K3CCDTools Home Page
"When I consider your heavens, the work of your fingers, the moon and the stars which You have set in place, what is Man that You are mindful of him?" -- Psalm 8:3,4

Introduction Features FAQ/HowTo Support Download

How to process a sequence affected by field rotation

When you take multiple photos of sky on alt-az mount or on static tripod, they usually suffer from field rotation. It's caused by the fact that stars seen from the Earth rotate around the celestial poles. This effect may appear even when you use an equatorial mount - when it is not perfectly polar aligned.

In the right picture you can see an example of stack of 57aligned frames (each with exposure 8seconds). The frames were taken by Nikon Coolpix 995 digital camera fixed on static tripod (Sagitarius area).
The effect of the field rotation is clearly evident. The alignment point was selected in the left bottom part of the frame. The stars near the alignment point are point shaped, but the more distant stars form arcs.


The following tutorial uses a sample AVI file compressed by lossy DivX codec (to make the sample AVI file size smaller for downloading). The original file was captured by Lumenera Lu075M camera in K3CCDTools in 12-bit mode (12-bit monochrome AVI K3CC Codec). Thus the sample file quality cannot be compared to the original 12-bit AVI file. But for explanation of using the Field Rotation filter it's sufficient. You can save the sample AVI by using the right click browser menu on the AVI link.
BTW, you are welcome to look at my
very simple Lumenera deepsky setup.


Step by step procedure:

1. Load the sample sequence M31Lum_05000ms_06-09-15_0000_divx.avi

2. Load the Dark frame M31Lum_05000ms_06-09-15_0001_DF_48b.png

3. Check your Dark frame processing settings. I usually use the Averaged Surround method:


4. Choose the "Select center point (FFT)" tool (using the main toolbar button) and select the first center point for automatic alignment.
The best way is to select it at one of the corners of frame (e.g. in the bottom left corner).
In this example I used FFT size 128 pixels. I selected the point [149, 416]:


5.
When the 1st center point is calculated, select the Field Rotation Filter (using the Input filter toolbar button):


6.
In the Field Rotation Input Filter dialog press the tool button (step 1). It will calculate the rotation center for all frames:

Note: For proper calculation the filter must be still deactivated (the left bottom green LED is off).


7. Now choose again the "Select center point (FFT)" tool (using the main toolbar button) and select the second center point for automatic alignment.
The best way is to select it on opposite side of the frame (e.g. in the upper right corner). The longer the distance between the 1st and 2nd center points, the better the acuracy of calculation of the field rotation.
I selected the point [455, 179]:


8. When the 2nd center point is calculated, press the tool button (step 2) in the Field Rotation Input Filter dialog. It will calculate the rotation angle for all frames:
:
Note: For proper calculation the filter must be still deactivated (the left bottom green LED is off).

You can check that the calculation is OK by selecting the last frame (Image 0089) in the frame list. You should see its rotation angle - it is -1.249 in our case:

Note: Don't forget to return to the first frame (Image 0000) after checking the rotation angle for the last frame.


9. Now the rotation is calculated for all frames so we can activate the Field Rotation Input Filter by pressing the left bottom green LED tool button:

Now the field rotation is corrected and the sequence behaves like it had no field rotation. Of course, only while the Field rotation filter is active, so all further operations are done with the Field rotation filter active.


10. Now it's time for the definite frame alignment. Choose again the "Select center point (FFT)" tool (using the main toolbar button) and select the definite center point for automatic alignment. It should be situated on the object which is the most important for you.
In our case I chose the star [286, 311] near the galaxy center:

11. At this step we can do a fine alignment. We just select a small rectangle around the alignment star using the Select Result Rectangle tool button . We can do it in higher zoom level:

Note: This zoom level also reveals the compression artifacts of the DivX codec used for compressing the sample AVI file.

Then we do the fine alignment by using the Alignment tool - the Difference alignment. Set the sky background level as threshold and press the Align Checked button.

At the end we must maximize the frame rectangle by right-clicking the tool button. For even better result we can use also the X2 mode, but at this example we don't use it.

12. The last but one step is to stack the sequence by using the Calculate result image (Sum) toolbar button .
We can save the all frame alignment parameters to a project file (e.g. for later use) by using the menu Sequence Processing - Save Project...

13. The final step is to do some post processing by using the Post-processing toolbar button .

And here is the result image. Just for comparison, on the right side you can see the result image when the original uncompressed 12-bit AVI file was used:


Stack result with DivX Sample

Stack result with the original 12-bit AVI

Note: The DivX and the original 12-bit sequences were processed with exactly the same parameters.


You can also try what becomes when the field rotation filter is switched off (just switch off the green LED button in the Field Rotation Input Filter dialog) and stack the frames again.


Field Rotation Input Filter ON

Field Rotation Input Filter OFF

The effect of using the Field Rotation filter is clearly visible.

We can explore the effect of the Field Rotation Input Filter also by using the Zoom tool:


Field Rotation Input Filter ON

Field Rotation Input Filter OFF

Notes:

  • In our example we used the frame "Image 0000" as the reference frame for alignment. You can use any frame as the reference frame (you should select the frame with the best quality)
  • You can save the stacked image to any 16-bit image format (PNG16, TIFF16, FIT16/32) and do the final post-processing in your favourite image processing program (e.g. Registax, Photoshop). You can also use a combined method - do some post-processing in K3CCDTools in Post Processing dialog and final post-processing in other program
  • You can store your sequence processing alignment settings by using the menu Sequence Processing - Save Project... command. The project file stores all alignment and rotation properties for all frames
  • When you open the sequence again later, set the darkframe and load the project file (by using the menu Sequence Processing - Load Project...), don't forget to activate the Field Rotation Input Filter if you want to use it - just open the Field Rotation Input Filter dialog and press the green LED button - all rotation settings are already reloaded from the project file.


Back to K3CCDTools - FAQ & How To page


Computer generated images, real images, drawings and texts are property of the author and may not be reproduced or used without permission of author. The Jupiter images are property of Christopher Go.


Home

Last Update: 21.12.2006