Sky and Telescope - March 2016 - 62

S&T Test Report

needed for my mount, and another plate to the top for
mounting my SBIG SG-4 autoguider.
You can loosen the tube rings to allow the telescope
to rotate for placing the camera or eyepiece at a convenient angle. But this proved diﬃcult to do without either
shifting the mount or having the tube slide down the
rings, compromising the balance. Instead, I settled on
an orientation that worked well for all the imaging I did
in the eastern and southern sky.

small detectors typically found in high-speed video cameras used for lunar imaging, the optional Quattro Coma
Corrector is an essential accessory for deep-sky imaging
with any DSLR camera or today's CCD cameras with
mid-sized detectors.
The Quattro Coma Corrector ﬂattened the ﬁeld
almost completely, leaving star images ever so slightly
elongated only at the extreme corners of a full-frame (24
× 36mm) DSLR sensor - performance that I consider
more than acceptable. While this telescope has a large,
3-inch secondary mirror, there is still a signiﬁcant
amount of vignetting in the outer third of the ﬁeld of a
full-frame DSLR camera. Also apparent is a darkening
along the bottom edge of the frame, and along the top to
a lesser extent, caused by shadowing from the camera's rectangular mirror box. This isn't any fault of the
Quattro - it's an eﬀect that shows up in all full-frame
DSLR cameras paired with fast astrograph systems. The
faster the optics, the worse the shadowing eﬀect, which
can vary depending on the camera model. It is less of
an issue for photographers using cameras with smaller,
APS-size sensors.
Nevertheless, imagers should be prepared to record
and apply ﬂat-ﬁeld calibration frames that eliminate
most, if not all, of the vignetting. Just be warned, this
is the price to pay for using fast focal-ratio astrograph
systems with DSLRs, especially full-frame cameras.

Imaging Performance

Visual Use

Without the optional coma corrector, the 8-inch Quattro presented a sharp ﬁeld of view over a central 12-mm
circle of the camera's frame, typical for an f/4 Newtonian. Outside of this small area, stars appear as progressively larger teardrop shapes radiating from the center.
While this is suﬃcient for lunar photography with the

Although the Quattro series is marketed primarily for
imaging, an f/4 Newtonian makes an attractive "richﬁeld" visual telescope. I spent some nights observing
with the scope to evaluate its visual performance.
Despite its small coma-free ﬁeld, I never found the
coma at the edge of the ﬁeld to be objectionable even

The primary mirror cell is well ventilated and includes knurled
collimation knobs that are easy to turn by hand and lock down
securely. Note that they protrude beyond the end of the cell,
requiring care in transport to ensure they do not get damaged.

Left: Without the optional coma corrector, the Quattro exhibits strong coma on full-frame cameras, with sharp stars in only the central 12-mm of the
frame. Right: While using the telescope with the Quattro Coma Corrector, some light falloﬀ toward the corners is inevitable in such a fast system. In
addition, the mirror box of a full-frame DSLR camera added shadowing along the top and bottom edges of the frame. This image is processed with
increased contrast to better display the vignetting.

March 2016 sky & telescope

Sky and Telescope - March 2016

Table of Contents for the Digital Edition of Sky and Telescope - March 2016