mars is dusty, but it is also windy, and if you’re lucky, the wind will blow off the dust. Take a look here. Be sure to use the green slider in both images for before-and-after comparisons.

Ms. Lakdawalla is hard at work on an upcoming book, tentatively titled “Curiosity on Mars”, which will certainly find an eager buyer in this author. One interesting side note: in her extended biography, she states that her two year experience of teaching convinced her that a teacher’s life is very hard!

Posted in Mars, Planets

Planets? Positively! Meteors? Maybe.

This week the sky offers some absolutely “gonna-be-there” viewing, plus a tantalizing possibility for a once every few decades special event. Both of these, of course, depend on there being clear skies at your viewing location.

First, the sure thing. Three naked-eye planets are visible in the early evening sky just after sunset, and will be for another couple of weeks. Two of them will be visible all summer long. Below is the sky at 9 pm EDT on May 21, 2014, looking southwest.


The sun has set, but the sky is not yet completely dark. No matter; these planets will be the brightest things in the sky. Farthest to the west (closest to the already set sun) is Jupiter, by far the brightest thing around. A little east of directly south is Mars, distinguished by its orangey color. And farther east is Saturn, with a yellowish cast. You certainly don’t need a telescope to see them, but even a modest scope will reveal Jupiter’s largest moons, the disc shape of Mars, and Saturn’s glorious rings.

The iffy event holds out the prospect of a spectacular sight that most people have never seen, and the timing couldn’t be better for a pre-dawn adventure: 3 am EDT on Saturday morning, May 24. In short, what we see could range from nothing at the most disappointing end of the scale to a meteor shower with one or two meteors per minute, to a seldom-seen meteor storm with one every three or four seconds. More details can be found in this post from Sky & Telescope. The Lynchburg forecast is for clear skies, and I plan to be up and viewing!

Posted in Planets, Sky Phenomena

Earth 2.0? Not quite yet.

Don’t get me wrong. The discovery of Kepler-186f is a big deal: a near Earth-sized planet in the habitable zone of its star, neither too close nor too distant from it for liquid water to exist on the planet’s surface. But is it another Earth, Earth 2.0, the long-sought twin of our beautiful blue planet? Not quite. Not yet. But that hasn’t stopped artists from speculating on what this planet might look like close up.

What exactly is it that we are looking for? More precisely, what planetary characteristics are necessary to be considered a twin of Earth? Let’s start with the characteristics of Kepler-186f to see how it almost-but-not-quite qualifies.

The name itself tells you that this is a discovery of the Kepler Space Telescope, dedicated to finding planets around other stars by the dimming of those stars’ light when the planet passes in front of them. Here’s how that works.

Astronomers can deduce a surprising amount of information about a star from a detailed study of its light. We know how big it is and how hot. From an analysis of the planetary transit light curve, we can determine the orbital period of the planet and its size. Put all of this together, and what do we know about Kepler-186f?

It is only slightly larger (10%) than Earth, and just close enough to its host star for surface temperatures to fall in the temperature range allowing water to exist as a liquid (0 to 100 degrees Celsius). The host star itself is much smaller and dimmer than our Sun, so the planet must orbit much closer to its star than does the Earth to find this warmth.

Therein lies a key difference between this planet and our own: the host star is not Sun-like. It is smaller, dimmer, and redder. The close-in orbit leaves this planet more vulnerable to stellar flares, and if the planet were just a little closer, tidal interactions with the star would lock it into an orbit where one side would always be facing the star. A planet with one side in perpetual frozen night and the other always in hot daytime is hardly another Earth.

The search for Earth 2.0 is of course also a search for Earth-like life, or at least for a planet where life as we know it could arise. A small dim star is much more long-lived than our Sun, so life would have a longer time period to emerge, take hold, and evolve. That much is a positive.

Is there liquid water on the surface of Kepler-186f? That would require the planet to have an atmospheric blanket for protection. We don’t know if the planet has such an atmosphere, and it is too far away (500 light years) to be able to determine that. The planet’s size is such that it almost certainly has a rocky surface instead of being a gas giant like Jupiter, so we can’t rule out the possibility of lakes and oceans of water.

The ideal Earth 2.0 would orbit a star like our Sun, hot enough so that its habitable zone is at some distance from the star, putting a planet in that zone at less risk from solar flares, and eliminating the possibility of tidal locking. A star that is hotter will have a larger and more distant habitable zone, but stars that are very much hotter are short-lived and allow little time for life to emerge or evolve to any level of complexity. The planet should be small enough not to retain a huge atmosphere of light gases like hydrogen and helium, but large enough to hold onto some atmosphere and allow liquid water to exist on its presumably rocky surface. In short, it would look pretty much like this.

