S&T Test Report by Richard S. Wright, Jr.
AL
L^ I
MAGES^ BY^ THEAUTHORCelestron’s
RASA 8
Schmidt
Astrograph
Lightning-fast imaging now
comes in a popular-size package.FAST OPTICS ARE LIKE FAST CARS:
Both are a thrill to experience fi rst-
hand. I have some previous experience
with fast astrographs and thus was
very enthusiastic when the opportunity
arose for me to try out Celestron’s new
f/2 8-inch Rowe-Ackermann Schmidt
Astrograph (RASA 8). Just as with a fast
automobile, though, there’s little toler-
ance for sloppy driving when it comes to
operating this instrument.
The RASA 8 is an optical design
inspired by the classic Schmidt camera,
in which a piece of fi lm is placed at
the prime focus of the primary mir-
ror. While Schmidt cameras produced
well-corrected stars across a piece of fi lm
using a custom fi lm holder that curved
the fi lm to match the curved focal plane
of the instrument, the RASA design
fl attens the focal plane with a set of
corrective optics located near the cam-era’s focus point. From a distance, the
RASA 8 looks a lot like one of Celestron’s
Schmidt-Cassegrain telescopes (SCTs),
but as you get closer you’ll notice some
signifi cant differences. The most obvi-
ous one is that a camera attaches to the
RASA 8’s front corrector plate — there is
no visual back to connect a star diagonal
or eyepiece. The telescope is also slightly
longer than a standard 8-inch SCT, mea-
suring 24.7 inches.
The optical design of the RASA 8 is
a tradeoff between focal length and
photographic speed. The RASA 8 has a
focal length of 400 mm, approximately
one-fi fth that of a standard 8-inch
f/10 SCT. While this translates to an
amazingly fast focal ratio of f/2, it also
means a much lower resolution. For
comparison, an 8-inch f/10 SCT paired
with a camera having 9-micron-square
pixels produces an image scale of 0.938 Rowe-Ackermann
Schmidt Astrograph
U.S. Price: $1,699
celestron.comWhat We Like
Fast photographic speed
Lightweight
Integrated cooling fanWhat We Don’t Like
No provision for standard fi lters
Short back focus limits camera options68 NOVEMBER 2019 • SKY & TELESCOPE
arcsecond per pixel, whereas the same
camera attached to the RASA 8 pro-
duces 4.64-arcsecond resolution. This
instrument should perform best with
cameras having very small pixels.
Being fast is nice, and there are
some great benefi ts that make imaging
easier. One is short exposures. You can
take pleasing photos of many astro-
nomical targets with exposures of just
a few minutes, or you can spend a lot
of time on an object to pick up really
faint details, such as the extended
dust or nebulosity that exists around
some popular objects. Short exposures
stacked together also mean less rigor-
ous demands on your mount’s trackingpCelestron’s 8-inch Rowe-Ackermann Schmidt
Astrograph (RASA 8) optical tube assembly
packs a blazing-fast f/2 imaging speed into a
lightweight package.