Category Archives: Trivia

Astronomy Photographer of the Year: 2012 Edition

The Royal Observatory, Greenwich has just announced the winners, runner ups and highly commended entries for this year’s contest.

You can watch two of the judges discuss this years winners and runners up.

The entire list can be seen on the Royal Museum’s winners page here and in person at an exhibit. Below are those that I really liked – displayed with permission, of course.

Simeis 147 Supernova Remnant

Simeis 147 Supernova Remnant

by Rogelio Bernal Andreo (DeepSkyColors.com)
Like me, Rogelio is a San Francisco Bay Area resident. Obviously Mr. Andreo has mad skills and dedication to astrophotography. See his portfolio for more work.

Lost in Yosemite [C_033706] Runner Up - Astronomy Photographer of the Year, 2012

Lost in Yosemite
by Steven Christenson

It would seem that Rogelio and I are linked somehow. We both won in our categories in 2010, and we both were runner’s up in 2012. Above is my runner-up shot. Click the picture and read the story about the lost hikers we met on our night hike up Half Dome.

You can view a slides show of all the photos submitted to the contest here. Warning: There are a LOT of them – 688 in the over teen category (I’d call it adult, but that word seems to have a different connotation).

Why your Streak is (probably) NOT a Meteor

Satellite or Meteor? [C_061879] So you took our advice or perhaps the advice of someone more clever than us and have captured a streaking bit of flaming cosmic stuff that some people call shooting stars. We do not want to rain on your parade, but let’s first get something straight: that flaming streak is more properly called a METEOR.  If it hit the ground, it’s a meteorITE.  If it in fact struck YOU, well you’re a lucky one!  No one in recorded history has ever been directly struck by a meteor EVER. We know what you’re thinking (really, we do). You’re thinking, but dudes: “What about the German boy who was hit in the hand, or the lady who had one bounce off her furniture and hit her in the leg or the man who suffered a broken finger when one crashed through his windshield and bounced off his steering wheel.” Sorry those were METEORITES apparently you weren’t paying attention when we explained the difference between meteors and meteorites.  Did anyone ever find a meteor on the ground? NO THEY DIDN’T… they found a meteorITE. Are we harping? Sorry.

Here is the sad news. You probably DID NOT catch a meteor (or meteorite) in your photo. Terribly sorry to tell you that. Go ahead, bring the photo and plop it in front of us. Claim what you want… but we are skeptics. Below are some things to rule out before we will conclude you have indeed caught a meteor.

Why Are We Such “Meteor Haters”

Hey, don’t put words in our mouth. We LOVE meteors. We just don’t believe you caught one. And here is why.

  1. Meteors move VERY, VERY fast across the sky and therefore across your image.
  2. Only exceptionally bright meteors throw out enough light in their rapid transit to even  register on your sensor or film.
  3. Just because you SAW a meteor occur in the direction your camera was pointing when  it was taking a picture doesn’t mean it registered.
  4. And it probably wasn’t a meteor.
  5. Besides, we think you’re wrong. So there.

Ok, so we admit to being a bit sour about it. After all, collectively we have shot about 20,000 (TWENTY THOUSAND) frames trying to catch meteors. And how many did we get? About 100.  We didn’t get so few just because we suck at it.

Below the Belt [5_020853] CARMAic Visitor from Cygnus [5_034154]
Dew Drop In [C_019416] Chiplet [C_034134]

Perseus slays Little Bear, oh my! [B_032691]

Of those 100, about 20 are readily noticeable. Of those 20, perhaps 10 are well captured. And of those 10, sigh, only a few really stand out.   But perhaps we should admit that we – like you – didn’t make all of those attempts under the best conditions. No, Like you, we took most of our shots when there was moonlight, light pollution, streetlights, and other impediments and the result was as you see at the left here: the meteor is almost impossible to see.  Like you we’ve SEEN a lot of meteors. And like you, most of the time the meteor we saw was regrettably not where we had pointed our cameras.  It’s a game of (very low) odds, after all.

