Great imaging tools for eclipse chasers: new software makes it easier than ever to create stunning total eclipse images.
Photographing solar eclipses is one of the most challenging forms of astro imaging. It requires a great amount of advanced planning, from organizing the equipment you tow along on your voyage towards totality, to recording a series of exposures that capture the massive range of brightness displayed by the Sun's ethereal corona. The age of digital photography has added a new twist; some photographers spend days or weeks processing their exposures to create the breathtaking results that have emerged from recent eclipses.
But now there's an even newer twist; modern software developed for conventional photography makes combining your bracketed eclipse exposures an automatic process. Photomatix Pro 3 (www.hdrsoft.com) is a $99 program designed to combine a series of frames progressively under-and over-exposed to reveal details beyond the range of brightness captured by a single exposure. This technique, known as high-dynamic-range (HDR) photography, has become popular with landscape and nature photographers. But it also lends itself perfectly to eclipse photography.
I've developed a workflow between Photomatix Pro 3 and Adobe Photoshop that eliminates much of the tedious steps previously needed to create an HDR eclipse image.
While the process is ideally suited to modern digital exposures, it works just as well with older film images that have been scanned.
Photomatix Pro 3 is available for download from the Internet, and a free demo version is fully functional, though images created with it are watermarked. Once you install the program, you simply start the process by clicking the Generate HDR image in the Workflow Shortcuts tool palette. A new window opens, and pressing the browse button allows you to search for your images. The program supports JPEG, TIFF, PSD, BMP, and all major digital-camera manufacturers RAW formats.
Once you've selected your series of images, you'll be off ered a few choices for aligning them and mapping the tonal curve of the final composition. If your images weren't aligned during capture (or when you scanned them), try using the "correcting horizontal and vertical shifts" option. Ideally you want a series of exposures made in rapid succession, so the Moon's movement relative to the Sun's corona isn't significant between the start and end of your series. I suggest using the default "tone mapping" option at the bottom of the palette, which allows the program to automatically calculate the exposure ratios of each image.
If your series has multiple images made with the same exposure, Photomatix Pro 3 will ask you to verify this before combining. If you're using scanned images, you'll have to assign each image its own exposure increment. The program offers several automatic options that tend to work well, but be sure to verify your photos are in order before proceeding. Once everything is ready, you just click OK to combine the frames into one raw HDR image.
Once combined, your image will display on the monitor appearing blown out. Don't worry; this is your unprocessed HDR result. A small HDR Viewer window also opens that will show the detail in any part of the image that you move your cursor over. At this point, it's a good idea to save the raw image so that you can reload it and try different tone mapping without having to start from scratch.
The real strength of Photomatix Pro 3 becomes apparent when you click the Tone Mapping button. This function allows you to compress the dynamic range of your combined image and reveal details across a huge range of coronal brightnesses in a single image. I have experimented with all the options available with tone mapping, and what follows is a description of the settings I find most useful for creating a great eclipse image. Feel free to experiment, as my tastes in photography will likely differ from yours.
I start by selecting the Details Enhancer tab, setting the Strength slider to 100 and the Color Saturation slider to about 50. Next, choose the right-most button under the Light Smoothing options (the other buttons tend to create strange vignetting effects).
I usually set the Luminosity option between 0 and 2. Changes here will be reflected in a handy histogram displayed in the window below the Luminosity setting and in the main image preview. As with most of my astronomical-image processing, I strive to keep both ends of the histogram from becoming chopped off or "clipped," so I monitor this step very carefully.
Below the histogram are four additional tabs. Under the Tone tab, because we don't want to clip any of the image, set the White Point and Black Point sliders all the way to the left, or 0.0%. Next, adjust the Gamma slider to about 0.66. Experiment with this setting to suit your own tastes. Under the Color tab, change both the Temperature and Saturation Highlights sliders to 0, and drop the Saturation Shadows to about -7, in order to tone down the unnatural blue color of the Moon that is common in long exposures.
The Micro tab allows you to change the micro-contrast settings in order to extract more detail from the highlight regions of the corona. It's in these bright regions that you often find interesting coronal detail such as magnetic loops above active regions on the Sun's surface. It's fun to play around with these settings, but keep an eye on the histogram after each change to judge if your changes enhance or detract from the final result. It's during this step that I try to make sure detail is visible everywhere in the image, from the lunar surface to the outer corona.
The final tab on the right is labeled S/H for shadows and highlights. I have found that it's not very useful for astronomical images because it tends to blur or clip detail.
Once you're content with your image, save your settings so that you can retrieve and modify them on another batch of images, and then click the Process button.
At this point you should have a spectacular image, and I suggest saving it as a 16-bit TIFF file and also as a JPEG. The entire process from start to finish will have taken you less than five minutes! Some eclipse chasers may be happy to stop here, but others may want to continue processing their images.
