Specifications

Section 3 - Advanced Topics
Page 22
Color imaging places some interesting requirements on the user that bear mentioning.
First, many color filters have strong light transmission in the infrared (IR) region of the
spectrum, a region where CCDs have very good response. If the IR light is not filtered out, then
combining the three images into a color image can give erroneous results. If your Blue filter has
a strong IR leak (quite common), then your color images will look Blue. For this reason, you
place an IR blocking filter in front of the three color filters.
Second, since you have narrowed the CCD's wavelength response with the interference
filters, longer exposures are required to achieve a similar signal to noise compared to what one
would get in a monochrome image with wide spectral response. This is added to the fact that
tri-color images require a higher signal to noise overall to produce pleasing images. In black
and white images your eye is capable of pulling large area detail out of random noise quite
well, whereas with color images your eye seems to get distracted by the color variations in the
noisy areas of the image. The moral of the story is that while you can achieve stunning results
with CCD color images, it is quite a bit more work. Fast optical systems, f/2 through f/6.3,
work best. Focal reducers also help.
3.4. Autoguiding
The CCDOPS software allows the CCD camera to be used as an autoguider through the
commands in the Track menu.
When used as an autoguider, the CCD camera repeatedly takes images of a guide star,
measures the star's position to a fraction of a pixel accuracy, and corrects the telescope's position
through the hand controller input. While autoguiding alleviates the user of the tedious task of
staring through an eyepiece for hours at a time, it is by no means an end-all cure to telescope
drive corrector performance. All the things that were important for good manually guided
exposures still exist, including a good polar alignment. Use of an auto guider is to correct for
the small drive errors and long term drift, not to slew the telescope or compensate for a poor
drive.
One of the reasons that autoguiders are often better than human guiding is that rather
than just stabbing the hand controller to bump the guide star back to the reticule, it gives a
correction that is the precise duration necessary to move the guide star back to its intended
position. It knows how much correction is necessary for a given guiding error from the
Calibrate Track command. The Calibrate command, which is used prior to autoguiding,
exercises the telescope's drive corrections in each of the four directions, measuring the
displacement of a calibration star after each move. Knowing the displacement and the duration
of each move calibrates the drive's correction speed.
To autoguide you need to connect the camera's Telescope port to your telescope's CCD
input. On Celestron Ultima, Losmandy and LX-200 mounts this is accomplished with the small
6 conductor "telephone" cable provided with the camera. Simply plug one end into the
Telescope port on the camera's CPU and the other end into the telescope. On Ultima and
Losmandy mounts you will need to unplug the hand controller and plug the auto guiding cable
in its place. On the LX-200 you plug the cable into the CCD jack. Finally if you have an older
telescope that does not support the 6 pin CCD port contact SBIG Technical Support and inquire
about custom hand controller modifications and whether you need an external Relay Adapter
box.
3.5. Field Operation
The ST-5C Advanced CCD Camera is supplied with a wall transformer for operation off
110 VAC that provides 12 VDC output to the camera. You can run the camera off of a car
battery when operated in the field. If you are interested in this please contact SBIG or refer to
Appendix A for the power jack pinouts.