Friday, 24 April 2020

ISS Solar Transit Captured

ISS Solar Transit Captured

This is the third instalment in a batch of blog posts which I'm putting out.  This time, I'm looking at imaging the ISS.
Since I've been interested in astronomy, I've held subscriptions to some astronomy magazine and my favourite part, regardless of which publication, is generally the section when people can submit their own images.  You can always find a great mix of targets.  Galaxies, lunar, solar, nebulae the list goes on.  I remember being stunned when I first saw a picture of the ISS taken through amateur equipment.  At first, I couldn't believe that you could photograph the ISS, and show anything more than a streak of light in the night sky as it passes overhead.
Soon though, I come to learn and understand how these things pan out and I grew a greater appreciation of the techniques needed to get these images.  For some years since, I have always fancied giving it a go.  It seemed all I needed was to be in the right place, at the right time, with the right kit in the right weather conditions.  That's a lot of 'right' things needing to be in place all at the same time.  Mmm, this could be the hardest part.

When do transits happen?

The type of image I wanted to capture was a transit image.  Either of the ISS transiting the moon, or the sun.  Either would do for me.  Logic kind of says a solar transit is more likely given that the phase of the moon is always changing whereas the sun is always present.  You just have the great British weather to contend with.  Then up steps the internet, and a plethora of sites and apps that are designed to help you determine when a visible transit might be due.  I found https://transit-finder.com/ to be most helpful when it comes to predicting solar transits.  You can enter your location, a time frame within which you want to check and how far from your position you are willing to travel to see the transit.  The site then takes that information, checks the latest ISS orbit data, and plots any relevant times and dates onto a map, showing the area where a transit will be visible from.
Let's not underestimate all the things that need to come together for this to happen.  The sky on a global scale is massive.  The Sun in comparison to the area of the sky visible from the whole planet is pretty small.  The ISS from the perspective of being on the surface of the Earth is tiny.  What you are waiting for is for your position on the surface of the Earth, the ISS (moving at a rate of 17,130mph) and the Sun to all line up at the same time.  The margins are tight to say the least.
Now imagine my excitement as I entered in my details into the ISS tracking website, only to find out that over a 2 week period, the ISS would be visible passing in front of the Sun (transiting) not once, not twice, but 3 times and all visible from my back garden.  A transit will last anything from under one second to around 2 to 3 seconds.  There's no messing.  That's all the time you've got to get that image.

Capturing that image... and failing.

My first opportunity came around the 13th April late on in the evening.  The sun was no longer visible from within the observatory, so, I took a spare mount and tripod into the garden, took my refractor off the mount in the observatory, and set everything up connected to my laptop.  It is at this point I need to put out the usual warning, but it would be somewhat remiss of me if I didn't give this a mention.  
Under no circumstances look directly at the sun, be it with the naked eye, or with any sort of optics such as a telescope or binoculars.  It will hurt.  It will blind you.  You will suffer irreversible damage.  Not even a pair of posh expensive sunglasses will protect your eyes.  Don't do it.  Just don't.  The only way it is possible to look at, or image the sun is by using specifically made filters or equipment that severely cut down the amount of light allowed through the device.
I used a white light filter specifically made to fit snuggly over the end of my refractor telescope.  Only when that was fitted was I able to turn the telescope in the direction of the sun, and start preparing to take pictures.  Anyway, back to the first attempt.
With everything set up and working, I took a couple of test frames and managed to frame the Sun pretty well in the centre the camera's field of view.  Then, up steps the wonderful British weather.  A single, although large cloud crept along the surface of the Sun, obscuring it from view.  This is where the problem started.  Not being a regular solar imager, I needed some time to test my camera settings.  Getting the sun in focus is one thing.  Getting the correct exposure times and gain settings is something else, requiring plenty of experience, or at least, time for trial and error.  Do you know, that single cloud hung around as long as it possibly could?  I watched my countdown to the time when the transit was due to happen.  I was running out of time.  I wanted to take 1 minute worth of exposures, approximately 30 seconds either side of the transit time, to make sure I caught it.  The cloud cleared the Sun with about 90 seconds left to go, and I simply ran out of any testing time.  I could only go with what I had.  I failed.  Miserably.  However, I have got a nice collection of around 200 frames of a bright white circle to show for my efforts.  There's no doubt the ISS did transit the Sun as it was predicted to.  My camera settings were hopelessly over exposing the images though.  I had nothing.

Capturing that image... and succeeding.

I would need to wait another week for my next chance.  Time I could spend gathering more information, and learning a bit more about the techniques required to get the image.  I decided to change tac slightly, and instead of taking a rapid series of still images, I read up on how I could use my equipment to capture a movie clip in .avi or .ser format.  I could then use some free software called PIPP which is designed for solar and planetary imaging, to take that video, break it down into component frames and safe them as .tif pictures.  There is a bunch of other clever stuff this PIPP software can do, but that would be enough for me, and would get the result I wanted.
So, come the morning of 21st April, I checked the weather, checked the transit website and verified that all was good.  This time, I didn't need to remove anything from the observatory as the Sun was gong to be high enough for me to see directly from inside the observatory.  This transit was scheduled to last around 0.8 seconds.  No third chance for at least another week.  After that, no more foreseeable chances for perhaps many months, even years.  At least, not from my position in the back garden.
I had the mount set up and tracking the Sun.  This time, not a cloud in the sky, so nothing was going to get in the way.  I was also able to run a few tests, and bring down the settings to allow me to see some features on the solar surface.
My plan of attack this time was to capture 1 minute of video, in the middle of which would be 0.8 seconds of transit time.  And that is what I did.  The countdown clock came to zero, I pressed the capture button, and waited, staring at the screen for any sign of the ISS passing in front of the Sun.  I didn't see it.  The video clip finished and was saved to hard drive.  I had no idea if things had worked.  All that preparation, and everything was over in a flash.  Quite literally the blinking of an eye.  Next I needed to copy the captured video from the observatory computer to my laptop, where I could start searching the footage.  It was only then, I realised that with my camera, and the resolution and frame rate it was recording at, that one minute of video footage took up around 21GB in size.  Positively massive when compared to your regular HD 150 minute classic Disney film!

Extraction.

So, PIPP did it's thing, produced hundreds of .tif files, debayered them and made them ready to view.  I started my search, getting rid of all the images I didn't need until I struck gold.  A sequence of around 8 or 10 images taken from the video where the ISS could clearly be seen passing in front of the Sun.  How excited was I?  Really excited.  Pleased as punch, made up, stoked.  Generally, quite happy.
Since I took these images, I have done two things.  I have arranged them in a short video, which shows the transit, slowed down to around 5 seconds.  I've also taken one of the images and introduced some false colour to bring the more expected, yellow colour of the Sun.



The ISS passing in front of the sun.  This transit passed in front of the sun on it's left edge.  This image is from approximately half way through the transit.

There is a third opportunity for me to try for another image, although the next chance in a few days time a quite early in the morning, possibly before the sun has made it up above surrounding roof tops, trees and fences.  I don't think I will try to image that one, but instead, start the search for an ISS transit of the moon.
Stay safe everyone!  Cheers.

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