Tuesday, 3 September 2013

Confessions of a Perseid Hunter

Confessions of a Perseid Hunter

All of my life, I’ve had dreadful weather on or around the peak of the principal meteor showers.  When I lived in London I rarely bothered because the light pollution made it very difficult to see much, but when I moved back to Lancashire in 2001 I was looking forward to having darker skies again.  For 12 years I religiously camped out in the garden late at night during the week leading up to the peak of all the main showers.  The most meteors I ever saw on those nights was 2 or 3 over a 2 hour period. However, on the nights immediately before and after the predicted peak I was always treated to cloudy skies and was so utterly disappointed each time.  I recently moved to rural Oxfordshire and the skies here are a huge and dark.  Once again I allowed myself to become excited in the run up to this year’s Perseid meteor shower, because from such a dark location I would surely see more than 3 meteors!  Over the weekend of 9th/10th August, we sat out until quite late but didn’t see very much.  

Sure enough, on the night of 11th August it was cloudy again.  I was beginning to think I was destined never to witness a meteor shower peak.  But the predicted rates were still good for the night of the 12th, so I kept everything crossed for a clear night!  The weather during the day was awful, and things were not looking good.  Then at around 6pm the sky started to clear. I was becoming so superstitious. Don’t even say out loud that the sky has cleared; don’t print out any observing sheets; don’t get the camera ready; don’t even look directly at the sky in case it makes it cloud over!  As darkness fell, the clouds stayed away and I began to believe that I might actually be in luck.

At 10pm I dressed myself in 4 layers of clothes, dug out my scarf and gloves, grabbed a blanket (I’m prone to feeling the cold!), got my camera focused, notepad at the ready and my chair in position.  By 10:30pm my eyes were dark adapted and I was ready to go.  Now there is a wealth of information on the internet about the origins of The Perseids, together with observing tips, but there was surprisingly little practical advice about photographing a meteor shower. We both had our cameras set to continuously take 30 second exposures and we tried to make sure we were pointing at different parts of the sky.  We knew that we should be pointing the camera at least 45 degrees away from the radiant and at about 45 degrees up. We knew that the later it got the higher the rates were likely to be, partly because the radiant was getting higher, but more importantly because after midnight our side of the Earth would be facing the debris cloud head-on, and effectively “scooping up” more dust particles.  Aside from that, it really was just a case of trial and error.  

From an observational point of view, nothing prepared me for just how amazing a night I was in for! I was out for 5 hours in total, keeping a tally of what I’d seen each hour.  In addition to keeping a tally of number of meteors observed, I wanted to try and distinguish between those which were definitely Perseids and those which were either sporadics, or possibly Kappa-Cygni meteors as this minor shower overlaps with The Perseids.  Over that period I saw 156 meteors! 148 of those were Perseids, 5 of which were fireballs.  I was beside myself with excitement!  At about 2:40am there was a really bright Iridium Flare so I was distracted for 10 minutes or so while I set the camera up for that, but during a test shot I had a really bright Perseid streak through my field of view! By 3:30am the cold finally got to me and I had to come back inside. I drew up a summary table of what I’d observed.

Perseids Observations 12th- 13th August, ‘13

10:30pm - 11:30pm
11:30pm - 12:30am
12:30am - 1:30am
1:30am - 2:30am
2:30am - 3:30am

For about 3 hours of this session there were 2 of us observing, so that gave us a greater chance of spotting more meteors. Had there been 2 of us for the final couple of hours, I’m sure that the rates would have been even higher!

Once I’d collated my numbers, I couldn’t resist a quick look through my photos before going to bed - all 400 of them! I knew I’d caught at least one bright meteor whilst getting ready for my Iridium flare, but at first glance it looked like I may have captured about 4 more too. But I was too tired to look at them all properly, so finally I went to bed around 5am.

After a few hours sleep I looked more carefully at the photos. At times I wasn’t sure whether what I was seeing was a meteor streak or a satellite. Having the camera set on continuous was a real advantage here, because it helped to identify anything that was a satellite or aircraft, as they would appear in previous and subsequent shots, therefore ruling out a meteor.  It turned out that I had a lot of photographs of satellites! However, all together I believe I caught 16 meteors on camera. For a first time observation, I was utterly thrilled with that number!

