Registax is a powerful program, and watching it work is amazing.
Registax can analyze, align, and stack thousands of images in just minutes.
And the whole procedure is displayed for you -- watching your computer think
has never been so much fun! Registax is a free download.
Click here to visit
the Registax website.
Registax is typically used with a video file (.AVI) captured using a webcam.
See the Capturing Images with a Webcam page for details on capturing
For this section, a 200-second video clip of Mars will be used. The
video sequence contains 2000 frames. The image was taken through Rick Auerbach's TMB 254mm f/9 triplet refractor, using a
5x Powermate lens and Lumenera webcam.
Above: On the left, a single raw frame from the video clip of
Mars. On the right, the final processed image using the best 750 frames
out of 2000.
The first step is to select the file you wish to process. For webcams,
you will use an .AVI file. Click on the Select button and navigate
to the folder where you saved your video file and open that file.
Note: Registax sometimes gives
error messages with AVI files containing more than about 2200 frames. Keep
the input file to this size or less to avoid getting an error.
Above: The Mars video clip selected in Registax.
Initial Frame Selection
There is a slider along the bottom of the window that allows
you to select which frame from the video clip to use for the starting frame.
Move the slider slowly along and watch for a fairly sharp frame or one where the
planet looks the most round (sometimes poor seeing squishes the planet).
Normally you will check the Color Processing box for planetary images.
For lunar images, you may leave this unchecked since the moon shows little
color. For color images, the LRGB option can be checked as well. The
differences the LRGB processing makes can be subtle, but there are benefits in
noise reduction and in making sure bright highlights do not appear overexposed.
There is no real disadvantage to checking the LRGB box since the processing time
is the same and you don't have to use the LRGB processing if you like the
default settings, so it is recommended that you go ahead and check the LRGB box.
The Processing Area can be changed if desired. If your image is very
large, you can select a subframe to be processed. This speeds things up by
just processing a small section of your larger image. You can see results
faster and decide on the processing that you like and then process the final
larger image after choosing the desired setting. For webcam images, which
are generally very small, the default 512 pixel setting is usually fine.
In 640x480 resolution mode, the edges of the image will not be processed.
For planetary images, this is fine, since the planet only occupies the center of
the field. However, for lunar or solar images, where the object fills the
image, you will want to increase the Processing Area to 1024.
The alignment box can be changed depending on the subject. If a planet
appears quite large in your image, 128 or 256 should be used. For small
targets, or for aligning on a small crater on the moon, use one of the smaller
boxes to keep other craters or features from interfering with the alignment.
For the example used here, the 256 pixel setting was used.
Move the cursor over the image and a box is drawn. Make sure the box is
larger (but not too much larger) than your chosen alignment feature. For
the image of Mars, the whole planet can be used. The 256 pixel box
surrounds the entire planet. Click on the center of the disk of the planet
and the Aligning window will appear.
Above: Selecting the alignment feature, in this case the whole planet.
After selecting the planet the FFT Spectrum window appears
showing the alignment
feature being used. In the case of a planet, you should see a distinct
small red spot in the center. In the case of a lunar image, where there
could be multiple alignment features to choose from, make sure only a single
distinct dot is visible. If more than one red spot is seen, Registax may
not know which feature to use for alignment.
Increasing the FFT filter number will decrease the spot size and potentially
increase the accuracy of the alignment. Usually a good rule is to increase
the FFT Filter until any colored areas around the edges of the window disappear,
leaving just a single dot in the center, as shown below.
Above: FFT Spectrum
Align Window Options
The only setting to change here is the Quality Estimate.
The Gradient method works well for planets. Setting the Lowest
Quality to 80 is a good starting point. This means all images with a quality level greater than
80% will be
aligned and included for further processing (or later rejection). If your
entire set of frames has a quality greater than 80%, all the images will be
included in the alignment. Normally, with good atmospheric conditions
during imaging, this will be the case. However, for less than ideal
images, the full range of quality may go below 80%. It might be best to
lower this value to 60% as a starting point. You can always raise the
quality settings in the next step, but it is best to try to include the entire
range of images at this point since you don't know what the quality level will
Click Align to begin the automated alignment procedure. The bar
at the lower left corner of the screen will indicate the progress of the
After running the initial alignment, the Align window displays the Registration Properties graph. This
graph shows a red line displaying image quality. The graph is not numbered
but each horizontal line represents 10%. In this example (see figure
below) the image quality line starts on the upper left at 100%, then drops off
to about 75% on the right. The green line indicates alignment error, which
will be discussed more below.
