Understanding the AllSky Camera
Discover how the camera works and why continuous observation can reveal things a conventional observing session may miss.
Explore the AllSky camera →LEARN & EXPLORE
What Is a Keogram?
A keogram turns a whole night of sky observations into a single image, allowing us to see how conditions changed as the hours passed above Billingborough.
A DIFFERENT WAY TO SEE THE SKY
Turning a night into one image
An AllSky camera can take thousands of images during a single night. Looking at those images one at a time can tell us what the sky looked like at particular moments, but it makes it difficult to see the whole night at once.
A keogram provides another way of looking at the same observations. It takes a narrow slice through each successive image and places those slices side by side, creating a visual timeline of the night.
The result is an image that represents the changing sky from one side of the night to the other.
A keogram compresses time into a single image.
Instead of viewing thousands of separate photographs, we can use one image to see how the sky changed throughout the observing period.
A REAL OBSERVATION FROM BILLINGBOROUGH
One night, seen from beginning to end
This keogram was produced automatically from observations made by the Billingborough Observatory AllSky camera.
Latest keogram from the Billingborough Observatory AllSky camera.
HOW IT IS MADE
From thousands of images to one keogram
During the night, the AllSky camera continuously records images of the sky. Each image captures a different moment in time.
To create a keogram, a narrow vertical section is taken from each image. These sections are then arranged in chronological order.
The first slice represents an earlier point in the observation. As we move across the finished image, we move forward through time.
The keogram therefore contains information about both the sky and the passage of time.
Every vertical slice represents a different moment.
Reading across the image allows us to follow how the appearance of the sky changed during the night.
READING A KEOGRAM
What are we looking at?
A keogram can initially look rather strange because it does not show the sky in the same way as an ordinary photograph. It is better thought of as a timeline of the sky.
The horizontal direction represents the progression of time. The vertical direction represents different positions within the slice taken from the AllSky images.
Features that remain visible for a long period can therefore appear as structures stretching across much of the image. Features that appear only briefly may produce much shorter features.
This makes the keogram particularly useful for looking at changes that happened gradually throughout the night.
WATCHING THE WEATHER
Clouds become part of the record
One of the most useful things a keogram can show is changing cloud cover.
A conventional astronomical photograph may show clouds at one particular moment. A keogram can show how those clouds changed throughout the night.
Cloud entering the field of view, periods of clearer sky and changing cloud structure can all leave visible patterns in the resulting image.
This makes the keogram useful not only as an astronomical observation, but also as a record of the conditions under which other observations were made.
The sky itself becomes part of the observing record.
Knowing whether the sky was clear, cloudy or changing helps us understand what could and could not have been observed during a particular period.
MORE THAN CLOUDS
What else can a keogram reveal?
A keogram can preserve evidence of many changes taking place during an observation.
Stars, bright astronomical objects, changing illumination and other features of the night sky can appear as structures within the image.
Because the observations are arranged according to time, the keogram can also help us identify when a particular feature appeared or disappeared.
It is therefore a useful companion to the individual images from which it was created.
WHY WE MAKE THEM
A permanent record of the night
The value of a keogram is not simply that it produces an interesting picture. It provides a compact visual record of a long period of observation.
At Billingborough Observatory, the AllSky camera can continue recording when nobody is standing beside the telescope. The resulting observations can then be examined later.
A keogram gives us a way of seeing that entire period at a glance and deciding where something interesting happened.
One image can tell us where to look more closely.
Once an interesting feature has been identified in a keogram, the original images from that period can be examined in much greater detail.
KEEP EXPLORING
Explore the observations
The keogram is just one way of turning the AllSky camera's continuous observations into something we can investigate.
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