Specifications

Section 1 - Introduction
Page 19
1.1.18. 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 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 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 each of our cameras 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 arcseconds per pixel. Also note that cameras with smaller pixels may be binned 2x2
or 3x3 to create larger pixels and expand the useful range of the camera. For example, an ST-4020M
with 7.4 micron pixels can be binned 2x2 to give 14.8 micron pixels. The overall field of view of the
CCD does not change however, and a camera with larger pixels and a larger field of view might be
preferable if it will not be used on shorter focal length instruments.