64 AUSTRALIAN SKY & TELESCOPE APRIL 2016
with the 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 × 3 6mm)DSLRsensor
— performance that I consider
more than acceptable. While thistelescopehasalarge,75-mm
secondary mirror, there is still a
significant amount of vignetting in
the outer third of the field of a full-
frameDSLRcamera.Alsoapparent
is a darkening along the bottom
edge of the frame, and along the
toptoalesserextent,causedby
shadowing from the camera’s
rectangularmirrorbox.Thisisn’t
anyfaultoftheQuattro—it’san
effect that shows up in all full-
frameDSLRcameraspairedwith
fast astrograph systems. The faster
the optics, the worse the shadowing
effect, which can vary dependingThe primary
mirror cell is well
ventilated and
includes knurled
collimation knobs
that are easy to
turn by hand
andlockdown
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 fall-off 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.
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.Visual use
Although the Quattro series is
marketed primarily for imaging, an
f/4 Newtonian makes an attractive
‘rich-field’ 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 with wide-field eyepieces.
Views with many premium
eyepieces, including Tele Vue’s
4 1-mm Panoptic, 31-mm Nagler
and 17-mm Ethos eyepieces, were
stunning. Crisp, round stars were
visible across all but the outer
25% or so of the field. (Note thatTes t Re por t