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Taming a bright star

October 2012: Follow these steps to reduce any overly bright and big stars.
tony_hallas
Exceptionally bright stars have a way of over-representing themselves. This is mostly due to flare, which occurs whenever light passes through glass or bounces off a mirror. Despite astroimagers’ best efforts to keep the optics clean, some scattering of light is inevitable.

This month, I’ll show you how to reduce a bright star until it is only a fraction of its original size and brightness — overkill for sure, but useful in demonstrating how powerful this technique is. It will be up to you to decide how far to take it.
 
For our purposes, we will select a bright star in the Pleiades (M45) and make it as “small” as some of the much fainter stars next to it. Note: You must be in 16-bit mode to do this technique, as we will be compressing tonal space around the star.
Bright-star
A bright star in the Pleiades (M45) creates some unwanted flare in this astroimage. All photos: Tony Hallas
Start by looking carefully at the star. Do you see how far the flare extends from its center? With the “Elliptical Marquee Tool” in Photoshop’s tool palette, hold down the shift key (to force a perfect circle) and draw a circular selection just inside the edge of the worst flaring. Now “Feather” (found under the “Select” menu) this selection about 40 to 60 pixels. Open “Curves” (in the “Image/Adjustment” menu) and pull down on the center of the curve until the density of the outer flare matches the surrounding areas. Careful — if you darken too much, you will end up with a dark circle.
 
Next, deselect and redraw the circle, closer to the core this time. Hide the selection if it makes it easier to see what’s going on, repeat the process, and then deselect.


Despite astroimagers’
best efforts to keep
the optics clean,
some scattering of
light is inevitable.


Darken-star
Darken the star by using Photoshop’s "Elliptical Marquee Tool," "Feather," and "Curves." This will reveal detail under the flare.
Notice two interesting results: The hot core becomes smaller, and hidden detail beneath the flare begins to emerge. This is already a huge improvement, but we can go even further.

With the image open, go to “Filter” and then “Liquify.” On the extreme left-hand side of the window, make sure you select the fourth icon from the top (the “Pucker Tool”). The brush size should be 50 percent larger than the core of the star. Set the brush density to 50 and the brush rate to 20. Under “View Options,” uncheck everything. Move the brush (circle) and center it over the star; then click the mouse and hold it. Note how the core of the star shrinks, much like with the common “Negative Spherize” technique, only minus all the hassle.
 
Move the mouse slightly while still holding the button down to accelerate the shrinking process. Just shrink the core a little bit; experiment with this technique until you feel comfortable with this tool. When you’re happy with the results, let go and press “OK,” then return to where you left off.
Stars-match
It’s possible to get all the stars to match (even unrealistically so) by darkening further and using the "Pucker Tool."
Make another circular selection around whatever flare is left, feather, and darken again. Note that the core of the star might now be a shade of gray, or it might have assumed a spurious color. If this happens, use the “Clone Stamp Tool” in “Color” mode to return the star to the surrounding color.
 
Dodge the core of the star with the “Dodge Tool” in “Highlight” mode at a strength of 10 percent to bring the core back to white.

After finishing, you should see details that were previously covered up by the flare and a star that is more representative of its true luminosity. Remember, I took this technique to extremes to show you how well it can work. In real life, my adjustments would have been more modest.
 
Happy star shrinking!
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