Sky & Telescope - December 2022 - 62

Imaging Technique
background subtracted (consecutively recorded
images are subtracted from each other) to
ensure the best possible comparison and to
(hopefully) allow the detection of any differences
between the two images, such as faint
arcs from the light echoes. This didn't reveal
anything of note.
I then subtracted the 2020 image from
the 2012 image using the PixelMath process
in PixInsight, in order to reveal any brightness
difference that might have occurred over the
eight years. At first, all I was presented with was a bland, gray
image, but when I stretched the result, I was surprised to see
very large and prominent arcs - the light echoes I sought!
These features appear as concentric light and dark arcs centered
on the precise location of SN 1987A. The light and dark
tones are simply an artifact of the order in which the images
are subtracted. However, the spaces between the light-and-dark
arcs clearly reveal how much the echoes have expanded during
the intervening eight years between the two sets of images.
The result shows what is possibly the first amateur detection of
light echoes from SN 1987A - fully 33 years after the event.
To present the light echoes in the beautiful context of the
sprawling clouds of the LMC, I then acquired a substantial
amount of narrowband image data in 2021 and used that
to create a deep, colorful picture of the region's complex gas
clouds. The broadband light echoes were then composited
onto this to achieve the result seen on page 60.
The full-size image is a mosaic of two deep fields centered
near the bright Tarantula Nebula in the LMC. The field is
filled with numerous bright, colorful nebulae and star clusters.
At the LMC's distance of 163,000 light-years, the field
of view seen here spans a massive 2,917 by 2,000 light-years.
At this large scale, the nebulosity seems to feature many
bubble-shaped voids of different sizes. These are formed by the
radiation pressure from young star-forming regions and shock
waves from ancient supernovae. The small but comparatively
PIXEL MATH Subtracting one image from another in order to
generate a difference image is easy to do in any astronomical image-processing
software. 1: Begin by opening the two images and aligning them
together. 2: Next, use the PixelMath function to subtract one image from
the other (seen here in PixInsight). The result will most likely look gray until
you stretch the data (3). Anything not found in both images will appear
either black or white. Stars appear as black spots with white halos due to
the difference in star sizes between the two pictures.
INNOVATIVE AMATEUR Author Rolf Olsen and
his 12-inch astrograph
recent remnant of SN 1987A is also visible as
a tiny pink dot at the focal point of the lightecho
arcs.
I acquired all the image data from my
observatory in outer suburban Auckland,
New Zealand. I used my 12½-inch telescope
to acquire the wider, narrowband context
photo of the Tarantula Nebula and surrounding
area in early 2021. This is a deep,
bi-color image comprising 7 hours and 15 minutes worth
of exposures through hydrogen-alpha and oxygen III filters,
totaling 14.5 hours.
Superluminal Illusion
The light echoes from the supernova appear to be moving
outwards faster than the speed of light, something that is
physically impossible but often observed in phenomena that
move close to our line of sight. This effect is most often associated
with relativistic jets of matter ejected from active galactic
nuclei - for example, the jet emitted by the supermassive black
hole in the center of elliptical galaxy Messier 87 in Virgo.
Using the formula s = d × v, we can calculate the angular
speed of motion of the light echoes across the sky, as apparent
speed (s) equals distance to object (d) times angular
velocity (v).
Between 2012 and 2020, the brightest arc had expanded
outwards by approximately 47 pixels. With an image resolution
of 0.764 arcseconds per pixel, this corresponds to a movement
of 4.49 arcseconds per year (47 × 0.764 ÷ 8). At the distance of
the LMC, this equates to around 28 light-years in 8 years - 3½
times the speed of light! So how is that possible?
This apparent superluminal speed is in fact an optical illusion
and doesn't represent the actual speed of the outward
travelling sphere of light, which
is, of course, moving at 299,792
1
2
62 DECEMBER 2022 * SKY & TELESCOPE
Sky & Telescope - December 2022

Table of Contents for the Digital Edition of Sky & Telescope - December 2022
