Santa Barbara Instrument Group STXL Advanced Series, Руководство по эксплуатации

Страница: 16 / 33

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 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 above 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 arcseconds per pixel. Also
16