From chi and h
Light pollution
| Physical parameters
|
| altitude:
| ≈5 km
|
| temperature:
| −15 °C
|
| air pressure:
| 540 hPa
|
|
| Image parameters
|
| camera:
| Canon EOS 300D
|
| detector:
| 22 × 15 mm
|
| focal length:
| 135 mm
|
| field of view:
| 6.5 × 10°
|
| aperture:
| f/2.8
|
| ISO:
| 400
|
| exposure:
| 30 s
|
| locations:
| Edinburgh
Earlyburn
|
|
|
|
City dwellers often don't know what they are missing, because the street lights brighten up their sky so much. In the UK virtually all street lights are orange, as they have sodium vapour instead of the more white mercury vapour. Country folk enjoy a much darker sky, even when it is brightened up by a Half Moon as in this image. The image shows the difference between Edinburgh and Earlyburn (about 20 km south of Edinburgh).
Noctilucent cloud (NLC)
 Noctilucent cloud 2009-06-17/18. |
| Physical parameters
|
| altitude:
| 84 km
|
| temperature:
| −135 °C
|
| air pressure:
| 0.003 hPa
|
| water vapour:
| 5 ppm
|
|
| Image parameters (2009)
|
| camera:
| Canon EOS 300D
|
| detector:
| 22 × 15 mm
|
| focal length:
| 18 mm
|
| field of view:
| 65 × 45°
|
| aperture:
| f/3.5
|
| ISO:
| 200
|
| exposure:
| 2 s
|
| location:
| Edinburgh
|
|
 Noctilucent cloud 2006-07-12/13. |
| Image parameters (2006)
|
| camera:
| Canon EOS 300D
|
| detector:
| 22 × 15 mm
|
| focal length:
| 18 mm
|
| field of view:
| 65 × 45°
|
| aperture:
| f/3.5
|
| ISO:
| 400
|
| exposure:
| 4 s
|
| location:
| Edinburgh
|
|
Regular clouds of deposited (frozen) water vapour occur anywhere between ground level (where they are called fog) and the tropopause at roughly 10 km altitude. At the tropopause, the temperature trend reverses with air being warmer both above and below this layer.
Similarly, there is a minimum in temperature in the mesopause at 85 km altitude. It is much colder there, and the air is much thinner as well. It is not quite clear why small amounts of water vapour come to exist at this altitude. Nor is it quite clear what seed particles are responsible for the vapour's deposition into ice crystals. What is clear, is that this can occur only when this coldest layer of air reaches a seasonal low in temperature far below the annual average of −85 °C. Contrary to intuition, this happens in summer and not in winter. Some years it may not get cold enough at all.
The clouds that form some nights of most summers are so thin that they cannot be seen in a daytime sky. They could also not be seen without sunlight falling on them. These nocticlucent clouds can then only be seen from certain geographical latitudes where the ground level is in twilight while the mesosphere toward the North is still bathed in sunlight. The clouds are then seen by the forward scattering of sunlight by the ice crystals. The stripy or stringy appearance of these clouds is due to thow the crystals form and dissolve. Nucleation particles at higher level drift down to an altitude where the water vapour can deposit. The forming ice crystals fall while they grow, and soon they have fallen through the mesosphere into warmer air where they sublimate again. Combine this with high-speed winds and you find small cloudlets stretched to the extreme in a horizontal direction.
- The technique for taking pictures of NLC is similar to photography of landscape and large phenomena like rainbows or halos; a wide field of view is preferred. The time to observe is strictly limited to the interval when the Sun is between 4 and 16° below the horizon. This is dusk or dawn, when the brightness of the sky changes rapidly. Use a wide aperture, moderate ISO, and exposures between one and 30 s. The longest sensible exposure depends on how deep twilight it is. Very bright NLC can be much brighter than that and may require exposures at the shorter end of the range. A very dark site is not necessary; during much of twilight the light pollution has only a small effect.
