How to observe NLC
How to observe NLC |
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How to observe NLC
Observing Noctilucent Clouds (NLC) is very simple and very rewarding. A complete novice, with the help of some observing notes, can venture outside to observe a NLC and produce accurate measurements of the apparition. The beauty of observing NLC is that the techniques learnt by the novice are also the techniques that are used by the veteran NLC observer, hence the techniques do not become more advanced over time, as with some branches of astronomy. The aim of these notes is to explain the techniques of how to observe NLC and how to correctly record the data obtained.
An example NLC observation report form, showing how all of the elements below are recorded, is illustrated in the section "An example NLC observing report form".
This section shows the general observing details which should be provided at the start of any NLC, or for that matter, astronomical observation.
The name of the observer should be provided. Also to aid with recognition and data analysis the observer code supplied to you should also be entered into the appropriate field on the observing form.
The address, and the location of the observer in latitude and longitude coordinates, should be supplied. It is an advantage to know the latitude and longitude coordinates as many observing groups, as well as astronomical computer programs, require this information. This information can be found from an OS map, or more accurately using a GPS and is vital for NLC height determination. If you observe from a different site to your usual site then this should be noted in the observing notes and the latitude and longitude of this site should be supplied.
The date should be given in the usual astronomical convention of year/month/double date, i.e. observing on the night of Sunday 10th June 2001 would produce a date of 2001/June/10-11 in this date format.
These are the observations which need to be performed for each NLC observed.
To allow correlation of NLC observations and advanced data reduction the time NLC observations are taken is standardised so that information is recorded at 15 minute intervals from the start of the NLC observing session. Therefore observations of an NLC display should be taken on the hour, 15, 30 or 45 minutes past the hour. It is very important that this is adhered to as if all observations are not performed in this manner then correlation between observations will be impossible. This also means that the observing session should also be started on the hour, 15, 30 or 45 minutes past the hour.
As with all astronomical observations time should be given in Universal Time (UT) 24h standard, i.e. British Summer Time minus 1 hour. An example of this is given below.
For example an observation is underway at 12.15am on Monday 11th June 2001. The double date convention described above deals with the fact that the observation has carried on into Monday morning. Using the UT convention the time is therefore recorded as 2315 UT.
There are four fields of basic, but highly important information, which should be recorded at each observation. These are elevation, azimuth, structure and brightness.
The elevation is measured from the horizontal (0° altitude) to the uppermost extent of the NLC display (Figure 2) and is measured in degrees. When visible it is also very useful to measure the elevation of the base of the NLC. There are two methods which can be used to measure elevation: 1) the hand method and 2) an alidade.
| Figure 2 Measuring the elevation of an NLC display |
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When an arm is held outstretched and the fingers of the hands are spread selected portions of the hand and distances between digits subtend a certain angular distance. These angular distances can be utilized as a crude, but very effective tool for measuring angles in the sky. Some approximate distances are given below.
1° = width of the little finger.
2° = width of the thumb.
10° = width of a clenched fist.
22° = distance between the thumb and little finger when the fingers are spread
out.
This is a device used to measure elevations of cloud phenomena more accurately than the hand method and is a common tool of more advanced observers. It is very easy to construct an accurate alidade for just a few pounds. This alidade consists of no more than a protractor mounted on a piece of wood with a plumb bob attached via a rod through the centre of the protractor. A simple plan can be found here.
The alidade is very simple to use. Firstly make sure that the plumb line reads 0°, i.e. the alidade is aligned with the horizon. To perform the measurement sight the lower or uppermost edge of the display along the upper edge of the mounting wood and then read off the measurement on the protractor which is indicated by the plumb line. This measurement is then noted accordingly.