My favourite Perseid 2013 photo

So what did I learn from this experience, and what will I do differently next time?  In no way do I profess to be an expert meteor spotter after only one meteor shower under my belt. But I did learn a few things which I think it may helpful to share (and I’m very happy to be corrected if you think any of this is wrong).

  • Dress appropriately - it’s an obvious one, but even in the middle of summer it can get really cold and damp when you’ve been sitting outside for 5 hours during the night. So dress warmly, use a blanket and keep a supply of hot drinks on hand.
  • Use a comfortable chair - another really obvious one, but if you are comfortable in a chair that tilts backwards, such as a sun lounger, you are less likely to strain your neck and will enjoy the whole experience more.
  • Have realistic ideas about what to expect - in the run up to any of the principal meteor showers, the press is full of quotes about the Zenith Hourly Rate (ZHR). For example, the often quoted ZHR for The Perseids is around 100 per hour. This does not mean that you will see 100 meteors per hour.  The ZHR is the number of meteors you would theoretically expect to see from a very dark sky site if the radiant of the shower was at the zenith, i.e., the point directly above your head.  It is rarely the case that the radiant of any meteor shower is directly overhead at any point during the night, so straight away you can probably cut that number down by at least a third.  Any light or atmospheric pollution near to the horizon will reduce your chances of seeing any meteors which are low in the sky, so you can cut this value down again. The actual rates seen will also depend on the phase of the Moon; a bright Moon in the sky will greatly reduce your chance of seeing fainter meteors. So let’s assume you are in a good dark sky site, the Moon is in the right phase, and the radiant is just above the horizon; even with the best will in the world you can still only see half of your sky at once. As great as it would be, we do not have eyes in the back of our heads, so once again cut the predicted rate in half.  So for The Perseids, a more realistic rate to expect would be around 30 per hour.  Meteors can also be detected by radio so you can actually listen to meteors as they enter our atmosphere. In the run up to this shower I was hearing stories of rates of up to 300 per hour being quoted by the media.  Radio will pick up meteor events that are not visible to the naked eye in addition to those which are, so don’t be misled by these values.  It is great fun to listen to meteors, but do not expect to see as many as the radio equipment can detect.  Rates always pick up after midnight, so expect to see more meteors if you observe in the early hours of the morning. Don’t forget to allow your eyes to become dark adapted too (it takes about 20 minutes for our eyes to fully dark adapt), and don’t blow it by looking at your mobile phone or camera LCD screen. If you have to use your screen then make sure the brightness is turned right down.
  • Don’t look towards the radiant - during a meteor shower, the meteors appear to originate from a particular part of the sky, but they actually appear anywhere in the sky. By looking directly towards the radiant, you may actually decrease your chances of seeing meteors.  Chose a point at least 45 degrees away from the radiant, and if there are 2 of you, look in different directions to increase your chances of seeing more meteors.
  • Be prepared - I wanted to keep an accurate record of what I was seeing, so I had a notepad and clipboard with me.  I used a new page for each hour of observing, and used one side of the page to record Perseids, the other side to record others.  Turning the pages and trying to write the time at the top of each page in the dark was a nuisance, especially when my red light torch fell to pieces and I kept dropping my pen! Then I wouldn’t remember if I had retracted the pen nib or not in between recordings, and ended up having to rely on counting score lines in the paper on one page.  Next time I will print out sheets for each hour, divide them in half, use a pencil rather than a pen, and attach it to the clipboard with a piece of string. And wear my red torch around my neck so I don’t manage to break it again!
  • Keep the camera pointing the same way - last time I was constantly moving my camera around and pointing at different parts of the sky every 15 minutes or so, and it became a real distraction from observing. A whole sky camera would be great in this situation, but sadly I can’t afford one of those! So next time, I will chose a direction which is at least 45 degrees away from the radiant and at a 45 degree angle (advice I’ve read elsewhere), set the camera to continuous and just leave it there for the duration.  This should not only give me a better chance of catching more meteors on camera, but it won’t interfere with the visual enjoyment of it as much. If there are 2 of us observing again next time, we will make sure we are pointing at different parts of the sky to increase our chances. You have to expect that the lens will end up covered in dew at least once during the night, especially in winter, so keep a lens cleaning cloth handy to dry it off when necessary.  By being on continuous mode, as I’ve already explained, you stand a better chance of identifying satellites and aircraft and eliminating them from any possible meteor captures.  I found that a setting of 30 seconds exposure at ISO1600 produced good results.  Some of the meteors were very faint, but by altering the levels a little bit afterwards, it was possible to bring the really faint meteors out a bit more.
I’m already looking forward to some more observing during the rest of the meteor showers this year. The Moon phase isn’t favourable for the main ones, but if it is clear, I’ll certainly be outside again, with my blanket and a flask of hot chocolate! 

