Sky & Telescope - March 2023 - 61

QUANTUM EFFICIENCY GRAPH: LEAH TISCIONE / S&T
Quantum efficiency (%)
100
IMX571M CMOS
20
40
60
80
400
500
600
700
800
Wavelength (nanometers)
SHOOTOUT AT THE QE CORRAL The newest CMOS detectors are
typically more efficient at counting photons than most CCD sensors that
amateurs use. This graph compares a popular CCD and CMOS sensor
with similar-sized pixels in cameras manufactured by QHYCCD.
There are, of course, limits to this resolution trick. Firstly,
one can potentially oversample the resolution possible with
your optics and sky conditions. You don't gain anything if
your local seeing rarely permits resolution of less than 2
arcseconds per pixel. You'll just have a bigger, blurrier image.
Another problem may have to do with the spot size of your
astrograph. Too small a pixel can begin to resolve optical
aberrations in your system that weren't noticeable when you
shot with a detector having larger pixels. (Don't worry, there's
nothing actually wrong with your telescope.)
Goodbye to Guiding?
There are other ways a new CMOS camera can potentially
simplify your imaging technique.
The lack of read noise with these detectors means exposures
900
1000
can be short enough that autoguiding becomes less crucial
and, in some cases, may be eliminated entirely.
With these new cameras, you can take hundreds or even
IMX814M CCD
thousands of 1-minute exposures and simply stack them
together to achieve an image just as deep and detailed as any
produced with a CCD camera and sub-exposures of 20-minutes
apiece. Most popular deep-sky image-processing software
(including PixInsight and MaxIm DL) can automatically
discard any trailed images. And eliminating the need for a
guidescope or off-axis guider, as well as a separate autoguiding
camera and its cables, is particularly attractive to imagers
who set up their equipment each night.
However, it's important to understand that this new
approach does come with additional costs. Recording hundreds
of 16-bit, full-frame images requires a lot of storage.
In addition, you'll need a fairly robust processor and lots of
RAM to reduce tens of gigabytes of data into a final image.
This may mean upgrading to a new computer. As usual,
there's no free lunch.
But aside from calibrating and stacking many more
images, processing CMOS camera data is practically the same
as for a CCD model. One approach that may not work for
some CMOS cameras is to scale short dark frames to calibrate
longer exposures. This was a useful shortcut that CCD imagers
employed to avoid recording lots of various-length dark
exposures (see S&T: Nov. 2019, p. 36). The technique permitted
accurate scaling of long dark exposures by measuring the
dark current and signal captured in a zero-length, or bias,
exposure in order to be used to calibrate shorter light and
flat-field calibration frames. However, this approach does not
work well with some CMOS cameras. If your CMOS images
show amp glow or don't calibrate well, try eliminating bias
CCD
CMOS
LOW NOISE The 5-minute dark frame from a KAF 16803 CCD at left is processed to display light and dark lines that arise from subtle differences
in sensitivity in the columns of the detector as well as a curved pattern resulting from an infrared flash needed to purge a residual bulk charge that
plagued this and other large CCD detectors. None of these are visible in the 5-minute dark frame taken with an IMX455 CMOS sensor seen at right.
sk yand tele scope .o r g * MARCH 2023 61

Sky & Telescope - March 2023

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