Sky & Telescope - May 2020 - 70

S&T Test Report by Johnny Horne

The Optolong L-eNhance Filter

The L-eNhance ﬁlter from
Chinese manufacturer Optolong
passes the light from hydrogen
beta (Hβ), doubly ionized oxygen
(O III), and hydrogen-alpha (Hα)
radiation while blocking most
sources of light pollution and
strong moonlight.

Optolong L-eNhance Filter
U.S. Price: $229
Available in the U.S. from optcorp.com

What We Like
High-contrast images of emission nebulae in full light pollution
Works well in bright moonlight

What We Don't Like
Blocks most reﬂection nebulosity

ONE OF THE FIRST ITEMS to check

on one's list of preparations for imaging emission nebulae is the phase of
the Moon. Bright moonlight typically
washes out all but the brightest emission nebulae, so most imagers simply
wait for moonless nights to shoot these
colorful objects. The typical exception to
this rule is to image them using monochrome cameras and special narrowband ﬁlters that isolate speciﬁc regions
of the visible spectrum where these
70

M AY 2 0 2 0 * S K Y & T E L E S C O P E

ionized gasses emit light. Narrowband
ﬁlters also block most moonlight, as
well as many sources of light pollution,
so amateurs equipped with these setups
could shoot pretty much any clear night.
But astrophotographers with one-shot
color (OSC) cameras were left to wait
for the Moon to leave the scene.
That is, until recently. The new
L-eNhance ﬁlter from Optolong Optical allows for narrowband imaging
with OSC cameras and results that
show a palette similar to natural-color
(RGB) images. This dual-passband ﬁlter
allows light from two astronomically
important regions of the spectrum to
pass through almost uninhibited while
blocking most sources of light-pollution. The ﬁrst passband encompasses
a 24-nm slice of the spectrum where
hydrogen-beta and the dual emission
lines of doubly ionized oxygen emit
light at 486.1, 495.9, and 500.7 nm,
respectively. Its second bandpass is
10-nm wide centered at 656.4 nm where

hydrogen-alpha glows. Outside of these
two bands, the ﬁlter blocks 99% of all
light between 300 nm and 1000 nm.
Holding the ﬁlter up to your eye and
examining a daylight scene reveals a
blue-green cast.
The 2-inch ﬁlter with 48-mm threads
makes it convenient to install on a camera's 2-inch nosepiece, coma corrector,
focal reducer, or extension tubes that
are threaded for standard 48-mm ﬁlters.
A 1¼-inch-format ﬁlter is also available.
For my ﬁrst imaging test, I targeted
NGC 7000, the North America Nebula,
using a William Optics 71-mm f/4.9
apochromatic refractor paired with
a ZWO ASI071MC Pro color CMOS
camera on a night with a bright gibbous
Moon. The ﬁrst 60-second exposure
clearly showed the shape of the nebula
with very little fogging from the nearby
Moon. I then took a series of 300-second exposures. The stacked result
rendered the nebula in its familiar reddish color though with a slight yellowish hue. For comparison, an unﬁltered
exposure just 15 seconds long on the
same night was hopelessly drowned
out by moonlight, with no sign of the
nebula at all.
Using the same setup, I turned the
scope toward M8 and M20 sinking low
in my southwestern sky. The emission
nebulosity of both objects stood out
well in 10-minute exposures despite
the pair being smack-dab in the most
light-polluted region of my sky. The only
minor disappointment was the weak
visibility of M20's blue reﬂection nebulosity due to the ﬁlter blocking most of
its wavelengths.
On a different night I set my sights
on NGC 7635, the Bubble Nebula in
Cassiopeia, shooting with a 14-inch
Meade ACF telescope at f/6.7 and the
same camera. Again, a bright Moon
was in the sky. My 60-second exposures

ALL IMAGES COURTESY OF THE AU THOR

This dual-bandpass ﬁ lter permits deep-sky imaging of nebulae with color cameras
even under light-polluted conditions.

http://www.optcorp.com

Sky & Telescope - May 2020

Table of Contents for the Digital Edition of Sky & Telescope - May 2020