Note: You can get a good feel
for the seeing conditions and the likely final quality of your image from the
quality graph. Average conditions tend to produce images with a lower
quality range of 75% or so. Poor conditions might lower the quality to
60%. In spectacular conditions, the worst images might be 95%, indicating excellent seeing
(and excellent optics).
Above: Registration Properties graph showing
the 40% cutoff described in the text
You can now select which frames will be used for the final image. The
best frames are on the left end of the graph and the worst images on the right.
By moving the green line (using the slider at the bottom of the Registax
window), you determine the quality cutoff. Everything to the left of the
green line will be used for the next step (stacking). A good rule of thumb
is to use the best 40% of the images. (See the box below for more details
on choosing the quality cutoff.) To select 40% for the cutoff, move the
green line left until it reaches the fourth vertical line on the graph, the 40%
mark. As you move the slider, note the number of frames being indicated
along the bottom of the screen (listed as stacksize).
How Many Frames?
Is it better to
use a smaller number of higher quality frames or a larger number of lower
quality frames? Out of, say, 1000 frames, should you use the best 30%
for a total of 300 images, or is it better to take 600 frames which are the
best 60%? Tests indicate that more frames is better. 600 frames
out of 1000 is noticeably better (less noisy and more detailed) than just
300 frames out of the same 1000 originals. In other words, a greater
number of frames is more important than a higher quality cutoff. There
is probably a point of diminishing returns, especially if the image quality
begins to drop off sharply toward the right side of the image quality graph.
The more images you have to begin with, the higher the quality cutoff you
can use and still end up with a large number of frames to stack. I
would recommend 400 frames as a minimum number. This means taking at
least 1000 original frames if you want the best 40%. If you want the
best 10% you will need to take 4000 original frames. Keep in mind
that, for Jupiter especially, the rotation of the planet becomes noticeable
in just a few minutes. At 30 frames per second (fps), 4000 frames requires
over two minutes. Jupiter rotates over 1 degree in that time span,
certainly noticeable with very high-resolution imaging. Also, remember
that Registax cannot handle more than about 2200 frames, so it is best to
limit the total number to this amount. Also, frame rates above 15 fps
cause most webcams to compress the data so faster frame rates are not
Above: Comparison of 100 frames (left) and 500 frames
(right). Note the significant decrease in noise in the right image.
Find the point where the image quality line crosses the 40% frame line.
In this example this is at about the 85% quality level. This means that the
best 40% of the frames are 85% quality or better. In this example, this
selects the best 800 images for stacking.
Usually the default settings in the Optimization window are sufficient.
Using the Resampling or Drizzling settings will enlarge the image.
Drizzling will enlarge the image after the next Stacking step. Resampling,
on the other hand, enlarges each individual frame before optimizing. This
allows a better optimization but requires much more processing time. Many
imagers will Resample to get a more accurate alignment, then resize the image
back down to the original size later. Lanzcos and Mitchell methods seem to
produce the least noisy results when Resampling.
Artifacts can creep into the image if it is resized, so my recommendation
would be to leave the image size alone and resize it later, if necessary, in
Above: Optimization window settings
The bar at the lower left will indicate the progress while
the images are optimized.
Above: Registration Properties window after optimizing.
The next step is to Stack the frames. Click on the
Stack tab to select the next window.
At the top of the screen you will see a checkbox to open the
The Stackgraph basically looks like the Registration Properties graph, but there
are two sliders that will be used to make the final selection of frames to be
included in the final image.
Above: Stackgraph with cutoff sliders.
The Stackgraph contains two cutoff sliders. One for Difference Cutoff
on the vertical axis, and one for Quality Cutoff on the horizontal axis.
As you adjust the sliders, notice that the number of frames to be included in
the final image changes.