Kepler returned massive amounts of data before the failure of two reaction wheels used to precisely point the telescope. It takes some time to sort through all this information, and when a star hosts several planets it can take weeks of supercomputer time for a full analysis. Earth 2.0 candidates will orbit far enough from their stars to make transits relatively rare events; if Kepler were to observe the solar system under the most ideal conditions, the Earth would block the Sun’s light for only about half a day every year. Clearly you have to observe for a year see even one transit, and Kepler needs to see at least three to confirm that the star’s dimming was indeed caused by a planetary transit. The first results from Kepler were, as expected, dominated by large planets orbiting quite close to their stars. As more data are accumulated, we are starting to see more and more planets that are closer to being Earth-like. Earth 2.0 is quite likely just waiting to be discovered in a light curve stored on NASA’s computers.


PHOTO CREDITS (all labeled for noncommercial reuse):


Tagged with:
Posted in Exoplanets

May/June 2014 Sky Watcher’s Guide

This will be an every-two-months guide to what you can see in the night sky, geared to mid-latitude northern hemisphere observers. The original post with more detailed guidance can be found here. What can you expect to see in May and June 2014?

MAY, 2014

Moon Phases: Full on May 14th; new on May 28th. Ancient people, who observed the sky much more closely than most modern humans, knew that Moon appeared larger at some times than others. We know now that this is because its path around the Earth is not a perfect circle, but an ellipse that brings it sometimes closer and takes it sometimes farther away. The difference really is noticeable, as you can see from these comparison images. Perigee is the point of closest approach to Earth; apogee is the point of greatest distance.

This is most apparent when a full moon occurs near perigee, since the Moon’s nearness makes it much brighter. The much-hyped “super moons”—more a media-invented term than anything astronomers actually say—take place at these times.

May’s full moon on the 14th is not a super moon, but it does occur near perigee on the 18th. Barring clouds, it should be nice and bright. That will be a good time to go outside and read a large-print astronomy book!

Planets: Mercury is the most difficult of the eight classical planets to spot because of its proximity to the Sun, both from our viewpoint and in actuality.


Mercury never gets farther away than 28 degrees from the Sun, about three times the width of your fist held at arm’s length. On the evening of May 25th, it will be 23 degrees away from the Sun in the western sky. Wait until the Sun sets to be safe, then use binoculars to spot the innermost planet about halfway between the Sun and the bright, hard-to-miss Jupiter. You can try for a few nights before or after this optimum date, but don’t wait too long. Mercury moves fast—it is, after all, named for the fleet-footed messenger of the gods.

Jupiter follows Mercury behind the setting Sun, getting lower in the western sky as the month progresses.

Mars is nicely positioned for viewing in the early evening and does not set until well after midnight. In mid-May, look for an orange-reddish “star” almost due south. Don’t be confused by the bright (real) star Arcturus! It is higher in the south and a little east (to the left) of Mars.

Saturn, the universal favorite planet to see through a telescope, lies farther to the east. It reaches opposition on May 10th.



A superior planet is simply one that orbits farther from the Sun than does the Earth; opposition just means opposite the Sun in the sky. In practical terms, a planet at opposition is due south at midnight.

Saturn is moving farther from the Sun in its own elliptical 30-year orbit, but it has been getting brighter with each succeeding opposition. The key to this apparent paradox is that the angle of tilt of Saturn’s rings has been increasing as we view them. Edge-on in 2009, the northern surface is now tilted toward us by 22 degrees. The rings are very reflective, since they are composed of mostly water ice.


Meteor Showers: There are a couple of nice showers this month. On the night of May 6th, the Eta Aquarid meteor shower provides a nicer show for our southern hemisphere neighbors than for those of us who live north of the equator. The dust particles responsible for it come from the most famous comet of all, Comet Halley. Meteor showers occur when Earth crosses the path of comet, which strews dust all along its path.


For northerners, the radiant (see this original post for an explanation) is highest in our sky after the sun has already risen; our best viewing hours are just before dawn. For folks in southern latitudes, this is one of the best showers of the year. It was unusually strong in 2013 and that may repeat this year.

An unnamed shower from a comet discovered in 2004 (Comet 209P LINEAR) MAY (and I emphasize may) produce a spectacular shower on May 24th. Some predictions are for a full-fledged meteor storm, with up to 400 meteors an hour, one every ten seconds or so. This one is better positioned for us in the north, as the radiant is well to the north.

Deep Sky: A view to the south this month is dominated not only by Jupiter and Mars, but by three bright stars as well. The red giant Arcturus is the brightest of these, and is the fourth brightest star in the sky, not including the Sun. Moving from east to west (left to right) we see Spica and Regulus, respectively the 15th and 21st brightest stars.

