Creating Keograms

[AstroBee]

I just thought I would share this for the group. I recently installed an All-Sky camera on my balcony to capture daily timelapses. Mainly to monitor the weather but also to capture the occasional meteor. Sadly, most of what I capture in my busy area is aircraft traffic from my nearby major airport. But the more I got into working with the timelapses I discovered keograms. You may be like I was and not know what they are. You've probably seen one before but didn't know that's what they are called. So first up, my first complete 24hr keogram and then an explanation of how they are made.

The keogram is created by taking a single vertical row of pixels from the center of the individual frames that are used to create your timelapse videos. In my case, it is taking a single row of pixels every minute and stacking them into an image with a timeline running along the bottom.
Here's an image taken at 10:58pm local time last night.

Now take a sliver of the pixels from this image and stack it along the timeline at 10:58. The red vertical line actually covers 4 pixels in this sample because a single row of pixels is very narrow and it would be difficult to see the red line if it were that small.


Later in the morning around 2:26am local time you can see the moon has moved a considerable distance and that strip in the center is covering nothing but clear, dark sky.

If you look back at the full keogram you can see at around 2:26am the sky was clear and dark.
The purpose of the keogram is to give you a 24hour snapshot of your weather in a single image. The time span of this particular keogram is from 9:30am to 9:30am and looking at my image you can see the sky was cloudy from around 9:30am yesterday until around 8pm (20:00) when it cleared up. Then, around 5am this morning it started to get cloudy once again.

It's a great tool to use if you are imaging all night unattended. When you finish your observing run you can look at the keogram for the night and see if any time period was obstructed by clouds. Even faint, thin clouds that might not stop an imaging session can be detected so you know to be leery of the data collected during that time period.
As a bonus, since my camera is oriented so that north is at the top, south at the bottom, the keogram takes that sliver of data roughly down the meridian. So you know roughly what time the moon transits the meridian. In this example, shortly after 10pm local time.

Here's a Wiki for Keograms that might explain it better than I did. https://en.wikipedia.org/wiki/Keogram
My timelapse videos only cover sunset to sunrise and here is the video that covers a portion of that 24hr keogram.

You can pause the video at any point in time and compare it to that sliver of time in the keogram!

[Gordon]

I learn something new everyday!!!

[Lady Fraktor]

Cloudy skies lately?
Definitely different.

[AstroBee]

Cloudy skies lately?
Definitely different.

Nothing but cloudy skies lately. Except when that moon is high and bright, then it's clear!

[Graeme1858]

Nice images Greg.

I saw these when I installed the AllSkyEye software and put them in the definitely going to do this some time box.

Good to see your creations. What are the fainter white vertical bandings between 0100 and 0500?

Graeme

[AstroBee]

Thanks Graeme,
Since this is my first keogram I am not 100% sure but my guess is it is either moonlight reflection on the crystal dome of the camera or high, thin clouds.

[AstroBee]

@Graeme1858 I just looked at the video timelapse and it is most likely reflection on the dome from the moonlight.

[JayTee]

Sorry, I'm dense!

I don't get it. What are you trying to achieve that the full frame doesn't provide, and what is the single row of pixels actually seeing?

Trying to understand,

[AstroBee]

Sorry, I'm dense!

I don't get it. What are you trying to achieve that the full frame doesn't provide, and what is the single row of pixels actually seeing?

Trying to understand,

I thought it was obvious but let me try and explain it again.
The camera is taking a photo every 15seconds. so you have 4 images per minute. The individual images are covering a 170° area of the sky, north is at the top, south at the bottom.
Every minute or every 4th photo, a 1 pixel wide sliver is extracted from that photo. In the case of my keograms, this starts at 9:30am local time. The timeline is read from left to right, covering a 24hr timeframe.
That tiny sliver is placed on the far left column of the final Keogram.
The next minute another 1 pixel wide sliver is extracted and placed next to the previous single row of pixels.
The final result is a sliver of pixels representing each minute of the 24 hour period that shows you the current snapshot of your weather for the previous day.
Looking at the final keogram you can see that it was cloudy from 9:30am when it started, there was a brief period of clear-ish skies around 3:30-4pm where the keogram shows it blue, then it got cloudy again.
At around 8pm (20:00) along the timeline you can tell the sky cleared up and was clear until just around 5am when the clouds rolled in again.
Again, between the 7am-8am portion of the timeline on the right you can see that it was sort of clear because that section is blue again. Then around 8am the clouds rolled in again.
The bright "spotlight" in the center of the image is the moon crossing the meridian. Remember, it is only taking a sliver of pixels from the meridian so that is why the moon shows up round instead of a smear across the entire image.

Now, to answer your question, what am I trying to achieve that the full frame doesn't provide?
The individual full frames are a single snapshot in time. So sure, you can watch the timelapse video and pause it at, say, 2am, if you want to know if the sky was clear. But the keogram in a single image shows you the weather for a full 24 hour period.
At a quick glance of this keogram I can tell the weather was clear from 8pm till 5am the following morning with the moon transiting the meridian between 10 and 11pm.
Once you understand how to read them, they are actually very helpful in understanding your weather on a night you were perhaps imaging unattended.

[hosshead]

dear Mr. Bee,

Way cool.
This is also very valuable data that the weather folks can use to quantify local weather over time from remote imaging locations.
It can record and record and then you can create a concise "weather sandwich".

This is pretty neat.

[hosshead]

p. s. and it kinda looks like Saturn rings.

[StarHugger]

That's neat, reminds me of how shg data looks rite out of the camera,

Thanks for sharing it...Indeed.

[AstroBee]

Today's keogram, covering a timeframe from 9:30am 01/25 to 9:30am 01/26 local times. A lot less clouds in this one. No, that's not two moons! That bright light on the lower left between 10a and 12p is the Sun.
The Sun is really low in the winter time, but remember, that's not the horizon because the camera is only shooting approx 170°. Between 13:00 and 19:30 you can see the occasional streak of clouds passing through. We had clear skies from 19:30 until around 5:50am when some light clouds appeared till just after 8am this morning. Hopefully this explanation compared to the original helps folks understand how to read a keogram.

[JayTee]

Thank you for the further explanation.

I thought it was obvious

but ouch!

My difficulty was trying to imagine how much of your sky is actually visible from that one pixel-wide swath.

[Old Photons]

I like to store a keogram along with my imaging data. It allows me to look at sky conditions to see what was going on when I captured my data. Often I have to toss a group of images that were captured around the same time of night and the keogram will show that a cloud rolled through at that time.