High Resolution Lunar Image Processing

Page 2

Alignment and stacking

My preferred program for alignment and stacking is Registax so my comments here will be specific to that program.  The biggest reason to use Registax however is the newly added support for Multiple Alignment Points. This makes what was formally a very time consuming manual process into something that anyone can use to achieve world class results. The implications and results for lunar imaging are nothing short of revolutionary.

The first step before getting into MAP processing is to load up your raw AVI into Registax and do a preliminary alignment and frame rejection pass. Use the largest centered alignment box that will fit the frame, change the quality method to Compression and run the alignment. To find your initial frame limit run the slider through the file starting at frame 1. Look for the point where the image quality begins to noticeably decrease. This will usually coincides with the 90% quality level on the graph. Set your limit at that point. Typically this will eliminate anywhere between 50% to 90% of your original frames leaving only the best for further processing. Click on the Stack tab and use the Registered AVI function to save your selected frames and write them to a new AVI file. The result will be a much smaller data set that's already fairly well aligned.  Preprocessing the raw file in this way eliminates problems from tracking and wind and makes the rest of the process run a lot faster.

Now it's time to start beating the seeing using the MAP (Multiple Alignment Point) method. One would think that simply going in and aligning the whole frame with the largest alignment box would do the trick. This works to a point but because of the nature of seeing effects this often will not deliver a sharp image across the full frame. This is because even though the imaging area is very small ( 3.3 x 4.5 arc min. in my case) seeing effects will still vary from place to place within the frame. If you watch your AVIs carefully the effect is easy to see as some areas of the frame distort or blur while other areas are sharp. The only way to offset this is to break the frame up into sub-frames, align the sub-frames separately and then reassemble back into the full frame. The new MAP method in Registax allows up to 39 separate alignment boxes of mixed sizes. Each box is aligned and optimized separately and then all the pieces are stacked and stitched to create a new full frame image.  If you consider for a moment how much work and time would be involved in doing this by hand, separately aligning and cutting and pasting each box, you'll realize just how much of a step forward the new MAP processing method in Registax really is. A process that would have taken days is reduced to hours, much of it unattended.

For our demonstration I've selected a 517 frame AVI of the Plato area taken on the morning of 7-27-05 in average seeing. Telescope used was a 6" F/8 Newtonian with a 3X barlow. Camera was an Atik 2HS. Here's a single unprocessed frame:

Slice and dice, doing the MAP dance

Once your AVI is loaded into Registax click on the Multi Align button and select your alignment points. You can use any alignment box size you like and sizes can be mixed. The general rule to follow when selection MAP points is, high contrast features and small alignment boxes give the most accurate results. Low contrast areas will require the use of larger alignment boxes so a bit of trial and error will be necessary to get a feel for what works for your images. Here's the alignment box distribution for our test frame:

Notice that the box size is fairly small and that high contrast features are favored. Don't worry about the low contrast areas that aren't covered by a box. All you're really doing here is setting points. As can be seen in the next graphic, the program will generate full alignment tiles based on your set points that will cover the entire image.

Once you're happy with your alignment box distribution, select your quality estimate method (I suggest Compress) and begin the MAP alignment process by clicking on Start Alignment in the Multi-Alignment window.  The program will then step through and run an alignment on each box you've selected. Once the process is finished, set the limit slider to the number of  frames desired in the final stack and click the Limit button. Keep in mind that because you've preprocessed the original AVI to eliminate bad frames in step one, chances are you'll use 100% of the available frames at this point.  Click the Multi-Optimize button and Registax will then refine the alignments for all selected frames. This process can take a while on large images and slow machines so be patient. At the end of the optimizing run click on the Stack tab, take the default settings and click on the Stack button. The program will reassemble and stack all of the separate tiles back into the final raw image, here's the result:

If you carefully compare this to the original single frame near the top of this page you'll see how much was gained from map processing and stacking even before any further processing has been applied. Much of the gross XY distortion is now gone, noise is reduced and bit depth is increased. The frame still looks soft but image quality is fairly uniform across the full frame.

Next page, Image enhancement