Why You Didn’t Catch a Meteor (or maybe you DID!)

So many times we have seen people post their “brilliant meteor shot”. Almost exactly as many times we noticed one or more of the following:

  1. There are tell-tale flashing white, green or red lights. The tale those lights are telling is “aircraft” but the gleeful meteor hunters have their fingers in their ears.  Look closely at your shot to see.
  2. The streak bends or changes direction and the curvature is not due to field warp (as with e.g. a fish-eye lens). Sorry, but only airplanes curve like that.
  3. The shot immediately before or the shot immediately after the prize has the continuation of the streak. There is a 0.000008% chance of capturing a single meteor that spans more than one frame.
  4. The shot was at low ISO (less than 400), a high f/stop (anything above f/4), a narrow field of view or for a very long time. For a meteor to register you’d need a super slow flaming fireball of a meteor. If in fact you got one, well good for you and we are jealous.
  5. After ruling out aircraft, most people fail to rule out the next most obvious possibilities: satellites, flare and moths.   Yep, moths or any other bug that might fly through a source of illumination. We’re pretty sure you’ll be able to tell if it was a firefly though. Satellites are a little sneakier. They can – and do appear, move through the sky and disappear.  And they can fade in and out, too.

Satellites

There are MANY satellites in the sky. So many that we catch them ALL the time.  About every shot that doesn’t have a stinkin’ airplane seems to have a bloomin’ satellite in it.  Most satellites are quite dim and you don’t see them easily with the naked eye, however there are a few bright ones and one family of satellites that is EXTREMELY bright for a brief time.  We’ll get to that in a minute.

Meteors and Meteorites Have A Signature

Star Man and Perseus [C_059960-1]

Perseid Meteor, Milky Way and Galen’s Arch, Alabama Hills, Lone Pine, California, August, 2012

Most meteor streaks have the following things in common:

  • They brighten rapidly and dim a bit more slowly.
  • They are asymmetric (the brightening phase and dimming phase rarely look exactly alike)
  • Because of the two things above, meteors streaks rarely, VERY rarely have nice round ends – generally one or both ends are tapered.
  • Often meteors are colored!  The Perseids, for example, are often green, the Orionids are often yellow.

Perseid meteor traveling from the lower left to upper right. Note the changes in brightness and color

About those Bright Satellites

Satellites seem to wink in and wink out because they are illuminated by sunlight.  You’ll rarely see a satellite at the (true) midnight hour because the earth prevents sunlight from striking the satellite. However for as much as 3 to 5 hours after sunset or before sunrise (and more at other elevations), a satellite may move quickly and stealthily out of the earth’s shadow into a place where it can be seen clearly against the dark sky.  Or it might do the opposite: streak across the sky and then wink out when it enters the earth’s shadow. But there is one spectacularly bright satellite. Sorry did we say one, we meant 90 of them!  The family of satellites named Iridium. The name Iridium refers to the planned 77 communication satellites – the atomic number for Iridium is, 77.  The Iridium satellites exist to service those big, bulky sat phones – about the only option you’ve got if you need phone service in the Bering Sea or on an ice shelf in Antarctica.

Satellite Flash (Iridium) [5_033852-4br]

Iridium and “Flares”

Because the Iridium satellites are highly polished, and because each of those 90 objects are circling the earth every 100 minutes or so at a relatively low orbit, it’s not at all unlikely that one will reflect the light of the sun toward you! If you happen to be in just the right spot the brightness is extreme.  How extreme? Astronomers use a stellar magnitude scale. On this scale the smaller the number, the brighter. The stars in the Big Dipper are around 3, the brightest star, Sirius, is -1.46; Venus, the brightest planet at its shiniest is -4.6 and the brightest Iridium flares are -9!  What this means is: Iridium flares can be more than 20 times brighter than Venus or about 400 times brighter than the brightest stars!