More of my Perseid photos here:  

What Are Iridium Flares?

What are Iridium Flares?

Recently I’ve become obsessed with observing and photographing Iridium flares.  A lot of people have asked me what they are. I knew the basics, but this question led me to find out more about them.
Basically, an Iridium flare is when the sun catches the reflective panel on an Iridium satellite, and this causes it to glint in the sky.  The flare can last anywhere from 5 to 20 seconds and may as bright as magnitude -8; that is 30 times brighter than Venus! Each satellite has 3 reflective panels, each called a main mission antenna, which are arranged in a kind of triangular structure and angled at 40o. Each panel measures just 188cm x 86cm x 4cm but they are highly reflective aluminium plates which have been treated with silver coated Teflon. 

Structure of an Iridium Satellite:

(Image: www.satobs.org)

When in orbit, the satellites are in a vertical position. When one of the main mission antennae catches the Sun, it produces a bright but small reflection which projects onto the surface of the Earth. This reflection is only 10s of kilometres in size, so this means that Iridium Flares are very location specific.

So what are these Iridium satellites? They are a network of 66 active satellites which are used for worldwide voice and data communication from satellite phones.  This means the network of satellites needs to provide full global coverage, so their orbits cover all parts of the Earth. This is called the Iridium Satellite Constellation” (ISC).  The ISC launched in 1998 with financial backing from Motorola.  Unfortunately they went bankrupt 9 months later, partly due to the huge costs involved with getting all 66 satellites functional before the satellite phone network could properly function.  Another factor was that the handsets were very bulky and would only work outside, therefore the popularity of mobile telephones overtook satellite phones.  At one point it was looking like the satellites would have to be de-orbited. Fortunately, the scheme was restarted in 2001 by Iridium Satellite LCC, who later merged with GHQ in 2009 to form Iridium Communications Ltd.  The system was largely used by the US Department of Defence.

The Iridium Satellite Constellation was originally planned to consist of 77 satellites, the atomic number of Iridium. However, it actually consists of 66 satellites, in 6 polar low-Earth orbital planes.  All satellite calls are routed via space. As well as communicating with the hand-held devices, each satellite is in communication with up to 4 adjacent satellites. This effectively forms a mesh network.  In addition there are 4 Earth base stations, which are in communication with any satellites within range.  They can only communicate if there is clear sky between the device and the satellite, therefore they don’t work well indoors or in areas of dense forest. 

Iridium are currently developing a second generation network, called Iridium NEXT. This will consist of 66 new satellites plus 6 spare in orbit and 9 spare on the ground. These are due to launch in 2015 via Space X Falcon 9 launchers.  Each satellite will also contain cameras and additional sensors.  The current ISC will remain in service until the NEXT network is fully functional, estimated to be during the 2020s.

There are currently over 8,000 satellites in orbit, around 3,000 of which are defunct.  With such high numbers in orbit it’s hardly surprising that on 10th February, 2009 there was a collision between Iridium 33 and Kosmos 2251. They collided at a speed of over 26,000 miles per hour!

So how can you observe an Iridium Flare? There are many flare prediction programs available, either online or in the form of smart phone apps. I use either www.heavensabove.com or the SatTrack app.  As they are so location specific, it is necessary to first input your exact latitude and longitude.  Then you will see a list of upcoming flares, along with their predicted magnitude and exact location in the sky.  If you are very lucky, you may get multiple satellites glinting at once, producing a double or even a triple flare.  There are some Iridium satellites which are out of commission and which are tumbling; flares from these cannot be predicted. The angle of the flare is dependant on the orbit of that particular satellite. If you want to photograph an Iridium Flare, you need to be organised. First of all, I manually focus my camera on a bright star or planet. Then using a remote shutter cable I start the exposure between 10 - 30 seconds before the peak of the flare (depending on the magnitude) and continue to expose for the same length of time after the peak. This will give you a really nice, symmetrical flare on your photo.  The result will depend on how dark the sky is at the time of the flare and will be affected by moonlight, etc. It took me quite a few attempts before getting decent results but it was worth persevering!

Some Iridium Flare Photos by Mary