| Part I The Plan |
Part
II Acquisition |
Part
III Sub-Frame Processing |
Part
IV Alignment |
Part
V Assembling the Mosaic in Photoshop |
Part IV - Alignment
The alignment of all the panels in your mosaic is a critical step. A misalignment, especially of a frame in the middle, will prevent you from properly processing the mosaic. I prefer to use RegiStar for aligning the mosaic frames. In my experience it does the best job for aligning frames, even frames with minimal overlap.
Open your original calibrated, gradient removed, un-stretched images in RegiStar.
Note: I have had some difficulty with RegiStar and stretched images. RegiStar has a little more trouble finding the star centroids when the image has a non-linear stretch done on it. So what I will typically do is perform the following steps first on the un-stretched images. Once this is complete and all the frames are aligned I will then open the stretched images and align them to the un-stretched images. This extra step has always worked for me.
I always start with the
middle frame and register each image to it. Depending on how your
mosaic is laid out, you might need to register some of the frame
together, combine them into a single image, and then use the combined
image to register other frames to the combined image. You have to
experiment around and find the best strategy for combining the
frames. Always think about how much overlap you have between the
frames you are registering. If there is not very much (like just
a couple hundred pixels in the corner of the images), you might need to
consider another strategy to give you more of an overlap.
For this particular mosaic, I knew that if I started with the center frame and registered the frames immediately above, below, left, and right (like a '+' pattern), I should get good results. So, referencing the mosaic layout in TheSky again:
For this particular mosaic, I knew that if I started with the center frame and registered the frames immediately above, below, left, and right (like a '+' pattern), I should get good results. So, referencing the mosaic layout in TheSky again:
I selected frame A2 and
registered it to frame A5. Next I registered frame A4 to A5, A8
to A5, and then A6 to A5. Here you can see the result of the A4
to A5 registration:
Now you can register the
corner frames (A1, A3, A7, and A9) to the A5 frame. RegiStar will
use the knowledge it gained from the A2, A4, A6, and A8 frames as well
when registering the corner frames. This will give accurate
positioning for these frames.
If this did not work, then one option available is to merge the A2, A4, A5, A6, and A8 frames into a single image, as I show here:
If this did not work, then one option available is to merge the A2, A4, A5, A6, and A8 frames into a single image, as I show here:
Before continuing, we
should do some cleanup. Close all the images in RegiStar, except
the registered images (and the original reference frame, A5 in my case):
The registered images
will have the extension "_reg" appended to the end of the file
name. Make sure to keep these open. You will see above that
the "VIRGOA5-GR.fit" file does not have the "_reg" extension.
This is the original reference frame that all the images are registered
to, so this stays open.
Now it is time to register the DDP stretched frames to the current registration group. While you still have all the registered un-stretched frames open, open the DDP stretched frames. Now register the DDP stretched frames to your un-stretched reference group. The order does not matter since RegiStar will use all of the un-stretched images to determine the registration pattern.
You can now close all of the un-registered DDP stretched images and the un-stretched images. You should be left with just the registered DDP stretched images. All of the images you now have open should have the "_reg" extension on the file name:
Now it is time to register the DDP stretched frames to the current registration group. While you still have all the registered un-stretched frames open, open the DDP stretched frames. Now register the DDP stretched frames to your un-stretched reference group. The order does not matter since RegiStar will use all of the un-stretched images to determine the registration pattern.
You can now close all of the un-registered DDP stretched images and the un-stretched images. You should be left with just the registered DDP stretched images. All of the images you now have open should have the "_reg" extension on the file name:
The next step is to pad
all of the images. Padding the images will make them all the
exact same dimensions. This way the registration between frames
is not lost when moving into Photoshop. Padding the images is
easy to do in RegiStar. Push the F8 button, or select the
"Operations->Crop/Pad Image" menu item. You will then see the
following window.
Select the options as
I've shown them above. This will pad all the images in the
current image group (the group is defined as the images registered to
frame A5 in this case). It will make all the images the size of
the union with all the open group images. In other words,
RegiStar will compute the size that an image needs to be to house all
of the frames in the group. RegiStar reports the size at the
bottom of the window. Finally, I select the option to close the
original images after padding. The originals aren't needed
anymore and they clutter up the RegiStar workspace.
Now all the images are
the same size, with blank information where the frame didn't exist
before. These images should be saved and the alignment is
complete. From RegiStar you can save the images as either FITS
files or TIFF files. Just be sure to save them as a 16-bit file
(you don't want to loose any information).
| Previous: Part III -
Sub-Frame Processing |
Next: Part V - Assembling the
Mosaic in Photoshop |