What Exactly is an Apochromat?

The photography/astronomy world is loaded with technical jargon that will undoubtedly confuse beginners. In the area of lenses, it is common to see the word “apochromatic,” or “apo” for short, used to describe high-grade lenses or telescopes. So what does “apochromatic” mean?

The term apochromatic/apo is used to designate any lens that minimizes color dispersion. In an even more basic description, apo lenses bring all colors of visible light to a single focus point. Before going any further, some basic scientific knowledge is required.

All colors we see are part of the electromagnetic spectrum. In fact, visible light is just a tiny sliver. From radio waves on one end to gamma rays on the other, each type of energy on the spectrum has a different wavelength, measured in nanometers (one billionth of a meter). Although covering just a tiny portion of the spectrum, visible light, red, orange, yellow, green, blue, indigo, and violet (think ROY G. BIV) all have a different wavelength, which complicates the process of bringing them to a single focus point over a short distance, which is exactly what a lens seeks to do. If all light is not focused at a single point, image quality will degrade. Images will appear soft and they will often show chromatic aberration, which appears as false color fringes in high contrast situations. The good news is that there are three main ways around the problem.

1. The first, and oldest, is to have a lens with a long focal ratio that gives the light a longer path to get in focus, thus minimizing focal length differences for different wavelengths of light. In the old days, refracting astronomical telescopes had long focal ratios, often around f15, and showed no false color except on the brightest of objects. In practical terms, a telescope with a four inch lens would be five feet long, which is not very portable. In photographic terms, long focal ratios require slow shutter speeds, which are often undesirable and necessitate use of a tripod in all but the brightest lighting for most people.

2. The second way, the quick fix, is through filter usage. Filters can be threaded onto camera lenses or telescope eyepieces to minimize the false color. Unfortunately, these filters can introduce inaccurate color casts to the image and also reduce the amount of light passing through the lens, which will result in longer shutter speeds on cameras and dimmer views in telescopes.

3. The third, and by far best method, is by utilizing special glass that will bring all colors to a single focus point in the first place.

Unfortunately, APO as an acronym could stand for “another paycheck obliterated.” Special high-grade glass that will bring all the colors to focus at the same point is expensive. Added to this is the fact that these types of glass are hard to work with, which means higher labor costs. The good news is that, for people who demand the highest image quality possible, apo lenses are worth the investment. Fully corrected for chromatic aberration, the false color will disappear. An added bonus is increased correction for spherical aberration, which results in sharper images.

Keeping in mind that pictures are worth a thousand words, I put two telescopes, one achro and the other apo, through a false color optical torture test: an extremely high contrast situation courtesy of tree branches and a bright sky.

Photo notes: All photos (except the last) are heavy crops taken from an original.

Photo 1 was taken with an Orion 120mm f8.3 non-apochromatic refracting telescope ($300 OTA). The false color is very obnoxious.

Photo 2 was taken with the same scope but with an aperture mask that made it into a 60mm f16. False color is greatly reduced, but exposure requires a shutter speed four times longer than photo 1.

Photo 3 was taken with an Orion ED80 f7.5 apo refractor ($500 at release). Absolutely no false color at all, but this 80mm scope costs $200 more than the 120mm model.

Photo 4 is the full, uncropped version of photo 1 to illustrate just how tiny the crops are. Viewed in full, the false color does not appear so extreme. However, the more the image is cropped, the more obvious it becomes. For astronomical uses, which are often far less forgiving than most terrestrial photography, even a full view photo will show ugly violet rings around brighter stars.

As a final note, apo lenses often go by many different names (LD (low dispersion), ED (extra low dispersion), (SAPO) super apochromat, SLD (super low dispersion), fluorite, and FPL-53). The terms all are more similar than different in their final goal: striving for better images, but that doesn't stop people, especially astronomers, from debating what constitutes a “true” apochromat. For me, an apochromat is any lens that shows no false color visually or photographically. Keep it simple.

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Mars Coming to Opposition, Won't be Size of the Full Moon

Mars on January 12. It has a long way to go in 2 weeks to be the size of the full moon.

Later this month, Mars will be coming to opposition, which is when Mars is directly opposite the Sun in the sky as seen from Earth, so Mars rises and sets opposite the Sun, making for an all night event.. Because of the straight alignment of Sun, Earth, Mars, this is the point in time where Mars makes its closest approach to Earth.

This time, the approach won't be too close, with the planets only coming within about 61 million miles of each other. Still, the change in Mars' brightness will be apparent. Of all the planets, Mars has the most elliptical (oval-like) orbit. This results in dramatic changes in brightness. At its farthest from earth, Mars shines at about +1.3 magnitude. When Mars made its very close 2003 approach, it was up to -3 magnitude, or about 40 times brighter than at its dimmest when the planet came to about 34 million miles from Earth.

This year, opposition will occur on January 27 with the planet peaking in brightness at around -1.3 magnitude, about as bright as the brightest star in our sky, Sirius. Obviously, this is not nearly as large or as bright as the full moon, which lights up the sky at -12 magnitude.
This opposition will also mark the first time since 1994 that missions to Mars haven't been launched at this close approach. Both the American Mars Science Laboratory and Russian Phobos-Grunt probes are being put on hold until the next opposition in 2012.

