Noctilucent Cloud (NLC) Camera

The components of the NLC Camera:

• a Canon EOS 550D digital Single Lens Reflex (SLR) camera body
• Tamron AF 17-50mm f/2.8 Di-II LD Aspherical lens

These have been chosen to allow:

• good quality images to be captured in low light levels
• a wide region of the sky to be imaged
• image capture to be controlled by an acquisition computer

The choice of lens was not based on its zoom capability but on:

• its widest angle capability, which gives a field of view of 67.9° by 45.3°
• its large maximum aperture size (f/2.8), which minimises the required exposure time
• its good image quality
• its cost relative to a fixed focal length lens with comparable specifications

The only requirement for the camera body was that it could be controlled from a computer using gphoto software. This removes the reliance on the camera’s free-running internal clock. The image capture time embedded within the image file is overwritten with a value generated by the computer, whose clock is synchronised using Network Time Protocol (NTP). The time stamp refers to the start of the image capture period, which can last for several seconds in twilight conditions.

The camera is operated using its aperture priority mode. This allows the the widest possible aperture setting (f/2.8) to be selected, whilst the the ISO speed and exposure time adjust automatically to the level of light available. In order to maintain image quality (at the expense of an increased exposure time), the ISO speed is not permitted to drop below 800.

The camera is operated from within the Receive Cabin of the Chilbolton Atmospheric Observatory (CAO) because:

• the surrounding landscape has a limited impact on observations at low elevation angles
• the camera can be operated from within a building, removing the need for a weather-proof enclosure
• the Receive Cabin offers a view towards an azimuth angle of 12°, which is close to the optimal angle of 0° (i.e. towards North)

The camera is placed as close as possible to the window. Nevertheless, it needs to be enclosed within a light-proof box in order to prevent it from seeing reflections from the window of the lights on other equipment within the Receive Cabin.

The coordinates of the camera (51.145168°N, -1.4397500°E) have been determined using a Global Positioning System (GPS) receiver. The camera is 4.3 m above ground level, which has been estimated (from an Ordnance Survey map) to be 84 m above mean sea level (amsl). This gives an approximate altitude of 88 m amsl for the camera.

An elevation angle of approximately 18° has been chosen to capture as much of the sky as possible, whilst covering some of the ground for reference purposes. The camera’s pointing direction (in terms of both azimuth and elevation angles) is not fixed and will change occasionally when the camera is moved for maintenance purposes. The actual pointing direction can be estimated by analysing the position of stars that pass through the field of view.

The relationships between x/y pixel coordinates and azimuth/elevation angles are not linear. This is mainly the result of perspective distortion, which is a consequence of the camera’s pointing direction being elevated above the horizontal. There is also some mild lens distortion, but this is much less significant.

Perspective distortion is a consequence of viewing geometry. If parallel lines are viewed at any angle other than at 90° to the plane in which they lie, they will appear to converge. The shallower the viewing angle, the more obvious the effect becomes. Everyone will be familiar with this effect on horizontal lines, e.g. from looking down a long corridor. Note that although the viewing angle to the floor, in the illustration below, is reduced by kneeling down, the viewing angle to the ceiling is increased. Consequently the effect of perspective distortion on the ceiling is increased rather than reduced, as it is for the floor.

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It might not be immediately obvious that vertical lines are also affected for any viewing angle than a horizontal one. For a photograph, the x coordinate of the convergence point will be in the middle of the image. The y coordinate will be above it for an upward viewing angle or below it for an downward viewing angle (in this context I am using the convention that the y coordinate increases from the bottom to the top of the image; the opposite convention is typically used for digital images). The convergence point will typically be outside of the field of view.

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Perspective distortion affects wide angle and telephoto images equally over the same field of view. Since wide angle images cover a larger field of view, the effect is more pronounced across the image as a whole. The NLC camera captures a comparably large field of view (67.9° by 45.3°, corresponding to a 35 mm equivalent focal length of 26.7 mm) as the camera used to take the photographs shown in the examples above (74.5° by 49.7°, corresponding to a 35 mm equivalent focal length of 23.7 mm).

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Although there are no actual vertical lines within the NLC camera’s field of view, implied vertical lines (the black lines in the illustration above) can be constructed by joining points with the same azimuth angle at different elevation angles. The small circles show the positions of 7 stars (at 5 minute intervals) as they pass through the NLC camera’s field of view. Their azimuth and elevation angles can be calculated for the time of each photograph. The black circles show the positions of the stars when they pass through azimuth angles of between -15° and +45° in 5° increments (by interpolating between measured positions to either side, if necessary). For each azimuth angle, the black line joins the circles with the highest and lowest elevation angles. Some points to note:

• only a vertical line at the centre of the field of view (i.e. at x coordinate 449.5 – this analysis was based on images scaled down to a width of 900 pixels) will appear vertical in the image
• the effect of perspective distortion becomes increasingly pronounced towards the sides of the image
• the top of the image covers a wider azimuth extent that the bottom
• the relationships between x/y pixel coordinates and azimuth/elevation angles are consequently not linear

As an aisde, the covergence point (449.5,-1561.8) was found by averaging the locations of the intersections between each pair of lines. The x coordinate corresponds to the centre of the centre of the image and the y coordinate to a point 2.6 times the image height above it.

The release of photos from this camera has been delayed whilst an NCAS metadata standard for images (NCAS-IMAGE) has been developed. The first version of the standard was published towards the end of 2022. The photos will be made available in the near future.

For the time being, two time-lapse videos based on the photos have been released. These have been published under a Creative Commons Attribution License. Anyone may download, view, and redistribute copies as long as they acknowledge the source.

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• Video start datetime: 2020-06-21 20:00 UTC
• Description: A time-lapse video showing Noctilucent Clouds (NLCs) seen from southern England (51.15°N,-1.44°E) during the night of 21st/22nd June 2020. In this video there is a mild display of NLCs during the dusk followed by a much more impressive display during the dawn. Note that British Summer Time (BST) is one hour ahead of Coordinated Universal Time (UTC). The solar elevation angles do not take account of atmospheric refraction, which is only noticeable when the sun is close to the horizon.
• Follow this link in order to download a copy of the video

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• Video start datetime: 2020-07-11 00:00 UTC
• Description: A time-lapse video showing Noctilucent Clouds (NLCs) seen from southern England (51.15°N,-1.44°E) during the dawn of 11th July 2020. Note that British Summer Time (BST) is one hour ahead of Coordinated Universal Time (UTC). The solar elevation angles do not take account of atmospheric refraction, which is only noticeable when the sun is close to the horizon. The brightest star seen in the video is Capella, which is in the constellation Auriga. It starts near the bottom-centre and moves in an arc towards the right and upwards. The comet NEOWISE can be seen following a similar path from approximately 01:20 UTC.
• Follow this link in order to download a copy of the video