Camera Resolution - SBIG STX Series Operating Manual

1.15. Camera Resolution

Resolution comes in two flavors these days. In the commercial world of digital devices, the
word resolution is often used synonymously with the number of pixels used in a device. You are
used to seeing ads for scanners with a "resolution" of 2,000 x 3,000 pixels, etc. Computer
monitors have various "resolution" settings which are basically the number of pixels displayed.
We use the word here in its literal sense, which is ability to resolve detail. This has nothing to
performed and the final display size desired. By way of example, if the atmosphere and optical
system allow the smallest star images of 2.6 arcseconds in diameter (FWHM) then one needs a
telescope focal length and pixel size that will let each pixel see 1/3 of 2.6 arcseconds. In this
example the pixel field of view should be about 0.86 arcseconds per pixel for an optimum
balance of extended object sensitivity to resolution of fine detail. If you aim for a pixel FOV of
about 1 arcsecond per pixel through a given focal length, then you should be fine for the majority
of typical sites and imaging requirements. If your seeing is much better than typical, then you
should aim for less than one arcsecond per pixel. If your seeing is much worse than typical, then
you can get away with 1.5 or even 2 arcseconds per pixel. The table at left shows the field of
view per pixel for several pixel sizes at various focal lengths. Select the focal length or range of
focal lengths of your telescope(s) and look across for a pixel size that yields a field of view close
to 1 arcsecond per pixel. Note also that the exception to this rule is planetary imaging where
sensitivity is not an issue and resolution is paramount. In this case, aim for 0.5 or 0.25
do with the number of
pixels, rather it is governed
by the size of each pixel and
the focal length of the
optical system. Typically,
seeing limits the resolution
of a good system. Seeing is
often measured in terms of
the Full Width Half
Maximum (FWHM) of a star
image on a long exposure.
That is, the size of a star's
image in arcseconds when
measured at half the
maximum value for that star
in an exposure of many
seconds. As a general rule,
one wants to sample such a
star image with no less than
2 pixels. It is preferable to
sample the star image with 3
or more pixels depending on
the processing steps to be
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