Sky & Telescope - March 2025 - 25

later). Without them, the nebula was too faint to be more
than a subtle stain in the eyepiece. It would take an exceptionally
transparent night to see much beyond the Rosette's
overall shape and the bright cluster at its center.
The O III filter greatly improved the visual contrast of the
nebulosity in both my 8-inch and 30-inch scopes. The filter
helped more than I expected, especially in the 8-inch.
I sketched the nebula and star cluster with the 30-inch
and then the 8-inch in quick succession. Because this was
mid-March, I only had about four hours before the Rosette
sank too low in the west to view. Nonetheless, I was able to
see some impressive detail and take the time to admire the
beauty of this striking object.
Checking the star chart on page 20, you'll see that the
Rosette is only about 5° south of NGC 2261 (Hubble's Variable
Nebula) and NGC 2264 that comprises the Christmas
Tree Cluster and the elusive Cone Nebula. This is a busy and
beautiful part of the sky.
NGC 2244 is a distinctive open star cluster, which I easily
saw with my 80-mm finderscope. The apparently brightest
star of the cluster is 5.8-magnitude 12 Monocerotis, but it's
a foreground star at only 1
/10 the distance of the cluster. Two
O-type stars, which are part of the cluster - 6.7-magnitude
HD 46150 and 7.3-magnitude HD 46223 - are up to 50 to
60 times more massive than our Sun. Overall, there are seven
O stars in NGC 2244, ranging from magnitude 6.7 to 8.2.
They're the perfect kind of stars to energize the hydrogen gas
of the nebula to glow, and which is why it makes sense that
they'd be at the physical center of Rosette. The nebula's fluffy,
circular shape with its central hole is quite beautiful, especially
if you can get the whole object into the field of view of
your telescope.
The NGC parts of the Rosette did not stand out in my
8-inch with the O III filter - instead, I saw a fairly even glow
of its entire wreathlike shape. Not surprisingly, the view was
significantly more detailed in the 30-inch. But wait! In addition
to the targets I already mentioned, just off the northeastern
edge of the Rosette is the final NGC object in the area,
the open cluster NGC 2252. It's elongated, and subtly follows
the curve of the Rosette. Easy to overlook in the 8-inch, it
stood out quite well in the big scope.
To me, one of the wonderful aspects of observing the
Rosette was enjoying the beautiful balance of nebulosity and
stars. They had a similar visual weight, which I appreciated
more the longer I observed. As I sketched, I saw more stars
and more detail within the Rosette. After just a few hours
though, I had to stop because I was pooped from the five-hour
drive to the observing site. So, I hit the sack in my van.
Encouragingly, the sky looked even more transparent the
next day, and I was ready for some Rosette Hα action.
Night Two: Hydrogen-alpha Observations
At this point, you might be asking yourself: " Hydrogen
alpha? For a faint nebula like the Rosette? " Aren't Hα filters
used for observing the Sun or by astrophotographers acquiring
multi-hour exposures? Yes, but not exclusively. Coupled
with a night-vision device, such a filter opens up the sky at
this very interesting wavelength. I think of it as using an
electronically assisted filter, so even though this is not visual
observing in the conventional sense, the intensified image is
presented in real time.
The way a night-vision device works is to attach it to the
top of an eyepiece afocally with an adaptor, with the filter on
the other end of the eyepiece. The night vision device has a
small viewing screen that looks like a built-in eyepiece, and
this is where you see the image.
I have a TNV/PVS-14, which is currently one of the more
popular night vision devices (for a review see S&T: June 2018,
p. 58). The PVS-14 has a spectral range of approximately 580
to 900 nanometers, which spans the spectrum from yellow
to the near-infrared. Given that Hα's wavelength is 656 nm,
it's well within the PVS-14's range, making it a great match
with an Hα filter.
As for the Rosette, the PVS-14 + Hα filter combination
completely transformed what I could see - the view was like
looking at a detailed, grayscale photo. The enhancement was
similarly strong in both scopes, and I devoted most of my
second night to observing the Rosette in Hα.
How Image Intensification Works
You might be wondering how the image-intensifier
technology inside night-vision devices improves
the view. As photons enters the instrument they're
turned into electrons, undergo amplification ( " electron
avalanche " ), accelerated through an electrical field,
and then the electrons hit a phosphor screen, where
they're converted into visible-light photons. Depending
on the device, the amplification may be on the
order of 30,000 to 60,000 times!
Generally, night vision devices produce a bright
green image. However, you can purchase the PVS-14
with a " filmless white phosphor tube, " which renders
a nearly color-free grayscale image. It's still very
slightly green though, which shows up much more in
images taken through the units afocally than what is
seen in the device's viewscreen.
Also, night vision devices have a " gain " adjustment
to fine-tune the image to produce the most pleasing
result with the least amount of electronic noise.
Their main drawback is cost - and availability (this
technology cannot be exported from the U.S.). Nightvision
devices will typically set you back several thousand
U.S. dollars, plus a few hundred for an hydrogen-alpha
filter, which limits their wider use. However,
resistance to opening your wallet probably depends
on how you're introduced to night vision astronomy.
In my case, it practically made my head explode.
sk yand tele scope .o r g * MARCH 2025 25
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Sky & Telescope - March 2025

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