Timelapes-video

Making a Time Lapse Astronomy Video

One interesting and fun way of starting down the road of Astrophotography with a DSLR is making Time Lapse Sky videos.  The process takes a minimal amount of hardware and software, and other than a computer and DSLR is relatively cheap. The best way to view the three examples on this page this is to enlarge the video box and increase the video setting to the maximum for your monitor.  Watch closely, you'll see the hot pixels!  You will also see some white flashes that come from car headlights.

Fifty miles from the nearest town and look at all the airplanes! 

Hardware needed:

• •     DSLR to take the pictures

  •     Tripod to hold the camera steady

  •     Computer to process and combine the pictures into a video

  •     Intervalometer/remote shutter release/camera to computer USB connection/Smart Phone Application to automate the picture taking process

Software needed:

• •     PhotoLapse or other jpg to AVI program to combine the pictures into a video (StarTrails.exe can do video with darks, but seems to limited in terms of size to 640x)

  •     PZapGUI from Tabaware to apply Dark Frame Subtraction to the pictures

  •     Canon's Digital Photo Pro or other software to adjust exposure, adjust white balance, and  resize a jpg to a smaller size

  •     Canon's Digital Photo Pro or other software to convert RAW files to jpgs if you shoot RAW

  •  

How To Take the Pictures:

Find an open area with good view of the dark sky.  While not necessary, scenic areas like National Parks, lakes, rivers, and parks offer many possibilities for re-creating a familiar scene under a different light.  Set up the tripod, mount and level the camera, focus, and frame the picture.  I personally like a wider field of view so I generally use shorter focal length lenses (10-24mm).  You may want to include foreground for interest or not as you see fit, although blur may occur if your shutter speed is too long.  Do check and make sure the memory card in your camera has enough room to capture hundreds of pictures.  I will mention that the individual pictures do not have to have the greatest depth.  Setting your camera to the smallest pixel size won't hurt, in fact, shooting at the largest pixel size will slow the display of the final video unless you resize the pictures prior to creating the video.  Set the ISO to 800-1600 and make sure the lens is wide open.  If you see aberations like coma in the individual pictures you may want to stop the lens down a bit.

One of the first decisions that has to be made is the length of the individual frames.  Longer exposures will record more dark sky detail, but exposures too long will record the stars as trails and not points.   A general rule from Barn Door mounts is the exposure should be no more than 600/focal length seconds suggesting about 30 seconds for my Canon T2i camera's 18-55mm zoom lens at the widest setting.  That's good, because other than bulb, the longest shutter speed setting on the camera is 30 seconds.  Try a few pictures and see if you like the results... are the stars points or blurs, is the sky too light because of light pollution, are structures blown out from inside or outside lights, can the picture be framed better?  Make any changes you think will improve your results.

You need to find some way to trip the shutter of your DSLR without moving the camera.  I've mentioned both an intervalometer and camera to computer connection.  Both of these way will allow you to automate the process.  With either you should be able to tell the camera to take X pictures, each Y seconds long, with Z seconds between pictures (to give the camera time to save each image).  A typical example would be 120 pictures, each 30 seconds long, with 2 seconds to save the results.  This would run for over an hour and show the stars move 15 degrees through the sky.  At any rate, determine how long you wish to capture images.  Normal video is 30 frames per second, so at that rate one frame every 30 seconds would give 120 frames, but would be condensed down to 4 seconds.  A whole 8 hours of recording would be condensed down to 32 seconds.  Don't be afraid to make the final frame rate as small as 5 frames per seconds.  Without one of the automated ways of capturing images, you'd get really bored quickly.  Lacking  a way to automate the process, plan on using some kind of remote release so you don't disturb the camera.   I usually plan on one sequence per night of 2-3 hours.

While I have not done it, it should be possible to put the camera on an automatic exposure setting and start the process before dark.  You should be able to see a gradual dimming with the shorter exposure during light giving away to progressively longer ones as it gets darker.  Let the capture process go on as long as your batteries hold out.  If you have an external battery pack or AC adapter you can even let the process run over night (IF you feel your equipment is secure).  Exposure ramping is also possible to get a more consistent sky.

How To Combine the Pictures into a Video:

First copy all your pictures to your computer.  Obviously, know where on the hard drive you've stored them.  Download and install the needed software and take a few minutes to read the docs. 

Current DSLRs can be up to 20 megapixels.  Even with jpg compression, these pictures are HUGE and using them to make a video at a high frame rate with no compression is an invitation to failure.  It isn't hard to create a video that is several GB in size.   Your computer can't load the individual frames fast enough to keep up with the display.  Plan for small pictures, either by taking the smallest size possible with your camera or by using software to reduce the size to something smaller like 1280x960.  Plan on reducing the frame rate to 10 or 15 frames per second and save the video with compression like MP4.  Play with these number using your computer.  The goal is to make the video run smoothly and get the size under 1MB.  If you plan on submitting the video to YouTube, you should probably see if they have any size guidelines.  If you've taken RAW images a bit of pre-processing to adjust the white balance and contrast may be in order.  Fire up the software needed to produce small jpgs of all your images, create a directory to place the pictures, and do the conversion.

A second problem is that long exposures (30 seconds) tend to introduce noise into the picture.  It isn't unusual to see hot pixels either.  Both can be removed with Dark Frame Subtraction where a dark frame is a picture taken at the same shutter speed, ISO, and temperature as your picture sequence WITH the lens cap on the camera.  Stacking astronomical pictures takes 20 or so darks.  I have yet to find a program that will subtract more than one dark for time lapse work.  It should be possible to stack multiple darks to create a master dark and then subtract it, but it seems too much work.  The result is I take one dark with the lens cap on at the end of my sequence.  I use Tabaware's PZipGUI batch mode to subtract the dark frame from each of the pictures in the sequence and it saves each picture with a modified file name.  Do you really have to do the Dark Frame Subtraction?  NO, but as you look closely at your video, you MAY see some stars that move (they are stars) and some stars that don't move (they are hot pixels).  If it bothers you, do the subtraction.  If it doesn't bother you, skip the step.  By the way, you could do this with PhotoShop, but it would take much longer.  If you find another piece of software that does the Dark Frame Subtraction (or Flat Frame Subtraction too) let me know.  I still looking for something better.

Once the pictures in the sequence have been saved to a common folder it is a simple task to run PhotoLapse.  Load and select the pictures in the sequence.  Choose a file name and output directory.  Create the AVI and select a codec with compression.  I use MicroSoft's MPEG-4 because it runs on my computer.  Once the pictures are combined PhotoLapse tries to open the AVI with Windows Media player.  If the display is slow/jerky you may need to adjust the frame rate.

You should be able to take the AVI to a movie editor and add a lead in, effects, and music.

I put the camera on a driven EQ mount in the above example so both the sky and foreground move.

While you have the sequence of pictures that you used for the video, consider running the free program Startrails on them to get a star trail picture.