SKY_September2014.pdf

SkyandTelescope.com September 2014 71

the adjustments to the DBE back over to the raw image

using the same steps you used to move them to the back-

ground map. You can then examine both the corrected

image and the new background model based on the

adjusted set of samples.

If you’re still seeing lumps in the background model,

continue adding, removing, or moving samples around

and reapply. After a few interactions with DBE, you

should end up with a background model that closely

resembles the gradients in your target image. At that

point, you can apply an STF stretch to the corrected image

and inspect it for any additional gradients.

In my example on the left of the Virgo Galaxy Cluster,

the resulting background isn’t evenly illuminated. But my

background model transitions very smoothly across the

entire fi eld, so these diff erences in “background illumina-

tion” must not be an artifact introduced by my eff orts to

remove gradients. This is the extremely faint dust and

nebulosity that permeates the Milky Way. Accurately

removing gradients enabled me to aggressively stretch the

image to reveal it.

If your original data was just a couple of hours of expo-

sure under suburban skies or through subpar conditions,

chances are that any existing faint signal wasn’t detected,

or is buried under several sources of noise. In other

words, a poor signal-to-noise ratio makes it impossible

to reveal faint objects. In my example, the image is more

than 25 hours of luminance under very dark skies. These

conditions allowed me to obtain suffi cient signal of these

extremely faint objects.

It might be daring for an amateur with a small tele-

scope to claim that the uneven illumination reveals faint

interstellar dust or nebulosity. And although we can never

be 100% certain that every background-intensity variation

corresponds to actual objects in our images, with this

accurate gradient removal technique, we can be very con-

fi dent that most leftover variations are indeed data-driven,

and not processing artifacts. ✦

Rogelio Bernal Andreo often images elusive nebulosity from

the darkest locations in the southwest United States.

Although we can never be absolutely sure that every brightness

variation in an image is real, removing the gradients in this photo

enabled the author to reveal what appears to be faint nebulosity.

You can use the DBE tool on any deep-sky image with

adequate exposure. The author captured this mosaic of M78

and LBN 1622 using a Takahashi FSQ-106EDX and SBIG STL-

11000M CCD camera and corrected it with his technique.

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