Iridium satellites move swiftly but nowhere near as fast as meteors so they are far more likely to leave a mark in your photo than a meteor. Iridium flares behave very predictably. They start dim, slowly grow brighter and then slowly fade all the while that they transit the sky. If you want to mess with someone, use an Iridium sighting tool, figure out when and where to look in the sky and tell people nearby: “I have this sense… that something strange is about to happen… right … up … there”.  If you time it well people will be so amazed they may fall down and worship you. Time it wrong and they will laugh. Either way it’s great fun.  [NOTE: That link will only work in MILPITAS, CA – you need to use your GPS location].

The thing is, however that your camera doesn’t know when the grand entrance is going to happen and it will dutifully record the event while you’re busy chatting with your fellow night denizens.

Meteor Radiant Point (Delta Aquarid Meteor Shower)
Unfortunately we ran out of space before we got a chance to explain to you that even your correctly identified meteor is probably incorrectly identified as a “Perseid Meteor”.

In summary, we TOLD YOU you didn’t catch a meteor!

But if you think you did and are willing to stand some public humiliation at being proved wrong, please post ONE alleged meteor shot below in the comments.  Please also give us the date, time, timezone and GPS location so we can make sure it wasn’t an Iridium Flare. Wait, why make us do that… do it yourself! The exposure information is important, too (length, f/stop, ISO, focal length).

Oh, one last thing… did you find this article interesting? Amusing? Alienating as hell?  Please share it!

600 Rule?

You may have heard it elsewhere as the “600 rule”.  I first heard about the rule while visiting the Looney Bean in Bishop, California in 2008.  Five photographers sitting in a coffee shop poring over their laptops reviewing what they recently bagged are bound to start talking.  It was my good fortune that one of those present was the very talented Brenda Tharp who first quoted the 600 rule to me.

I, however, have repeated the rule as the “500 Rule” because I think 600 is overly optimistic.  What is the rule?  The rule states that the maximum length of an exposure with stars that doesn’t result in star streaks is achieved by dividing the effective focal length of the lens into the number 600.  A 50mm lens on a 35 mm camera, therefore would allow 600 / 50 = 12 seconds of exposure before streaks are noticeable.  That same 50 mm lens on a 1.6 crop factor camera would only allow 7.5 seconds of exposure.

But Wait. The Rule Isn’t All That Great!

The real number is quite subjective.  A little math reveals that on the Canon 5D Mark II (a full frame camera), with a 16mm lens a pin point star on the celestial equator moves from one pixel* to the next in 5.3 seconds.  But the 600 rule would allow 37 seconds of exposure and the 500 rule 31 seconds.  Both rules will produce streaks on the sensor! The visibility of those streaks will depend on the finished print size and viewing distance.  Print it large and stand close and the streaks will be obvious.

So what does a 30 second exposure look like at the pixel level:

3 Stars at 30 Seconds, 16mm on 35mm Sensor

Clearly those stars are streaking across about 5 pixels* just as the math would bear out.

What is going on here?  The Canon 5D Mark II images are 5634 x 3753 pixels* from a sensor that measures 36 x 24 millimeters. Dividing 36 by 5,634 reveals that the distance from the center of one pixel* to the next is a scant 0.00639 millimeters (or 6.4 microns).

The formula for calculating the distance in millimeters (d) that a star travels across a sensor due to the earths rotation looks like this:

d = t * f / 13750

Where t is time in seconds, f the effective focal length and 13750 is, well 13750.  I’ve simplified the above from the full equation. Is the math scaring you a bit… don’t worry… we’re almost done. Earlier we calculated the pixel* to pixel distance as 0.00639, what we want to find is how long (t) it takes for a star to move that far on the sensor.

0.00639 = t * f / 13750

Solving for f = 16mm we get a t value of 5.3 seconds as I asserted earlier.

But how does that calculate out on a different sensor, the Canon 50D, for example?

