Santa Barbara Instrument Group ST-5C, Руководство по эксплуатации

Страница: 9 / 42

Section 1 - Introduction to CCD Cameras
Page 5
the temperature at a user determined value for long periods. As a result, exposures up to an
hour long are possible, and saturation of the CCD by the sky background typically limits the
exposure time.
The sky background conditions also increase the noise in images, and in fact, as far as
the CCD is concerned, there is no difference between the noise caused by dark current and that
from sky background. If your sky conditions are causing photoelectrons to be generated at the
rate of 100 e
-
/pixel/sec for example, increasing the cooling beyond the point where the dark
current is roughly half that amount will not improve the quality of the image. This very reason
is why deep sky filters are so popular with astrophotography. They reduce the sky background
level, increasing the contrast of dim objects, and can be used to advantage with the CCD
camera.
1.2.2. Dark Frames
No matter how much care is taken to reduce all sources of unwanted noise, some will remain.
Fortunately, however, due to the nature of electronic imaging and the use of computers for
storing and manipulating data, this remaining noise can be drastically reduced by the
subtraction of a dark frame from the raw light image. A dark frame is simply an image taken at
the same temperature and for the same duration as the light frame with the source of light to
the CCD blocked so that you get an "image" of the dark. This dark frame will contain an image
of the noise caused by dark current (thermal noise) and other fixed pattern noise When the
dark frame is subtracted from the light frame, this pattern noise is removed from the resulting
image. The CCDOPS software can automatically perform this.
1.3. The Various CCD Parameters and How they Affect Imaging
If you scan the CCD related literature you will see a slew of new terms describing CCDs and
their performance. In this section we will discuss the more common CCD parameters and their
effects in an imaging application.
1.3.1. Pixel Size
Every CCD, independent of the manufacturer, is divided into a relatively large number of small
pixels. In your CCD camera the imaging area of the CCD is 3.2 mm x 2.4 mm and the pixels are
10 microns square (1 micron is one thousandth of a millimeter or roughly 0.00004"). If you
looked at the CCDs available from the various manufacturers you would see that their pixels
typically vary from 7 microns on the small end to 25 microns on the large end. There are
advantages and disadvantages associated with the size of pixels in a CCD.
While having small pixels may seem advantageous in terms of offering "higher
resolution", large pixels gather more light and are thus "more sensitive". You can also adjust
your telescope configuration to accommodate various size pixels, using faster telescopes to
increase the speed of small pixel CCDs or longer focal lengths to increase the resolution of
larger pixel CCDs.
Often times the basic goal is to match the CCD resolution to the telescope resolution and
to the overall seeing. It would be a waste to use a pixel size of 7 microns on a telescope with a
spot size of 25 microns or to configure the CCD/telescope to produce an image scale of 10 arc-
seconds per pixel when you're looking for fine planetary detail.
1.3.2. Full Well Capacity
The full well capacity of a CCD is the number of electrons each pixel can hold before the pixels
are full. While this may seem like an important consideration in choosing a camera, you need
to think about how the camera is used.