Sometimes it may not be possible to quote the full elevation of the NLC as the base of the display may be lost behind objects obstructing the view of the display or the horizon. In this case only measure the elevation of the uppermost extent of the display and make a note of this fact in the observing records (Figure 3).
| Figure 3 Measuring the uppermost extent of an NLC when its base is obscured |
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This is the left and right hand angular extent of the NLC display (Figure 4). It is measured in degrees, clockwise from north, where north = 000º, east = 090º, south = 180º and west = 270º. Azimuth can be measured quite accurately using a pocket compass.
| Figure 4 Measuring the azimuth of an NLC display |
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Observations of structure are probably the most important non-numerical observations of an NLC that can be made. They describe the overall morphology of the cloud display. There are four major forms of NLC which will be encountered.
| Type I - Veil | A sheet form which shows no discernable structure. It is akin to a glowing fog or mist. It also occurs as a background upon which other forms of NLC are superimposed. |
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| Type II - Bands | These are lines or streaks in the NLC display. They commonly run parallel to each other of cross at very small angles. Two sub-types of this group exist and these should be reported when observed. Type IIa Bands with blurred edges. Type IIb Bands with sharp edges.
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| Type III - Waves | These NLC display a chevron-like structure, which resemble sand ripples, and are very unmistakable. Type IIIa Linear, narrow streaks. Type IIIb Undulating wave-like structure.
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| Type IV - Whirls | Twisted or looping NLC. Type IVa Small angular radii whirls (0.1-0.5°). Type IVb Angular radii of 3-5°. Type IVc Larger scale whirl structures. |
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Many NLC which may be observed in a display do not fall into the categories stated above. In this case these are defined using four extra classifications. These are as follows:
Type S Bright knots or clumps are visible in an NLC.
Type P Billows which cross banded or streaked NLC.
Brightness is measured using a standardised five point scale. It is susceptible to observer bias but is still a useful method in ascertaining the brightness of a display.
1 Very dim NLC. Just visible in the twilight sky.
2 NLC display is clear but with a low surface brightness.
3 NLC display is clear and has a higher surface brightness, standing out clearly against the twilight sky.
4 NLC display is very bright and instantly noticible.
5 NLC extremely bright and illuminating objects in front of it.
When a display of brightness 4 or 5 is seen it is never forgotten!
Photographing NLC is very rewarding, both aesthetically and scientifically. NLC make beautiful photographic targets and are easy to photograph with a basic SLR camera. If a certain degree of scientific rigour is applied to the photography the resulting images will be of great use scientifically as the altitude, velocity and direction of movement of the NLC can be determined using triangulation.
The requirements for photography are a SLR camera (e.g. the Olympus OM series), a 28mm wide angle lens is preferred as this is the lens of choice for the triangulation program. If you only intend to photograph the display for aesthetic reasons, and do not want to take part in triangulation a 50mm lens is more than adequate for most displays. Other requirements are a good, sturdy tripod, a cable release and good quality film. Fuji Sensia or Velvia slide film ISO 200-400 is highly recommended, though Fuji Superia or Reala (colour print) are excellent films. With ISO 200 film and the lens set at f2.8 an adequate exposure time should be in the region of 10 seconds. For every doubling or halving of the film speed a doubling or halving of exposure time results. Bear in mind that this is an average time and that it is best to bracket your exposures according to the brightness of the NLC and the sky. Try not to exceed 15 seconds as movement of the NLC may be visible on the final image.
Using the observation recording sheet record the film details of the film used in the "Exposure details" column. For every exposure record the exposure number, exposure time and the lens and aperture used. The time of exposure should be the same as the time the visual observation was taken. This time should be every 15 minutes past the beginning of the observing session with an accuracy of 15 SECONDS, as for visual observations. The accuracy is very important and any deviation from it must be noted. Also an indication of the azimuth of the exposure should be recorded. The time of the exposure and observation should always be noted.
On certain occasions two or more observers situated at separate locations several kilometres apart may photograph the same display concurrently. This will allow triangulation to be performed and the height of the NLC to be calculated. This is called triangulation. The use of a 28mm lens by all observers at all times will maintain consistency between observations, allowing more accurate triangulation to be achieved. As triangulation is a major aim of the program it is hoped that all observers will photograph NLC displays with the sole aim of triangulation (images which are taken solely for triangulation purposes still make extremely pleasing shots!).
Acknowledgement
The author of the four NLC type images used above is unknown. Full credit is given to the author of these superb images. If anyone knows the name of the author, or you are the author, then please contact me so I can credit the images properly.
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