The Canon 50D has 4770 pixels across 25.1 mm or an inter-pixel* distance of 0.0053 millimeters.  Substituting into the earlier equation we find that a star marches across a pixel on the 50D with the same 16mm lens in 2.83 seconds.  With a 50mm lens on the same camera… the bad news is the star is speeding from one pixel* to the next in less than a second!

What does an image look like with a 30 second exposure at 16mm on a full frame camera? Remember that the streaks will be 40% longer on the cropped Canon 50D.

Milky Way over Black Rock Desert, Nevada

30 Second Exposure – a close look shows elongated stars.

Scaled down to only 16% of the original image size or seen from a distance no streaking is obvious! We will try not to twitch knowing – because we pixel peeped – that the stars are really dashes not nice round pinpricks of light. And indeed only the eagle eyed are likely to notice the dash-like nature of the stars until the photo is printed large, say at 20 x 30 inches.

What Can We Conclude?

  1. Streaking starts a LOT sooner than any rule you may have learned.
  2. The time it takes to streak depends on the inter-pixel* distance (sensor density / mm) and the focal length.
  3. How much streaking to allow depends on your aesthetic tolerances.
  4. You can not get more or brighter stars by exposing longer; starlight has already given up on one pixel* and moved on to the next in just a few seconds.
  5. The longer the focal length, the more impossible it becomes to prevent streaking.
  6. Gaps in your star trails may be unavoidable if the inter-shot delay (normally 1 second) is long enough to skip pixels*.

Final Note

I carefully added asterisks* to every location where I wrote the word “pixel” in a way that might imply your camera collects light in pixels. You might be wondering why I did that. The answer is: your sensor is comprised of sensels, not pixels. It takes 4 to 9 sensels to create a single pixel depending on the de-mosaic-ing algorithm your camera uses. Maybe you aren’t that picky, but I didn’t want to hear complaints from the purists.

I particularly relish this epiphany because I reported long ago that “longer exposures do not result in more stars“.  I just never got around to doing the extra bit of math – or the experiments – to prove out my assertions.

Real Final Note

A commenter has rightfully taken me to task by pointing out that the perception of a streak is dependent on many things other than just the actual sensor values recorded. In particular, if the image is not enlarged much some streaking will be scarcely or completely unnoticeable because the feature will be too small for the eye to perceive.  The problem with this assertion is that it assumes a lot of preconditions: e.g. how large the print is, how far from the print a viewer stands, and the subjective experience of the viewer.  My real world experience has led me to conclude that it is a reasonable goal to keep the streaking to 2 to 3 pixels or less because that will provide the greatest possible usable magnification (finished viewing size).  There would be no point to collecting a high megapixel image if you can not produce a print proportionately larger or more detailed than a lower megapixel image!

Here is an example that makes my point. I love this image captured on a Canon 5D Mark II. When printed at 20×30″ and viewed at 4 feet there is some streaking. Perhaps only a critical eye would notice, but even an untrained eye will notice when viewed from two feet away.

Famous III [C_035478]

Stratospheric Exercise for Moonatics

The moon setting behind the US Capitol Building, Washington, DC

If you’re going to chase the moon (or the sun), there are problems that you need to solve.  Here are some exercises to hone your sun and moon chasing skills so you can turn the chasing into catching.  The questions get progressively harder.  Those who have taken our Catching the Moon Webinar will find the answers and much more detail on the private course materials page.  The tools you will need to solve the problems include

You might also want to read some of our past articles on the topic, especially part 1 and part 2.

The Stratosphere Tower, Las Vegas, Nevada

I’ve picked a place that I hope few people are intimately familiar with.  Many of use have been to Las Vegas, Nevada and know that there is one of the worlds tallest towers there. I’ve even had the thrill of hopping on the “Big Shot” ride – the tallest ride in the world.  The Stratosphere Tower is second in the Americas in height only to the CN Tower in Toronto.  The Stratophere’s height above relatively flat surroundings makes it an easier target for catching a sun or moon set or rise from a distance far enough to make the tower seem small.  Even though the Stratosphere is tall, there are complications – including surrounding buildings and surrounding mountains. The farther you move away from the tower, the more significant those obstructions and potential obstructions become.