Aurora borealis
 Aurora borealis 2005-01-21/22. |
| Physical parameters
|
| altitude:
| 100 km
|
| temperature:
| −80 °C
|
| air pressure:
| 0.0003 hPa
|
|
| Image parameters
|
| camera:
| Canon EOS 300D
|
| detector:
| 22 × 15 mm
|
| focal length:
| 18 mm
|
| field of view:
| 65 × 45°
|
| aperture:
| f/3.5
|
| ISO:
| 1600
|
| exposure:
| 5 s
|
| location:
| Roslin
|
|
Aurora (aurora borealis in the northern hemisphere, aurora australis in the southern hemisphere) is a bright glow in the atmosphere in a wide range of altitude above the mesosphere, 90 to 120 km. The light is emitted by particles of air after they have been excited or ionised by collisions that are ultimately caused by the continuous stream of ionised particles from the Sun known as the solar wind. Due to the way the magnetic fields of Sun and Earth interact, the aurora is limited to an oval region of the Earth some distance from the magnetic poles. Related to solar activity, the intensity and spread of this aurora oval can temporarily increase. Aurora can then be seen further from the magnetic pole, e.g. in Britain and the mainland United States, as opposed to Scandinavia and Canada.
The intensity of the oval changes with the time of day, and the best time to observe aurora is from midnight onwards. But this also depends very much on the timing of activity in the solar wind.
Relatively quiet aurora can be an extended green glow changing very little for an hour or more. More active aurora will exhibit vertical rays and curtains of such rays. Individual rays can form and disappear in a matter of seconds or less, and the curtains can move on times scales of between seconds and minutes.
- The technique for taking pictures of the aurora has similarities to photography of noctilucent clouds. Use a wide field of view. The best time of day to observe (for Britain and places of similar geomagnetic latitude) are the hours around and after midnight. But it is more important that the aurora be at its most active. There are forecasts and near real-time measurements of magnetic disturbance online. As aurora at its best changes very rapidly, use high ISO rating and widest aperture, so that the exposure time can be as short as possible. Depending on the brightness of the display, exposures will be between a few and a few tens of seconds.
Artificial satellites
 Iridium flare 2005-01-13. |
| Physical parameters
|
| altitude:
| 780 km
|
| velocity:
| 7.5 km/s
|
| revolution period:
| 100 min
|
| flare magnitude:
| −2 to −8
|
|
| Image parameters
|
| camera:
| Canon EOS 300D
|
| detector:
| 22 × 15 mm
|
| focal length:
| 50 mm
|
| field of view:
| 25 × 18°
|
| aperture:
| f/2.8
|
| ISO:
| 400
|
| exposure:
| 30 s
|
| location:
| Edinburgh
|
|
A fleet of about 70 satellites orbit the Earth to support satellite phones. Each of these has three sizable, roughly flat, shiny aerials. The orientation of these antennas is known and it can be predicted which points on Earth receive sunlight reflected by them. The illuminated spots move rapidly across the Earth, and an observer in a suitable location will see only a brief flare lasting on the order of 10 s. The brightest flares can reach −8 mag.
- Taking pictures of satellites is made difficult by the rapid movement of these objects across the sky. The satellite illuminates a given pixel in the camera only for a very short time; even a bright satellite has at best only a moderate impact on the pixels along its path. In addition, satellite passes occur mostly during twilight, when the sky in general limits how high the ISO rating and how long the exposure can be. The fast movement of satellites drives us toward high ISO ratings, but the brightness of the twilight sky then limits how short the exposure must be and how short an arc of the satellite track can be recorded. An option is to take several frames in rapid succession and to layer them into a single image making each output pixel as bright as its maximum brightness across the sequence of frames ("brighten only" layers in the Gimp). All pieces of satellite track will then come through at full brightness, while the unchanging sky background will come through roughly at its average brightness. In general one should use a wide field to catch a long arc of the satellite's path, perhaps including its ingress into or egress from the Earth's shadow. An Iridium flare covers only a shorter path, but the camera should still provide 20 or 30° of field.