Even though the celestial sphere looks like a dome, with stars of different brightness all the same distance away on a curved surface, it does in fact have a third dimension. A star’s brightness depends not only on its luminosity (its inherent brightness), but also on its distance. Not surprisingly, Arcturus is the closest of this triplet at 37 light years, and Regulus is 79 light years away. That leaves Spica at a distance of 260 light years. The easy conclusion is that Spica is the most luminous of these three.

To the north are two other bright stars. Capella lies low in the northwest early in the evening on its way to slipping below the horizon. Vega lies low in the northeast, and is rising higher in the sky as the night progresses. Almost directly above the north star is the upside-down Big Dipper asterism.

A deep sky object that requires a telescope but is well worth the effort to get yourself to one (here is information about Lynchburg College’s May observatory open house) is M51, the Whirlpool Galaxy. Many are familiar with the spectacular Hubble Telescope image:


But no one actually sees anything like this looking through a telescope; such images are the result of long exposures on a digital camera. Viewing time on a world-class telescope is too valuable to assign to a feeble instrument like the human eye! This is more like what you would see with the Gilbert 20-inch telescope at the Lynchburg College Belk Observatory:

M 51 in a small telescope

The Wikipedia article on M 51 is a great source for more information.

Sky Charts:

Looking south, 10 pm, May 15th.

May South Sky

Looking north, 10 pm, May 15th.

May North Sky

Southern-facing view in chart form.

May South Chart

Northern-facing view in chart form.

May North Chart

JUNE, 2014

Moon Phases: Full on June 13th; new on June 27th. In the summer, full moons are lower in the sky (more southerly) and the Sun is higher (more northerly). In the winter this is reversed: full moons are high on winter nights and the Sun is low during a winter day. All of this is from a northern hemisphere perspective.

Planets: A testament to just how fast Mercury moves is this: it was at its greatest distance from the Sun from our Earthly perspective on May 25th. Less than a month later, on June 19th, it passes on our side of the Sun, 4 degrees south of it.

Jupiter is moving ever closer to the setting sun, only one hour behind it at the end of June.

Mars and Saturn are the two brightest planets in the southern sky, and they move closer to each other throughout the month. They will pass each other in August. Saturn, the more westerly of the two, sets at 3:30 am on the first of the month and 1:30 am at the end. On the night of June 10th, Saturn and the nearly full moon will pass very close to each other.

Venus is the bright “morning star”, rising two hours before the Sun in the east.

Meteor Showers: The June Boötids are a notoriously variable meteor shower that may (or may not) peak on June 27th. The radiant is overhead at 9 pm and visible all night long.

Deep Sky: Another of those get-to-a-telescope-if-you-can objects is M 101, the Pinwheel Galaxy. A telescope of 16-inch or greater aperture will reveal structure in the spiral arms, provided you are under a dark sky, preferably with no moon.


Seeing structure in a dim object requires patience, and reveals as well as anything the difference between just looking (giving it five seconds and not seeing anything) and seeing (taking the time needed). Your eyes should be dark adapted, and you should use averted vision, not looking directly at the object. This lets you see more detail. It works because the central region of the eye has only cones, cells which detect bright light and color and are not particularly useful at night. Just a little off to the side, the concentration of rod cells is much higher, and rods detect dim light in black and white.

Here is more about the Pinwheel Galaxy.

Summer Solstice: Occurs at 6:51 am EDT on June 21st.  This marks the farthest north that the Sun travels in our sky, the official beginning of summer in the northern hemisphere and winter in the southern hemisphere, and the longest day of the year for the north.  The latest sunset, however, occurs on June 27th, so it seems like the longest day.  If you want to celebrate the solstice in a truly old school way, this is how to do it.

Sky Charts:

Looking south, 10 pm, June 15th.

June South Sky

Looking north, 10 pm, June 15th.

June North Sky

Southern-facing view in chart form.

June South Chart

Northern-facing view in chart form.

June North Chart

Posted in Sky Phenomena

Total Lunar Eclipse

In the early pre-dawn morning hours of Tuesday, April 15th, most of the Western Hemisphere will witness a total lunar eclipse.  Totality (when the Earth completely blocks the Sun as seen from the Moon) begins at 3:07 am EDT and ends at 4:25 am EDT.  Those in other time zones can adjust as needed.  The official NASA eclipse information is here.

Solar and lunar eclipses occur when the Sun, the Earth, and the Moon line up so that one body is blocking another as seen from the third body.  A lunar eclipse such as Tuesday night’s requires this sort of alignment.