If you want to reject some of the images that are not registered as closely,
you can bring the Difference Cutoff slider down. This will reject images
with a greater alignment difference. Bringing this down to 90% rejects
some of the more poorly aligned images. You can also limit the frames by
sliding the Quality Cutoff slider to the left to remove some of the lower
quality frames, but this was already done when we Limited the frames after
aligning them. The settings used here reduced the number of frames for
the final image to 750.
Click on Stack and Registax will stack the images and display the final
stacked image. Now it needs to be processed. Click on Wavelet
Processing to begin enhancing the final image.
Above: On the left a single raw frame from
the video clip, on the right 750 frames stacked. Note the significant
reduction in noise.
Notice that the final stacked image is pretty blurry. However, it shows
significantly less noise than the original. The image now has 6 layers
that can be adjusted using the Wavelet sliders at the right of the screen.
Every image is different, and some experimentation is definitely necessary.
Normally, most of the adjustment is done to layers 2 and 3, with just a little
on each other layer. Try running the sliders to the right to see what
happens. If you start to see noise creeping into the image, back off on
Tip: One of the most common errors in processing planetary images is to oversharpen. This leads to a noticeable
halo effect where there is a dark line around bright portions of the image.
This is most noticeable on Mars around the edge of the planet. On
Saturn it might show up where the rings meet the edge of the planet, and it
might appear as artifacts around high-contrast craters on the moon.
Above: Wavelet Processing settings for the Mars example
A handy feature of Registax is the ability to save wavelet schemes that you
like. To do this, set the wavelet sliders how you like then click on the
Save Scheme button below the sliders. I find that settings tend to apply
to a given planet, so I have schemes named "Mars" and "Jupiter" and so on.
Once you find a good setting you can easily use it on a new image by clicking
the Load Scheme button.
There is still a little work to be done, as can be seen in the image above. Four other processing features in the Wavelet Processing window were used on
this image: Contrast, Gamma, Histogram, and the L=R+G+B mixer.
Techniques for using these features are outlined below.
The stacked image tends to be a bit bright, especially after Wavelet processing.
Usually lowering the brightness and contrast is helpful. For this image
the Contrast had to be lowered quite a bit. This brings detail back into
the overexposed bands near the middle of Mars.
Above: Contrast/Brightness settings
This feature is ideal for controlling color balance. The
red, green, and blue Colorweight sliders adjust the color balance of the
image. This image of Mars is a little off in color. Adjusting the
color balance is a matter of moving the sliders until the planet appears as you
Above: Histogram settings for the Mars example.
This adjusts the mix of each color channel in the luminance
channel of the image. It does not affect the actual color of the image.
It just changes how much weight each color contributes to the overall intensity
of the image. Changing these sliders can help enhance certain details.
For Mars, increasing the blue slider tends to increase the brightness of the
clouds but some contrast is lost on surface features. For this example,
red and green were increased to bring out more detail.
Above: LRGB Mixer settings for the Mars example.
When finished with the Wavelet Processing window, click on the Final
tab to finish processing the image.
Final color adjustments can be made in this window using the
Saturation sliders. Since we corrected the color using the RGB
in the last step, changing the Hue is probably unnecessary for this image.
Saturation was increased to 10 to help intensify the color. The rotation
dial can be used to align the planet differently, if desired. For this
image, the Flip Y option was checked to put the planet's north pole on
top. The Flip X option would mirror the image, if desired.
The last step is to save the file. Normally you will want to save your
file as a TIFF file. This will prevent any compression from being applied
(as would be the case for a JPEG file). This is preferable if you plan on
opening the file for more processing in Photoshop. Go to File > Save and select
TIFF from the pulldown menu after naming the file.
Above: The Final window
Post-Processing in Photoshop
Normally, some slight post-processing is done in Adobe Photoshop. A
little more sharpening, noise reduction, and color balance adjustments can be
made easily in Photoshop. Various techniques are possible and the methods
used and results achieved will depend greatly on the particular image being
processed. See the Photoshop section of
this website for more details on the different processing methods that can be
For this image, the Despeckle filter was run twice to remove any
remaining noise, a slight
Unsharp Mask was applied, and a few adjustments were made using Curves
and Levels. The Saturation was boosted slightly.
Above: The final results of processing using Registax and
Photoshop. Note the pink-colored dust storm on the right side of the
planet. Image by Rick Auerbach and Scott Tucker.
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