Capturing and Processing Mosaics

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:



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:



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:



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

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