Derotation in Astrophotography [Field Rotation]

Derotation in Astrophotography is Amazing

I learned something new this weekend.  I learned about a technique in astrophotography called Derotation.  I’m amazed at what a little imagination and scientific skill can accomplish.  Grischa Hahn, had the bright idea to reduce motion blur in photos of the planets.  Especially those with days slightly greater than 9 Earth hours.  Derotation in Astrophotography is a powerful technique and is brilliant.  Derotation is built into his software called WinJupos, is an algorithm which does the following:

1. takes the frames of a group of images or video and flattens them out into a cylindrical shape.
2. Compares those cylinders to one another and matches them up
3. takes each aligned cylinder and recreates the image of the planet.

Is it really 3 easy steps, no and when you add a database of planetary positions, this becomes an indispensable tool in Astrophotography.

Initial Steps to Derotation

First capture one or more AVI videos of the target planet.  Generally speaking a minimum of two minutes each.  Next, open PIPP and upload the captured video.  Make sure planetary is selected and process the video.  You will use this new video in WinJupos.  Once open you will see the following.

Within the software select Program, Celestial Body, finally the planet of choice. Under Recording, select Image Measurement and finally open the video file from PIPP.

A wire frame version of your planet will be on top of or near your planet. On the left you will see boxes for time entry. Input the date and UT time the video was taken. WinJupos uses this information to resolve the rotation of the planet.

Next Steps

Great! It’s time to make adjustments to the wire frame. In the top left corner of the screen are tabs. Click the Adj. tab. Similarly, you should see this:

Next find the Outline Frame button and click it. Additionally, If Automatic Detection is not greyed out, select it. Once selected, it will resize and place the frame on the planet as shown above. Consequently, you may be required to use the arrow keys.

Finally, using the N and P keys, rotate the frame to north and you’ll be ready for the derotation step. Once you’re satisfied, select the Imj. tab. Find the save button and select.

How to Derotate

Recall where you saved the file for the next step. Select Tools from the menu bar then find De-Roration of Video Streams.

Fill in the boxes.

• Original Video = Location of Original Video Taken
• Start Time/End Time = Enter Date/UT Time of Original Video
• Image Measurement of a preliminary image from the video = File you just saved

Finally under Output check corrected video and select the Start Button. WinJupos will now create a new de-rotated video for you to stack in your favorite stacking software.

My Results

Here’s a photo on the left of Jupiter I processed without derotation.  I was happy to see the GRS (Great Red Spot) but could not bring out the detail beyond what you see here.

On the right is the derotated picture and the details are much sharper.  I was also able to brighten the photo in the process.

I’ve tasked myself to derotate many of my previous photos of Saturn and Jupiter.  So far with little success.  I’m learning that I need to improve my video capture techniques.  So look out for more of my work in the near future. Good luck and remember the sky is only the limit when your mind is unwilling to fly. Go beyond!