Sky & Telescope - December 2018 - 67

(MPSAS). For example, a typical, suburban night sky is about
19 to 20 MPSAS if there's no bright moonlight. For context,
a pristine sky with no light pollution yields a value of 22,
whereas an inner-city sky is often below 17. Knowing my sky's
value helped me determine how much exposure I need for a
given target. A 1-magnitude decrease in the meter's value, say
from 18 to 17, means the sky brightness is 2½ times brighter.
So it takes 2½ times more exposure time to get the same SNR,
assuming everything else is equal.

Eliminating Gradients
Finally, once I improved my raw-image quality as much as I
could, the ﬁnal step in the battle against light pollution was
to remove skyglow gradients from my images during postprocessing. Even imaging under a dark sky produces subtle
gradients that need addressing, so this step is essential no
matter where I shoot.
Gradient removal is done on each stacked result but before
combining into a ﬁnal color image. This is perhaps the most
important step to ensure a good astrophoto, because images
with strong gradients are impossible to color balance. How
you perform this step depends on the image-processing software you use. Most of these programs include tools to deal
with gradients. I prefer the Dynamic Background Extraction
(DBE) tool in PixInsight (pixinsight.com), and a helpful tutorial on using this powerful tool can be found on page 68 of
this magazine's September 2014 issue.

p COLORFUL RESULT Once you address gradients in each color-filtered image stack, you can then combine the results into a dazzling result
virtually indistinguishable from a shot captured at a dark-sky location.

If you're shooting with a one-shot-color camera, stack your
images ﬁrst. Then split the result into its red, green, and blue
individual color channels; remove the gradients in each; and
then recombine and color balance the result. After that, the
processing steps are no different than for images taken under
pristine skies.
These important steps each have greatly improved the
quality of my images, even from the murky glow of city lights.
And it's hard to beat the convenience of shooting from home,
which can open up many nights that you otherwise might
have stayed indoors.

¢ JONATHAN TALBOT, a retired flight meteorologist for the
U.S. Air Force Reserve, spends all his free time these days
imaging the night skies.

ttt SHOOT THE ZENITH One tried-and-true technique
for mitigating the effects of light pollution in astrophotos is
to target objects that pass very high in your local sky. This
deep image of M33 was recorded over several nights as
the galaxy passed near the zenith.
tt PUSHING THE ENVELOPE Imaging relatively faint
targets, such as the colorful object NGC 2170 in Orion
seen here, is difficult but possible from urban locations. The
author recorded each sub-exposure as the nebula crossed
the meridian over the course of many nights.
t LOTS OF SPACE Light pollution is often easier to address in an image when the target does not fill the entire
frame. This shot of M63 had strong gradients in each color
channel that the author easily addressed in PixInsight
before combining them into the color result seen here.
sk yandtelescope.com * DECE MBE R 2 018

http://www.pixinsight.com http://www.skyandtelescope.com

Sky & Telescope - December 2018

Table of Contents for the Digital Edition of Sky & Telescope - December 2018