Still, don't be dismayed at the fact that this opposition will not be one of the best. Due to the timing, Mars will be very high in the sky, which is great for high-power telescope observing, which was anything but the case for the “great” 2003 approach. Unfortunately, don't expect to see any trees on Mars.

Humble request
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The genius, the stupid, and the historic

As the first decade of the third millennium ends, it is irresistible to take a look back at what has transpired over the past year, and decade for that matter. Here are three look-backs at the world of photography that showcase how we got to where we are today.


The genius: The Cammys
This is a quick look back at the best photography had to offer in 2009. Explore the best in cameras to the greater impact of photography on the world.


The stupid: The Golden Garbage Cans
Here is a quick look back on the worst photography had to offer in 2009. From flop cameras to useless features to camera-inspired stupidity, it's all here!


The historic: Reviewing the Digital Decade
As the the first decade of the third millennium closes, one can't help but take a look back. 1999 was the true birth of digital photography. In the intervening 10 years, many innovations were made, but only a select few were industry-altering.

January Skies: What's Up This Month?

Yes it's January and yes, that's Vega of the Summer Triangle. Things will get better!

Now that it is January, we are deep into the cloud season. Like December, clear nights in January should be cherished, especially considering the sights to be seen and the fact that January nights are still among the longest nights of the year. The unique fun of observing (or even better, photographing), the same star on the set and rise during one night is something you should try should the sky allow. Vega and Deneb, two bright stars of the Summer Triangle, make great targets.

On to more traditional observing. Because of the Long nights, January allows for observing a wide range of constellations from three of the four seasons. Despite it being January, at least as the month starts, Lyra and Cygnus are still visible under dark skies low in the Northwest! Take a last look at the fall constellations as the sun sets, probe the winter constellations in the dead of night, and race the light to behold the spring and early summer constellations (the aforementioned Lyra and Cygnus) as they race the Sun in the Eastern sky. There's way too much to be seen for going into detail here, so grab your sky atlas and some warm clothing to prepare for an all night observing session that will take you through the majority of the year.

Last but not least, planets. This is really the last month to view Jupiter telescopically. By the end of the month, Jupiter will be a twilight object low in the Southwestern sky, so forget the telescope by then. Mars will continue to brighten as it passes from mid way between Leo and into Cancer (to about 3 degrees from the Beehive). It will reach opposition on January 29. Grab a telephoto lens and your SLR for and take advantage of this great photo-op. Neptune is still relatively close to Jupiter, so now is a great time to see it, but you'll need to crank up the power to distinguish it from a star. Saturn is still holding station in Virgo and last but not least, Mercury will put on one of its best morning shows of the year toward the end of the month after first appearing around January 12.

There's a lot to see this month if the clouds cooperate. So if the night is clear, go on out for a look.
January Visual Observing Highlights
January 1: Sirius is almost due South at midnight (Happy New Year from the Dog Star!)
January 2: Like last month, the nearly full Moon is very close to the Beehive Cluster in Cancer
January 3: A cosmic triangle of the Moon, Mars, and Regulus
January 4: Date of latest sunrise
January 10: Mercury starts to rapidly ascend in the Southeast
January 11: The Moon scrapes Antares
January 16: Young moon just after sunset. Test your Southwest horizon.
January 17: The thin waxing crescent makes its closest approach to Jupiter
January 20: Mercury is at its highest, about 10 degrees up in the Southeast, and it'll hand around awhile
January 24-25: The Moon approaches the Pleiads to either side
January 29: The Moon, Mars (at opposition today), and the Beehive are within six degrees.
Late January: Jupiter is now a dusk object, and makes a nice addition to sunset photos.

The Star of Bethlehem Explained?

One of the greatest mysteries of the Bible and astronomy is the Star of Bethlehem, which guided the Magi on their journey to the baby Jesus. The description of the star in the Bible leaves a lot of questions, and just as many possible answers to its true identity.

One problem, by far the biggest, must be confronted right before we can even start to narrow down the possible identities of the star: no one knows exactly when Jesus was born. Our current calendar is based on the birth of Christ. Unfortunately, this is wrong. It is now thought that Christ was actually born in the span of 8 to 4 B.C. In the Bible, the Holy Family fled into Egypt to avoid the wrath of King Herod, who died in 4 B.C. Thus, that year is the last possible year in which Jesus could have been born.

Now that our time frame has been narrowed down, we can start looking to the sky. There are two schools of thought about the Star of Bethlehem: it was either astronomical or astrological.

Astronomical possibilities include supernova, planets, comets, and conjunctions. In the year 7 B.C., there was a conjunction of Jupiter and Saturn. However, this is very unlikely as the planets were about a degree (little finger at arm's length) apart. Unless the Magi had very poor eyesight, there is no way that two planets this far apart could be mistaken for a single object. Comets have been suggested, especially since they were often described as “hanging” in the sky. This is exactly as the Star was described in the Bible. A last possible interpretation is a supernova. A bright object was seen for about 70 days in 5 B.C. By Chinese and Korean astronomers. This may just be it, but there's a problem: the star was described as moving, which leads into the other school of thought: astrology.