The terms penumbra and umbra can be confusing, but here is what they mean.  Within the umbra, the Sun’s disk is completely obscured from view by the Earth.  Within the penumbra, only part of the Sun’s disk is obscured.  A total lunar eclipse occurs when the Moon passes into the umbral shadow of the Earth.

Since the Earth is much larger than the Sun as seen from the Moon, the Moon can spend a fairly long time passing through Earth’s umbral shadow.  Tuesday morning’s total eclipse lasts well over an hour, and that’s not even counting the time when the partial eclipse begins (1:58 am EDT) and ends (5:33 am EDT).  The exact size of the shadow depends on the distance between the Earth and the Moon at the time of eclipse, but the difference between the maximum and minimum shadow sizes is not that large.

Total solar eclipses are an entirely different story.  The necessary alignment is this.

As seen from the Earth, the Sun and the Moon are very nearly the same apparent size.  Another way of saying this is that the umbral shadow of the Moon is quite small when seen from the Earth.  Total solar eclipses do not last long (the longest possible is around seven minutes), and totality occurs only along a narrow strip of Earth’s surface.  One generally has to travel to see a few fleeting moments of the Sun’s being blocked out during the day.  But speaking as one who has done so, the sight is well worth the effort.

The good news is that lunar eclipses are much more accessible.  And if you don’t want to stay up that late or get up that early next week, you can mark your calendar for the night of September 28th, 2015.  A total lunar eclipse will begin that night at 10:11 pm EDT, and end at 12:27 am EDT the next morning.

Tagged with:
Posted in Sky Phenomena, The Moon

The Latest News From The Big Bang

It won’t make you richer. It won’t organize your calendar. It probably has no practical application whatsoever. But the scientific discovery announced on March 17th is one of the great discoveries of the new century, and virtually certain to result in a Nobel Prize. (Appropriate note of caution: assuming this is confirmed and stands up to peer review.) Here is a link to an article by Dennis Overbye, the long-time science writer for the New York Times.

In the question-and-answer format that follows, I’ll try to explain what the new discovery reveals and what was actually discovered. I hope it will neither annoy those who really, really know this stuff nor blow away those whose last science course was in high school! Get comfortable and settle back, because this will take a while. Maybe set aside two sessions.

What exactly was discovered?

Patterns in the polarization of radiation from the cosmic microwave background (CMB). Yeah, I know—geek talk. Let’s try and break that down to standard English.

In the early 1960s, two competing cosmological theories stood on roughly equal ground. Steady State theory acknowledged the expansion of the universe that had been detected decades earlier, but it still maintained that the overall universe did not change with time. “Continuous creation” filled in the gaps left by universal expansion; the universe billions of years ago and billions of years from now would not look any different over galactic scales of distance.

Steady State

The competing Big Bang theory discarded the idea of continuous creation, and asserted that the expansion meant that the average separation between galaxies would be greater five billion years from now than it is now. More to the point, the theory “ran the clock backward” to a hot early universe that was much smaller than today’s.  The average separation of galaxies would increase over time.

Big Bang

A successful scientific theory is one that makes testable predictions. A hot early universe would have cooled as it expanded, not by radiating heat away like a coffee cup, because there is nothing to radiate it away to–there is no “away”. You just have the same amount of energy distributed throughout a much larger universe. A universe with an average temperature of 3000 K expands by a factor of 1000, and its average temperature becomes 3 K.

Read more ›

Tagged with: , , ,
Posted in Cosmology

March/April 2014 Sky Watcher’s Guide

This will be an every-two-months guide to what you can see in the night sky, geared to mid-latitude northern hemisphere observers. The original post with more detailed guidance can be found here. What can you expect to see in March and April 2014?


Daylight Saving Time: March is one of the two months of the year in which we have to account for this abomination. Since the 15th of the month comes after the Congressionally determined switch back to standard time, the times are all Eastern Standard. Don’t get me started.

Moon Phases: New on March 1st and March 30th ; full on March 16th

Planets: Jupiter continues to dominate the early evening sky in the far western reaches of Gemini. Although he is traveling a little farther east each night relative to the background stars around him, Earth’s orbital path around the sun puts those background stars a little farther west each night, at a faster pace than Jupiter’s motion. The net result is that Jupiter will be quite low in the west in a few months–but not for a while!

Neither Mars nor Saturn show on the chart, but you should know that Mars rises around 11 pm in mid-month, and Saturn about two hours after that.

We need to give a nod to Venus, impossible to miss in the morning sky before sunrise. Because Venus is an inferior planet (it orbits closer to the Sun than does Earth), it goes through phases similar to the Moon’s. But because, unlike the Moon, Venus varies considerably in its distance from us, its apparent size in each phase is noticeably different. This should help you see why.


Read more ›

Posted in Sky Phenomena