Sky & Telescope - July 2020 - 71

CCD. My colleague Sean Walker, who
processed the deep-sky images with
this review, noticed this as well. It was
especially true of images made through
a hydrogen-alpha ﬁlter. We don't have
apples-to-apples examples, however, so
I don't want to put too much emphasis
on our observation.
I feel the jury is still out on what
will be the best way to calibrate CMOS
images, but I did note a couple of differences compared to my usual way of
working with CCDs. The ﬁrst is that
dark frames should be made with the
same sensor temperature and exposure
time as the light frames they are used to
process. The typical method of scaling
CCD darks did not work well with this
CMOS sensor. And dark frames made
a few days before or after light frames
also didn't work as expected. This would
be an issue for those of us who build
dark-frame libraries in the course of our
normal workﬂow.
In the best of times, I ﬁnd making
high-quality ﬂat-ﬁeld calibration frames
challenging, especially when it involves
shooting through narrowband ﬁlters.
I had better luck shooting ﬂats with a
relatively low signal, rather than the
usual ones I use with CCD images that
have pixel values of at least half of a
pixel's full-well capacity. Also, I found
it better to shoot ﬂats at the camera's
full resolution and downsize them for
processing astronomical images that
were made with the camera binned 2×2,
rather than shooting ﬂats binned 2×2.

Under a Microscope
When I initially set up the camera
indoors, making test exposures with
a Nikon camera lens, I was astounded
by the QHY600M's image quality.
Snapshots of equipment at the far end
of my darkened workshop were ﬁlled
with almost unimaginable detail. The
QHY600M is indeed a full-frame camera
with remarkable sensitivity and the resolution of ﬁne-grain ﬁlm that many of us
wished for years ago. But these attributes
have a downside, since I soon discovered
that the camera's sensitivity and resolution were putting my imaging systems
under a virtual microscope.

p Three hours of total exposure captured the nebulosity in central Auriga, including IC 410 at bottom and the Flaming Star Nebula, IC 405, at upper right.

Over the course of testing the
camera, I used four astrographs that
perform well with cameras ﬁtted
with Kodak KAF-16803 CCDs having
9-micron pixels. The resolution of the
QHY600M's 3.76-micron pixels, however, revealed that three of them had
off-axis star images that really weren't
that great. The setup that delivered the
best star images across most of the chip
was the 4-inch f/5.4 Tele Vue NP101is
refractor used for all the celestial
images in this review.
And there were other issues that
arose because of the QHY600M's
extraordinary resolution. One is the
need to have the chip absolutely square
to a telescope's optical axis to achieve
maximum performance. This is true for
any large-format sensor, and fortunately the chip in the QHY600M was
notably square to the camera body. But
most imaging setups involved adapters that can wiggle around, and even
a tiny wiggle can cause star images to
appear of uneven quality across the
QHY600M's chip.
The same goes for telescopes that
change focus as nighttime temperatures
drop. One of the scopes I used typically
has to be refocused every half hour or
so during the springtime evenings when
shooting with a KAF-16803 CCD camera. But the QHY600M revealed obvious
focus changes during the course of a
single 10-minute exposure because of
the camera's resolution. Tracking was

also an issue, since even tiny guiding
errors were made obvious by the camera's resolution. And there were other,
more subtle issues that cause me to
rethink the way my imaging setups performed when subjected to the scrutiny
afforded by the camera.
The solution for many of these
issues is to simply use the camera
binned 2×2, which created pixels that
are effectively 7½-microns square.
That's still better resolution than the
KAF-16803's 9-micron pixels, but it
was enough to mask some of the issues
mentioned above. And, as an added
beneﬁt, the image ﬁ les were reduced to
a far more manageable 31 megabytes
each - something my computer hard
drives, image-processing software, and
I myself all appreciated.
I've been in the astrophotography
game long enough to remember well all
the things that confounded amateurs as
the hobby was switching from emultion-based to CCDs. To a lesser degree,
that is happening again as we transition
to CMOS detectors. But the ride will
likely not be as bumpy this time, and if
you want to get a head start on learning what the future will be like, I can
recommend the QHY600M camera; it's
a bit of the future today.
■ DENNIS DI CICCO spends a lot of
clear nights testing equipment from his
backyard observatory in Boston's western suburbs.
s k y a n d t e l e s c o p e . o r g * J U LY 2 0 2 0

http://www.skyandtelescope.org

Sky & Telescope - July 2020

Table of Contents for the Digital Edition of Sky & Telescope - July 2020