People at this time were almost universal believers in astrology. A notable exception here were the Jews, who were forbidden to practice astrology at numerous spots in the Old Testament. As far as everyone else was concerned, heavenly bodies had special meaning.

One thing we know was that the Magi came from the East. Considering the geographical location of Judea, “East” almost certainly meant Persia. In Persian language, the word “magi” referred to Zoroastrian priests, who practiced medicine and magic (“magic” comes from “magi”), which could also include astrology, at which the Persians were very sophisticated.

One particular passage in Matthew can greatly narrow down possible candidates for the true Star of Bethlehem. According to the Gospel, “the star which they had seen in the East went before them till it came and stood over where the young Child was.” If this is to be believed, the Star was a planet. Over the course of months, a star's position will change as it rises about four minutes earlier each night. Stars don't stand still, but planets do.

Observe a planet over the course of a year, noting where it is in the constellations. For most of the time, it moves forward. However, there are times where it stops, reverses course, stops again, then continues forward. This apparent change in direction called retrograde motion is an optical illusion caused by the Earth passing the slower planet as both orbit the Sun. A comparison can be made to passing cars on the highway. As you pass, the slower car seems to travel backwards. The same is true of planets.

Besides retrograde motion, there is more. Planets and constellations had different significances. Jupiter was widely considered to be associated with kingship. The constellation of Aires the ram was often associated with Israel/Judea. Putting this information together with the knowledge that the Star of Bethlehem was almost certainly a planet allows one to start putting the puzzle together.

In 6 B.C., an astronomical/astrological event that fits the bill very nicely occurred. In that year, the planet Jupiter (planet of kingship) moved into the constellation of Aires (the constellation for Israel/Judea). Thus, this could be interpreted as a sign that a new king of Israel was born. To add even more weight to the hypothesis, Jupiter first appeared as a morning object in the East. At this time, the Sun was also in Aires (Jupiter was rising just ahead of the Sun). In astrology, any constellation is at its most influential when the Sun is in it. Also, it was believed at the time that planets were at their most powerful as they emerged in the East after a period of invisibility in the Sun's glare.

As it would have taken the Magi months to reach Bethlehem from Persia, this also explains the motion of the Star. As time progressed, the Magi could have observed Jupiter slow down and stop before going into retrograde motion. The stoppage could have coincided with the arrival of the Magi in Bethlehem after stopping in Jerusalem and being told of the prophecy predicting the Messiah's birth there.

This is by no means more than a hypothesis. The Star of Bethlehem will probably never be conclusively explained. Either way, merry belated Christmas if you celebrated on the 25th, or merry Christmas if you celebrate on January 7.

Vega setting, Vega rising

On the last post explaining what was going on in the sky in the month of December, I set a unique challenge: in this season of long nights, try and observe (or even better, photograph) the same star on the set and on the rise. It is possible now and will be possible even into February. Above are my efforts from three years ago with my old HP Photosmart 945, so get out and give it a try. Click on the top picture and that cross shaped constellation (pointing straight down) is Cygnus the swan.


Setting


Rising

December Skies: What's Up This Month

The months of December through February are the cloudiestof the year, averaging only about 30% of possible sun, and star light. Needless to say, clear nights in December should be cherished, especially considering the sights to be seen and the fact that December brings the longest nights of the year, which allows for some unique fun. Only at this time of year is it possible to observe something twice on the same night: once on the set in the evening and again on the rise in the morning. Vega and Deneb, two bright stars of the Summer Triangle, make great targets for this once a year opportunity. I've done it, you should, too. On to constellations, December allows for observation of the sky for three quarters of the year. Take a last look at the fall constellations as the sun sets, probe the winter constellations in the dead of night, and race the light to behold the spring constellations as dawn nears. There's way too much to be seen for going into detail here, so grab your sky atlas and some warm clothing to prepare for an all night observing session that will take you through the majority of the year. As two last planetary notes, keep an eye on Mars, it will nearly double in brightness by the end of the month. Want to see Neptune? Look near Jupiter. This is the way Galileo saw it 400 years ago.

December Visual Observing Highlights

Early December: Jupiter emerges due South at twilight, no better time for telescopic observation. Also be sure to catch Venus at dawn low, extremely low, on the Southeastern horizon.
December 1: The full moon splits the sky between the Pleiades and Hyades
December 5: The waning gibbeous moon, Pollux, and Castor line up.
December 6: The moon is within 5 degrees of the Beehive Cluster. Use binoculars.
December 7: Cosmic triangle: Regulus, moon, Mars. Grab the camera. Earliest sunset of the year.
December 13/14: Geminid Meteors peak. Keep an eye to the sky about a week either side of this date.
December 18: The moon meets Mercury low in the Southwest. Test your horizon.
December 21: Winter Solstice, the longest night of the year.
December 28: The waxing gibbeous moon scrapes the Pleiades
December 31: Blue moon (second full moon of a month)
All month: Morning planets: Mars in Leo and Saturn in Virgo