If you have never seen the tower, above is a relatively close up shot captured from Google Street View. Take note of the height of the mast above the “bulge” in the tower – that’s where you find the ride “Big Shot.” To my thinking it would not be terribly interesting to get an alignment with the sun or moon behind the mast of the tower. On the other hand, if the moon/sun diameter is not as large or larger than the bulge, the shot may not be all that interesting either.

On to the questions, starting from the basic data you need to collect, and on through to solving a “real life” alignment problem.

  1. What is is the correct GPS location for the Stratosphere Tower in Las Vegas, Nevada?
  2. The base of the Stratosphere Tower is at what elevation?
  3. Looking west from the base of the Stratosphere Tower, what is the azimuth and altitude of the tallest natural obstruction in the range of West, south west, to west north west (235 to 295 degrees)?
  4. From the tower at ground level: sunset on Tuesday, August 28, 2012 occurs in line with which of these natural features:
    1. La Madre Mountain
    2. Griffith Peak
    3. Lone Mountain
    4. Frenchman Mountain
    5. Mt Charleston
  5. On what day in August, 2012 will the sun appear to set on (not behind) La Madre Mountain peak?
  6. How far is the summit of La Madre Mountain from the base of the Stratosphere Tower?
  7. Can the Stratosphere Tower be seen from the intersection of Boulder Hwy (Nevada Route 582,aka Fremont Street) and East Sahara Avenue?
  8. If the tower is visible from the above intersection, which part of the intersection provides the least obstructed view?
    1. East
    2. North
    3. South
    4. West
  9. How tall is the Stratosphere Tower (excluding the antenna/mast on top)?
  10. How far is the Stratosphere Tower from the Fremont Street/East Sahara avenue intersection?
  11. What is the difference in altitude between ground level at the Stratosphere, and the ground level at the intersection?
    1. The intersection is 279 feet lower
    2. The Stratosphere is 279 feet lower
    3. No change
    4. The Stratosphere is 1,402 feet higher
  12. What is the altitude (angle above ground) from the intersection to the tip of the mast of the Stratosphere?
  13. On Wednesday, December 19, 2012 from the intersection, the moon will pass closest to the Stratosphere tower at what time:
  14. From the intersection the apparent moon size is about:
    1. Equal to the tower height, excluding the mast
    2. Half the height of the tower, excluding the mast
    3. 1/6 the height of the tower, excluding the mast
    4. Twice the height of the tower, including the mast
  15. On Wednesday, December 19, 2012 at the time calculated in question 13 the moon will:
    1. Pass just under the bulge in the tower
    2. Pass just over the bulge in the tower
    3. Pass behind the bulge of the tower
    4. Pass through the mast of the tower
  16. As seen from the intersection: what is the first day after June 13, 2012 when a nearly full moon (at least 95% illuminated) will appear to set behind the Stratosphere Tower?
  17. What is the NEXT day after the date found in the previous calculation that a nearly full moon will appear to set behind the Stratosphere Tower? (Hint: it’s more than a year later than the previous event).
  18. You want to catch Venus crossing the face of the sun as the sun sets behind the Stratosphere tower on June 5, 2012. In what publicly accessible location would you stand, and at what time so that:
    1. The sun is as large as possible relative to the tower (i.e. you’re standing as far away as practical).
    2. You are confident there is a visible line of sight to the tower.
    3. There are as few obstructions as possible in your line of sight.
    4. There is no mountain, hill or other building behind the tower along the sightline.
    5. You have at least a little bit of room to move to correct for misalignments in your calculations (e.g. standing on a manhole cover in the middle of the freeway is not advisable!)

Good luck!

PS If you’re stumped, I recommend our Catching the Moon (and Sun) Webinar.

NOTE: You are free to ask or answer any of the questions in comments, but those comments will remain private so that those who come along later won’t be tempted to cheat!