skyandtelescope.com • NOVEMBER 2019 71
Flat-Field Challenges
Proper image-calibration is essential
for good astrophotography results. This
includes not just darks, but also quality
fl at-fi eld calibration frames. The RASA 8
has a large central obstruction, which
produces a “doughnut” of illumination
on larger detectors that cannot be cor-
rected by using post-processing tricks.
Quality fl at-fi eld calibration will reliably
correct this odd illumination, but fl ats
can be tricky to shoot with the RASA 8
for two reasons.
First, at f/2 it’s diffi cult to shoot twi-
light fl ats because it’s very easy to over-
expose the sky. A more reliable option
is to shoot fl ats using an electrolumi-
nescent panel or even T-shirt fl ats, but
the camera itself is jutting out past the
front corrector plate, and there’s no way
to seat the panel or stretch a T-shirt on
the front of the tube assembly. A practi-
cal solution is to place a dew shield on
the RASA 8. The scope really requires
a dew shield anyway, and even in dry
climates, stray light from the side can
very easily enter the telescope, creating
unwanted glare and gradients.
Celestron’s optional Dew Shield for
C6 and C8 Tubes ($26.95) provides a
very handy mechanical means to cover
the front of the scope with a fl at-
fi elding accessory, or even to completely
cover the scope in order to shoot dark
or bias frames with cameras that don’t
have a mechanical shutter.
qAn additional accessory that should be purchased with the RASA 8 is a fi rm dew shield (left),
such as Celestron’s Dew Shield for C6 and C8 Tubes. This ensures the corrector plate stays dew-
free but also allows a fi rm seating to place an electroluminescent panel (right).Recommendations
There are no shortcuts to good results.
Shooting with a fast instrument makes
some things easier, including track-
ing and guiding, but you still need a
compatible camera, sharp focus, and
precise spacing of your camera and sen-
sor. You also need to properly calibrate
your images, and this includes fl at
fi elds. Without the integrated cooling
fan, the scope cools down very slowly,
so don’t forget to run it for about a half
hour. It’s really such a good idea that
Celestron should fi gure out how to add
the option to all of its SCT telescopes.With the 8-inch Rowe-Ackermann
Schmidt Astrograph, Celestron has
delivered a scaled-down version of its
premier 14- and 11-inch astrographs
that can work well for amateur astro-
photographers who are primarily
interested in wide-fi eld, one-shot color
imaging of deep-sky objects. Once you
get it all dialed in for your equipment,
the sky is literally the limit.■Software Bisque software engineer
RICHARD S. WRIGHT, JR. can often be
found managing a small fl eet of equip-
ment at most major star parties.pLeft: Once everything is dialed in, shooting at f/2 makes short work of many bright Messier objects. This image of M16 in Serpens consists of 31
unguided 1-minute exposures using the Starlight Xpress Trius-SX814C CCD camera. Right: With proper spacing, the RASA 8 delivers a reasonably
wide and well-corrected fi eld. This image of globular cluster M13 in Hercules consists of 74 stacked 30-second exposures using the Trius-X814C
camera at the Texas Star Party.