Sky & Telescope - January 2025 - 63

FWHM GRAPH: LEAH TISCIONE / S&T
Pixel value
resolution that the seeing will support, assuming everything
else is optimal.
In my experience, a higher sampling rate - more like 1
to 1 of the seeing - is needed to ensure that even the tini/3
/5
est
stars fall on more than one pixel and appear round. For
instance, if the seeing hovers around 3 arcseconds (fairly
typical conditions), an image scale of 0.75 to 1 arcsecond per
pixel will capture all the resolution the seeing allows. Are you
one of the lucky few who enjoys seeing in the 1-arcsecond
range? You'll be able to take advantage of a higher-resolution
system providing a pixel scale of 0.25 to 0.3 arcsecond. My
current setup (3,940-mm focal length and 7.52-μm pixels)
produces 0.4 arcsecond per pixel, allowing me to take advantage
of those rare nights of excellent seeing conditions when
my FWHM is below 2 arcseconds.
When considering how these numbers might apply to your
equipment, it helps to calculate combinations of focal length
and pixel size that produce an image scale of 1 arcsecond per
pixel. For example, a camera with 3.76 μm pixels paired with
a focal length of 775 mm yields precisely 1 arcsecond per
pixel. My QHY367 Pro camera's 4.88-micron pixels produce
the same 1-arcsecond pixel scale with a focal length of 1,000
mm. Systems with larger pixels require long focal lengths to
achieve that same image scale.
Undersampled or Oversampled? So What?
An astrophoto is said to be undersampled when there aren't
enough pixels to capture all the detail available. The easiest
way to find out if your deep-sky image is undersampled is
to simply zoom in and look at the smallest stars. If they are
square, you could have obtained more detail with a higherresolution
system - one with a smaller pixel scale.
Conversely, an oversampled picture is one that has more
pixels than needed to show all the available information. In
Half maximum light
Full width at
half maximum
Maximum light
Pixel position on detector
 " SEEING " STARS Atmospheric seeing is commonly estimated by
examining unsaturated stars in a deep-sky image. One of the most common
metrics is the Full Width at Half Maximum (FWHM) measurement -
the width of the star's brightness profile at half the maximum brightness,
expressed in arcseconds. The lower the FWHM value, the better the
seeing conditions.
oversampled images, stars look bloated. Sometimes grossly
oversampled images reveal optical aberrations that aren't
visible in properly sampled images, particularly when using
field flatteners and focal reducers. This happens when your
pixel scale is smaller than the spot size produced by your
optical system and will show up as distorted stars most
often in the corners of the frame.
Due to the affordability of large detectors these days, imagers
are often tempted to buy cameras with detectors that have
large arrays of very small pixels. When paired with long-focallength
instruments, the resulting images are extremely oversampled.
But that same camera can still be useful, particularly
if you have a wide range of instruments with both long and
short focal lengths. The solution in these scenarios is to bin
the pixels in the camera, which groups adjacent pixels to act
1.1 arcseconds per pixel
0.4 arcsecond per pixel
sk yand tele scope .o r g * JANUARY 2025 63
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Sky & Telescope - January 2025

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