pAttached to the RASA 8’s corrector plate is
an additional set of corrective optics and the
camera mount, seen here with its threaded
M42 adapter in place. Three push-pull screws
and smaller lock-down screws used to adjust
the camera’s squareness are visible through
three curved slots.
qThe rear of the RASA 8 includes the focusing knob at right and its integrated cooling system
fan. Its 12V DC MagLev fan draws outside air in through three fi ltered ports (one is seen clearly at
left) to quickly bring the scope to ambient temperature.abilities, and they are also very forgiv-
ing of polar alignment error. A very fast
telescope combined with an accurate
tracking mount can even eliminate the
need for autoguiding.
Overall, a fast focal ratio can make
imaging a lot of fun and in many ways
easier. The road ahead can be slippery,
though, so you’ll need to take some care.
Cool It
Any telescope will perform poorly until
it has acclimated to the ambient air
temperature, and unassisted acclima-
tion can take hours with a closed-tube
system such as this. The RASA 8 comes
with an integrated fan system ready to
go. A 12V DC plug on the back accepts
the included battery pack requiring
eight AA batteries to power the fan. I
usually started the fan as soon as I set
up in the fi eld, and by the time twilight
ended the RASA 8 was ready to go.
Focusing at f/2
The range of focuser positions provid-
ing sharp focus is called the critical
focus zone (CFZ). The CFZ gets smaller
at faster focal ratios, and at f/2 it is tiny
— only several microns wide. Achiev-
ing focus by hand is extremely diffi cult,
and with the RASA 8 the focus posi-
tion shifts as the temperature changes
throughout the night. You’ll need to
refocus a few times per evening in most
circumstances. A focus motor, paired
with the software of your choice, is
strongly recommended. My fi rst few
efforts with the RASA 8 were manually
focused, and the results weren’t quite
perfect. I then ordered and installed
the Celestron Focus Motor for SCT and
EdgeHD telescopes ($199.95), which
produced consistently better results.
While I’m not the most mechanically
inclined person, installing the focus
motor was quick and easy. It can be
powered via a USB cable attached to a
powered USB hub, though I recommend
using an external 12V power supply, as
I did. Like most focusing systems, this
one has a slight amount of mechanical
backlash. Fortunately, most focusing
software allows for manually input
backlash compensation, and it was
relatively easy to fi nd the right value to
produce sharp focus every time.Connecting the Camera
While proper cooling and focus are
important, the interface between the
camera and the RASA 8 is perhaps most
key to producing good images. There’s a
threaded assembly for connecting your
camera at the front of the telescope’s
corrector plate. Within this port is a
removable clear-glass optical window
that can be replaced with Celestron’s
dedicated Light Pollution Imaging Filter
($219.95). Having quite a collection
of 2-inch fi lters for fast astrographs, I
was overjoyed when I saw this, until I
realized that the thread in the RASA 8
is not an industry-standard 2-inch fi lter
thread but instead one that only accepts
Celestron’s proprietary fi lters.
Connecting a camera is fairly easy
with the M42 adapter that was included
in my evaluation package, as most of
my spacers are M42 threaded. But here’s
where you need to choose your cam-
era carefully. The RASA 8 has a fi xed
29-mm distance requirement from its
front corrector element to your camera’s
sensor, and that drops to 25 mm if you
use an M42 adapter.skyandtelescope.com • NOVEMBER 2019 69