Noctilucent Cloud triangulation
Noctilucent Cloud triangulation |
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Photography for triangulation of an NLC display
The method of photography described in the section "How to Observe NLC" is the standard method of producing aesthetically pleasing images of a NLC display. To produce scientifically useful images a more rigorous set of procedures must be followed and adhered to. These images can then be used to triangulate the altitude of NLCs.
Firstly the camera must be positioned on a sturdy tripod. Theoretically the camera should be mounted in a fixed position at all times. As this is really impractical (however, hopefully in the future a few observers could try this) then the position of the camera mounting on the ground should be noted and marked for future reference to maintain consistency in results. A 28mm lens should be utilised. This will provide a large field of view, approximately 65x46°, and so should cover a large enough area of sky to allow many reference stars to be included in the final image. A 28 mm lens offers a maximum exposure time of 18 seconds according to the revised formula of 500/f (f = lens focal length) and therefore offers longer exposure times before stars start to trail on the final image, which will allow fainter stars to be included in the final image for more accurate triangulation. It is also worth noting that exposures of this magnitude will be infrequently used as most NLC can exhibit visible movement over exposures of this time scale. This lens should be used at all times so as to maintain observational consistency which will reduce errors on the final results.
It is then necessary to position the camera to photograph the NLC display. While positioning the camera it is imperative that several bright stars (or even the moon or bright planets, if applicable) are included in the field of view. Try to place these stars as close to the centre of the field of view of the camera as possible without missing some of the NLC display. This will help to reduce distortion of the star field caused by the wide angle (28mm) lens. These stars act as "reference stars" which will allow the triangulation of the display. Alpha Aurigae (Capella) is probably the most prominent star in the sky during the NLC season, and is visible on many images of NLC. Placing Alpha Aurigae near to the centre of the camera field of view will also include many other reference stars in the camera field of view. As can be seen from the reference star table several reference stars are visible in the sky. The table shows the visibility of the stars at 0000UT (0100BST) at 54°N for different times during the NLC season. As well as Alpha Auriage, Gadsden and Parviainen (1995) recommend Gamma Andromedae (in the eastern portion of the sky) and Alpha Bootis (Arcturus) (in the western portion of the sky) as suitable reference stars should the NLC be positioned in these areas of the sky. Another procedure Gadsden and Parviainen advocate is the use of an overexposed image of the NLC to aid in determining and visualising reference stars. This procedure should be implemented during any NLC observing session and should be noted accordingly in the observation records for the display.
Once the procedure of alignment is completed then the time of exposure of the image must be accurately recorded. Photographs should be taken every 15 minutes after the start of the observing session on the hour, 15, 30 and 45 minutes past the hour to an accuracy of 15 SECONDS, as for visual observations. This procedure must be rigorously adhered to. Any deviation from the time should be clearly noted as an error in the time of exposure can seriously affect the accuracy of the triangulation.
Each image should be clearly recorded in the observing notes with its exposure number (so that it can be recognised in its developed form) as well as the (accurate) time the exposure was taken, the reference stars which should be visible on the final image and an approximate measurement of the azimuth the camera is aimed towards.
If all observers always follow these simple rules then when several observers have taken images of the same display at the same time from two widely seperated locations the height of the NLC can be calculated. If this situation occurs then the individual observers will be contacted.
The reference star table shows the reference stars, tabulated by azimuth, which are visible throughout the NLC season (in the Northern hemisphere) and should be visible on images of NLC. These stars will be used to calculate the altitude of the NLC as explained above. The table only shows bright stars > = 3 magnitude, with the vast majority being > = 2 magnitudes. These stars are definitely visible on exposures of NLC. The altitude range of the stars is 5 - +30° for a latitude of 54°N, hence these stars will be seen at lower or higher altitudes depending upon your latitude but should all be visible from the UK and Northern Europe. It is worth noting that all of these stars are circumpolar from the UK.
A quick rundown of the main procedures of triangulation
- The camera must be mounted on a sturdy tripod.
- When photographing a display several bright stars must be included in the field of view.
- Alpha Aurigae (Capella), Gamma Andromedae and Alpha Bootis (Arcturus) are excellent reference stars.
- During a display one image should be purposely overexposed to record reference stars. This image should be recorded in the observing notes.
- Photographs must be taken on the hour, 15, 30 and 45 minutes past the hour (as with visual observations) to an accuracy of 15 seconds.
- In the observing notes each image should be clearly recorded with exposure number, time, reference stars visible and azimuth of camera, along with the other exposure details.
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