Section 1 - Introduction
Page 20
1.1.19. Camera Field of View
The field of view that your camera will see through a given telescope is determined by the focal
length of the telescope and the physical size of the CCD chip. This also has nothing to do with the
number of pixels. Through the same telescope, a
CCD that has 512 x 512 pixels at 20 microns
square will have exactly the same field of view as
a CCD with 1024 x 1024 pixels at 10 microns
square even though the latter has four times as
many pixels. One can vary the focal length to
vary the field of view. Using a focal reducer to
shorten the focal length will increase the field of
view (and make the image brighter in the
process). Using a barlow or eyepiece projection
to effectively lengthen the focal length of the
telescope will decrease the field of view (and
make the image dimmer in the process). In order
to determine the field of view for a given CCD,
note the CCD's length and width dimensions in
millimeters (from the camera specifications) and
use the following formula for determining the
field of view for that CCD through any telescope:
(135.3 x D ) / L = Field of View in arcminutes
where
D
is the length or width dimension of the CCD in millimeters, and
L
is the focal length of
your telescope in inches. So, for example, if you wanted to know the field of view of the new STL-
4020M camera when attached to a 5" F/6
telescope you would first determine the focal
length of the telescope by multiplying its
aperture, 5 inches, by its focal ratio, 6, to get its
focal length, 30 inches. The CCD dimensions are
15.2 x 15.2 mm. To calculate the field of view
multiply 135.3 x 15.2 = 2,057 and then divide by
30 = 68.6 arcminutes. By way of comparison, the
field of view of the longest dimension of the STL-
11000M through the same telescope would be
135.3 x 36 = 4,871 divided by 30 = 162.4
arcminutes. The table above shows the calculated
field of view in arcminutes for each of the large
format CCDs at various focal lengths. Keep in
mind however that when you vary the CCD field
of view you are also varying the field of view for
each pixel and are therefore also affecting the
resolution of your system.