Partial Eclipse From Afar

Back in December I was approached by the BBC about launching a high altitude weather balloon flight during the forthcoming solar eclipse which was total over the Faroe Islands but approx 85-90% over much of the UK.

After some discussion we planned the flight to be from Leicester Racecourse where the BBC were holding a event for the public, with mainly school groups in the morning and open to everyone in the evening. So no trip to the Faroes for me!

The aim for the launch was to get photographs of the eclipse from above the clouds (there are always clouds during a UK eclipse!). To see the moon’s shadow would mean launching much further north, especially as the forecast winds meant that we had to limit the maximum altitude to about 30km. We did though hope to get some filtered shots of the sun, to reveal the shape of the moon crossing.

path720

For most balloon flights, it’s easy enough to delay the flight if the wind predictions compromise the flight or if the launch winds are too strong to launch in. This time though we had one chance and one chance only! Even delaying for an hour would mean that there’s little point in launching. Flight predictions start coming in about 6 days before launch, though they usually do not start to get accurate until about 3 days prior. It’s always tempting to check those initial, unreliable forecasts and sure enough I did so. They looked OK, however as later predictions came in, the forecasts got worse, with the flight path going over London (bad) to land in Kent (a small target at that distance). Worse, if the balloon burst early (which happens) the flight would land worryingly close to Gatwick.

I ran a few different scenarios, using larger or smaller balloons, each with more or less gas, and found no good flight paths at all. A short flight would miss London but land in the rather built up area west of the M25 (and too close to Heathrow). It became obvious that, if we were to launch from Leicester, the only safe flight would be a very slow ascent, to go south from Leicester, missing London, then turn east over the English Channel to make landfall near Dieppe! We seriously considered doing just that, with a chase team setting off early to be in France ready for the landing, but logistically this was always going to be a pain (especially getting the video files back to Leicester in time). We also looked at possibly launching from somewhere else.

Showing Tim just how crap a cheap Chinese aerial really is (answer: very)

Fortunately though, predictions continued to move the flight westward. About 2 days prior to launch we could see the possibility of doing a “normal” flight to about 30km, landing not that far south of Leicester. The only remaining worry was overshooting the landing spot and getting close to Luton Airport, so to avoid this we aimed for a point SW of Milton Keynes and used a Totex balloon (chosen for their reliability in bursting when they are supposed to!). So, finally and to everyone’s considerable relief, we had a nice safe flight plan that would work. As a bonus, the forecast ground winds were very low, so it looked like we would have an easy launch.

As this was a photographic flight, the payload was designed to allow for multiple cameras – both video cameras and Pi cameras connected to A+ boards and the PITS+ tracker board. We used 4 Replay XD Prime X cameras, each pointing in a different direction:

Downward-facing wide-angle
Upward-facing moderate angle
Outwards-facing wide-angle (moderate downward angle about 22 degrees)
Outwards-facing narrow-angle (canted upward about 24 degrees) with solar filter

These are particularly nice cameras to use for HAB:

Field of view can be set to 70, 110 or 140 degrees
Can do full HD (1080p) at 50/60 fps
Easily mounted using plastic clips and strong 3M sticky pads

We mounted all 4 in a wide payload box (the idea being to reduce spin by increasing angular mass when compared to a normal payload box. We placed 2 of the ReplayXD cameras at the ends of the box, alongside the Pi/PITS trackers, with battery packs a little way inboard. The up/down cameras were placed more centrally for better views. Here’s the complete payload prior to launch:

With the payload built, we checked the live images (working) and backup tracker (no GPS lock). With a box full of electronics it was possible that the GPS was struggling with electrical noise, so we tidied the cables away from the GPS aerial, and added an extra tracker externally in its own ball. Once that had a lock it was time to launch. We’d been asked to let some schoolkids launch, so once Tim had posed with them …

… before launching …

Anthony then reported that the live images weren’t working. Nothing we could do of course, as the flight was in the air and beyond reach! It was only days later that I discovered that a recent change meant that, depending on timing, it was possible for the code to delete a converted image just before starting to send it. Once this happens, it doesn’t send that image and thus doesn’t trigger the conversion of the next image, so no more images are sent until a reboot. However it didn’t stop the images being taken so we did get plenty of images once the flight was recovered.

I didn’t get much time to think about what was wrong, or worry about it, because I was soon needed for a live TV appearance. We’d practiced the interview (live TV is rehearsed a lot!) but we had to change the plan slightly because of the lack of live images, and decided not to show any images (we had some from other flights). However nobody told the interviewer so she still asked to see an image and I had to swiftly load up a suitable image on the laptop! You can see the entire program on iPlayer or just my section.

With that out of the way, it was time to watch the eclipse itself! Here’s a short timelapse taken by Tim, using my 600mm mirror lens with a sheet of solar film taped over the end:

Then came another TV spot:

After that, it was time to check up on the flight. Here’s the map on the big screen during my interview …

and here it is on a rather smaller screen inside …

As I mentioned, we aimed for a “standard” flight and the exact targets were a 5m/s ascent rate, 5m/s descent rate and a burst a little past 30km. I knew that we’d be slightly off because of the extra tracker added at the last moment, and this reduced the ascent rate slightly and moved the landing point a few miles south. The balloon popped pretty much when expected, at 30,750 metres and the predicted landing spot was just south of Leighton Buzzard. Here’s the burst video which we retrieved later:

Ironically, having launched from a racecourse devoid of horses, the flight landed in a random field with several of them in attendance!

Here’s a view of the landing from the upward-facing camera. Keep watching till the end 🙂

About lunchtime, I was collared by the BBC Radio Leicester team, who wanted me to do a live interview for them. This was actually much nicer than the TV stuff – instead of having to cut what I said down to as short as possible, I was allowed to rabbit on and on! Click here for the 5-minute slot.

Meanwhile, our chase team (Anthony Stirk and David Bowkis) were busy collecting and returning the payload. The landing spot was in a field near to a road, so their task was very easy! Once they got back to Leicester, we started copying all the SD cards from the cameras, and I booted up the Raspberry Pi boards to copy the still images from those.

With the SD cards copied, Tim went off to see the BBC production guys to help choose what footage would be shown in the evening live TV show. By now we were all exhausted and hungry, so we got some food at the canteen and sat outside in the sun to relax a bit before the evening show started. As before, there were lots of rehearsals and we must have cut the slot down to about half the time of the first take. I don’t know if it was the rehearsals or the lack of rest, but by now I wasn’t at all anxious about appearing on live telly and I was surprisingly relaxed as I waited for us to go live.