Additional Sharpening in Photoshop
If you'd like to enhance your images and reveal details often overlooked when viewing the corona by unaided eye or with binoculars, here are some of the tricks I use to bring out the subtle coronal structures.
I begin by opening the 16-bit TIFF file in Adobe Photoshop CS. Earlier versions of this program will also work, but only CS and later will allow you to work in layers with 16-bit data. With the image displayed, I open the Layers Palette with the pull-down menu Windows>Layers. From the Tool Palette, I choose the Elliptical Marquee Tool and change the feathering option to 5 pixels.
To select a circle that encompasses the entire Moon, I start at a corner of the image just beyond the lunar disk itself. While holding the shift key and mouse button simultaneously, I drag the cursor diagonally across the image until I create a circular selection that roughly encompasses the Moon. I can then move this selection to properly center the Moon within it by using the arrow keys on the keyboard. To grow or shrink the selection, I simply use the pull-down menu Select>Modify>Expand or Contract, and adjust the circle's size until it's just a few pixels larger than the lunar disk.
Next I use Edit>Copy, then Edit>Paste, to make a new layer with only the Moon visible. This allows me to adjust its appearance separately from that of the solar corona.
To enhance the corona, I click on the Background layer in the Layers Palette, copy it, and paste it onto the image five times. Four of these new layers will be used for sharpening the image, and one will be used to suppress any highlight clipping during the final stages.
I begin the process by clicking on Layer 2 and pulling down the High Pass filter (Filter>Other>High Pass). The window that pops up controls the radius of sharpening applied to this layer. For the four sharpening layers I usually use filter radii of 100, 50, 25, and 12, which increase the visibility of coarse and fine detail in the corona. Again, you can experiment with these settings.
To apply these sharpening layers to the final image, I change the mode in the Layers palette from Normal to Overlay. The result will be immediate. I have to make sure that all my sharpening layers are below the layer containing the Moon, or else the lunar disk will become over-sharpened relative to the corona.
When I'm satisfied with the sharpening levels, the inner corona is often blown out. To fix this, I simply select the layer with the High Pass filter set to the largest radius of sharpening, then chose the Eraser tool and erase the over-blown area near the lunar limb. I can also change the eraser tool's radius to control how much I erase from each successive layer.
Finally, I select Layer 1 and apply a curve (Image> Adjustments>Curves) to the image to suppress the brightest regions, and bring out the faintest reaches of the corona captured in the series. I can also use the Curves palette to colorize the sky background and simulate the deep-blue sky visible during totality.
When I'm done, I save this layered image as a Photoshop document (PSD format) in case I want to come back later and experiment with some of the layers or apply other techniques to further enhance the image.
With the new digital-processing tools available to eclipse chasers, creating breathtaking portraits of a total solar eclipse has never been easier. Results that once took weeks of careful manipulation in a darkroom can now be accomplished on your laptop computer before the end of the partial phases!
ULTIMATE ECLIPSE DETAIL
While just about any eclipse photographer adept at digital processing can create beautiful pictures with the software described in the accompanying article, enterprising individuals such as Miloslav Druckmuller and his daughter Hana Druckmullerova have developed custom programs for combining and enhancing exposures. They created the image here using data recorded by Constantinos Emmanouilidis while on an eclipse expedition to Novosibirsk, Russia, last August. It combines 31 exposures recorded through a Takahashi FS60C refractor with a Canon 350D camera set to ISO 100. Exposures ranged from 1/30 to 6 seconds. Each image was flat-field and dark-frame calibrated before the final composition was generated.
Once combined, an unprocessed high-dynamic-range (HDR) image is displayed along with a sub-frame preview window showing the full range of detail visible in user-selected regions of the photograph. The HDR image can be further processed with Photomatix Pro 3 as well as saved as a raw HDR image and brought into other programs capable of working with 32-bit images, such as Adobe Photoshop CS.
Photomatix Pro 3's greatest strength resides in its tone-mapping features. This allows you to compress the dynamic range spread across several exposures into a single view that reveals details in shadow and highlight regions. It's somewhat akin to a non-linear stretch commonly applied to astronomical deep-sky images.
Applying a few layers of high-pass filtering in Adobe Photoshop is an easy way to accentuate the delicate streamers, prominences, and magnetic loops in the solar corona.
Each layer in which high-pass filtering is applied needs to be changed from Normal mode to Overlay in the Layers palette in order to achieve the benefits of this sharpening technique.
High-pass sharpening also makes the Moon appear unnaturally bright. To subdue this effect you can make a selection with the Elliptical Marquee Tool and copy the lunar disk from the bottom layer, then paste it on the top of your layer stack. This way you can then adjust this layer independently from the rest of the image.
Sky & Telescope imaging editor Sean Walker is looking forward to a chance to stand in the Moon's umbral shadow.
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|Title Annotation:||Imaging Technique|
|Publication:||Sky & Telescope|
|Date:||Jun 1, 2009|
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