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Comet freeze: here's a great technique for stacking your comet photos in ImagesPlus.

Comets are rare visitors to our night sky. Often arriving with little warning, they put on a brief show and then slowly fade as they recede from the Sun. The brighter ones grab the public's attention, while amateurs grab their cameras. A bright comet is easy to shoot --simply point your camera or telescope at it, open the shutter for about 5 minutes, and you're pretty much done. Imaging the fainter ones can present an interesting challenge; they require much longer exposures to reveal any streaming ion or dust tails, which leads to more complex issues when assembling your image.

Perhaps the most appealing images of comets portray them as a brilliant greenish coma with a long, streaming tail that trails away against a field of sharp, round stars. But since comets move noticeably with respect to the stars in only a few minutes, long exposures tracked on the background stars are limited to only a minute or so before the comet begins to shift against the star field. The resulting photo often shows a nice star field with a blurry streak that was the comet as it moved during the exposure.

One fix to this issue is to guide on the comet itself with a separate guidescope. This produces a deep image of the comet surrounded by long streaks of trailed stars. So what can you do to get an image that has the best of both worlds: a deep, detailed comet with gossamer ion streamers against a perfectly tracked star field? My solution to this dilemma is to do both, and then combine the result using post-processing software. Here's how I do it with ImagesPlus (

Double Alignment

Part of the difficulty of freezing a typical comet's motion in deep photos is the fact that you need to take exposures tens of minutes long to bring out any ion or dust tails present. But as noted earlier, the target comet will trail in exposures longer than a minute or two. The solution is to shoot lots of short exposures that you'll later combine in two different ways: one registered and stacked on the comet, the other aligned and stacked on the background stars. Stacking many short exposures has the benefit of producing a deep image with signal comparable to a single long one.

The technique described below works on any DSLR or CCD images. (As with any deep-sky imaging, make sure to calibrate all your files before moving on to stacking.) To register your photos in ImagesPlus, select Image Set Operations > Align Files > Align Files - Translate, Scale, Rotate. This action first requires you to select the images you wish to combine. Navigate to the folder containing the files, and then hold down your control key and click on all the images you want to align, and press the Open button. Next, the Align TSR window opens, where you'll select a few options. Under Feature Selection Type, choose On Each Image, then select Translate Only under the Alignment Type section. Next, select the Common Point or Star and Reference Image in the Alignment Feature Selection area.

With all these settings chosen, move your cursor over to the first image displayed and click on the comet nucleus; the next image in the set automatically appears. Select the comet in each of your images until you're through the set. Once completed, the Align button becomes active at the bottom-right of the window; click it, and in a few moments all of your images will be aligned on the comet. Click the Done button.

Now let's align the images on the background stars. Open the same Align Files window as before, but this time select Translate, Scale, Rotate under the Alignment Type section, and Common Angle Defining Point or Star in the Alignment Feature Selection area. This time you'll need to click on two stars as your alignment points before the Align button becomes active. Click it, and in a few moments your second alignment set is complete.

Removing Stars

Once both sets of images have been aligned, you can stack each set together. Start with the comet-aligned images. Select the Image Set Operations > Combine Files/HDR Add ... function, and select your comet-aligned files. When the command window opens, select Minimum as the Combination Method. This works great if the stars are well separated between each image. Often, however, the stars overlap a little bit, and you end up with some faint star trails in the combined image. They'll be easiest to see if you stretch the image using the Auto Stretch or Digital Development functions in the pull-down menu. I recommend Digital Development because you can control the aggressiveness of the tool and avoid an "overcooked" appearance.

You can remove these residual star trails using the Feature Mask [C] tool. Open it by selecting Special Functions > Mask Tools > Feature Mask [C] from the pull-down menu. When the tool window opens, increase the Star slider to about 50, and increase the Mask Area Size slider to 5. Click Enable under Fill Radius, and increase its value to about 15. Under the Special Star/Area Processing section, check the Select box, and move your cursor to the image and click the comet nucleus. Then click the Include button and expand the Radius slider to its maximum setting. This will exempt the comet nucleus from the mask. Finally, make sure to click the No Stars radio button under Output. Now press the Apply button, and in a few moments the last bit of star trails should be removed.

If the comet didn't move very much between your individual exposures, you may need to isolate it and remove the stars in each individual exposure before combining. This would require enhancing each aligned photo before removing the stars to bring out more of the comet. You can do this with Curves, Digital Development, or any of the tools found in the Stretch pull-down menu. Just remember, try not to be too heavy-handed --only enhance the comet slightly at this stage before stacking, or else the comet's head will become overexposed in the stacked result.

Generate the Star Field

Our next task is to make the star-field image. This is done using the same Combine Files/HDR Add ... tool and settings used to create the comet-registered image. The stacked photo will have a trailed comet image, so you'll need to do some minor adjustments to the result. Open the stacked star-field image and stretch it to display its full dynamic range. You'll notice there's quite a bit of the comet left over in the image besides the trailed head; that shouldn't affect the final outcome.

Now we'll use the Feature Mask [C] tool with most of the same settings as we previously used for the starless comet result, with a few modifications. First, skip the Fill Radius section. In the Special Star/Area Processing section, click the Select box, then move your cursor to your image and click the middle of the trailed comet nucleus, and check the Remove button. Move the Radius slider all the way to the right, and finally choose the Stars button in the Output section. Now we're all set to hit the Apply button. When the tool is complete, save the result and we can reassemble the results.

Bringing It All Together

Because you're working with the same set of images, your comet image should already be accurately aligned to the star field image. ImagesPlus offers several options for combining the results, though my preferred routine is to use Special Functions > Combine Images Using > Blend Mode, Opacity, and Masks. In this tool you have the option to combine the images using average, median, or min (minimum) options. (My best results are often achieved with the Blend Mode set to Merge Split.) Adjust the opacity of the blend until it suits your tastes, and then click the Flatten button and you're pretty much done!

Comets are often challenging targets that require special attention to get the most out of your images. But having a robust set of tools in your image-processing arsenal can put you on course to take some of the most memorable photos of these rare and wonderful visitors to the inner solar system.

Tim Jensen is an avid astrophotographer and a research project supervisor at Swinburne Astronomy Online.
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Title Annotation:Imaging Technique
Author:Jensen, Tim
Publication:Sky & Telescope
Date:Sep 1, 2015
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