Imaging has by far the
best star registration package available to the amateur
astrophotographers called RegiStar. The latest
feature of this software will automatically align an entire folder of
images. This is extremely useful for CCD imaging where there are
usually many sub-frames. Many have expressed difficulty in
understanding the use of RegiStar. I hope this document will help
others in using this powerful tool.
The first step is to make sure that all the images
you wish to combine are in a single folder on your drive. The
best thing to do is reduce your images and save them to their own
folder. Make sure there are no other images from another part of
the sky in the same folder - this will cause the alignment to fail.
Run RegiStar and open one image from your series to
be your reference image. It does not matter which image you open
first - except that the stars should be good and round. If you
have drift visible in the image, you should not use that as the
reference. Once you have the window open, you should see
something like this:
Now you can begin the registration process.
Press the <F2> key or go to the Operations->Register
menu. The "Registration Control" window will open:
If you opened the
reference image from the same directory as the rest of the image (you
should have), then this window is ready to go. RegiStar
automatically sets the directory, but you can change it if you need
to. The reference group is the image you just opened. The
remaining options I don't change, but you can experiment with them if
Click the Register button! RegiStar now does its stuff.
After a bit of processing, you'll end up with a screen like this:
You now have a bunch of
aligned images. However, they are not necessarily in a format
best used by other programs (like PhotoShop). If you were to stop
here and save these images, the origin and size of each image will not
be the same. This will result in the images appearing not to
match in another program. So, we need to crop and/or pad all
these images. Push the <F8> button, or click the
Operations->Crop/Pad Image menu.
You will see the
Crop/Pad control window come up. Above you can see the settings I
use for this.
The "Image selection" tells RegiStar to apply this action to all the
images that we just aligned.
The "Target dimensions" tells RegiStar how to crop or pad the
images. I like to set this to the union of all the images in my
set. This will make the final images large enough so that all the
data from every image is still present. This allows me to crop
the image as I see fit after the final combine is completed.
Alternately you can set this to the intersection of all the
images. This setting will automatically crop all the images to
include only the area where they all overlap.
I don't need the original aligned images any more, so I check the
"Close original image(s) after cropping/padding" check box.
Finally, in the bottom of this window RegiStar tells you the origin and
size of the final images. Below you can see the difference when I
set the target dimensions to the intersection of all images.
Click the "OK" button
and wait for RegiStar to do its stuff!
After a bit, you will now have a series of padded (and/or cropped)
images. Now you just need to save them.
Unfortunately, the Save All command in RegiStar will not let you create
a directory, so using Windows Explorer (or the like) create a new
directory to save your images. Now in RegiStar, click the
File-Save All menu:
Now if you happened to have another image open and you didn't want to
save it you could select the "Save all open image from the current
image group", but normally I only process one set at a time so I use
the first selection. Use the "Browse" button to select your new
directory. Leave the other options at their default and click OK.
You now have very accurately aligned images! You can combine
sub-exposures using any other program now using your normal combine
One point of note. If you have images of different binning (e.g.
luminance at 1x1 and RGB at 2x2) I recommend first aligning all the
luminance using the above procedure, then closing the files, then
aligning all the red, green, and blue images. Once you combine
your luminance into a single frame and the RGB into a single frame,
come back into RegiStar and align the RGB to the luminance. The
reason for this is that RegiStar will change the resolution of your
binned images to match the luminance. This can cause the
registration process to take a lot longer. By aligning the binned
images to themselves, the resolution remains the same. Then only
the single RGB image has to be up sized to the luminance size.
I hope this is useful to some of you!