Specifications

Section 3 - At the Telescope with a CCD Camera
Page 40
3.9.4. Autoguiding and Self Guiding
The CCDOPS software allows the Research Series cameras to be used as autoguiders and self-
guiders through the commands in the Track menu. While these systems are not stand-alone
like the old ST-4, but require a host computer, they can accurately guide long duration
astrophotographs and CCD images with equal or superior accuracy. Their sensitivity is much
greater than an ST-4, and the computer display makes them easier to use.
When functioning 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. While autoguiding alleviates the user of the tedious task
of staring through an eyepiece for hours at a time, it is by no means a cure to telescope drive
performance. All the things that were important for good manually guided exposures still
exist, including a good polar alignment, rigid tubes that are free of flexure and a fairly good
stable mount and drive corrector. Remember that the function of an auto guider is to correct for
the small drive errors and long term drift, not to slew the telescope.
One of the reasons that SBIG autoguiders are often better than human guiders is that,
rather than just stabbing the hand controller to bump the guide star back to the reticule, it gives
a precise correction that is the duration necessary to move the guide star right back to its
intended position. It knows how much correction is necessary for a given guiding error
through the Calibrate Track command. The Calibrate Track command, which is used prior to
autoguiding, exercises the telescope's drive corrector in each of the four directions, measuring
the displacement of a calibration star after each move. Knowing the displacement and the
duration of each calibration move calibrates the drive's correction speed. Once that is known,
the CCD tracker gives the drive corrector precise inputs to correct for any guiding error.
When self-guiding is selected by invoking the Self Guiding command under the Track
Menu, the computer prompts the user for the exposure time for the tracking and imaging
CCDs. Once these are entered, the computer takes and displays an image with the tracking
CCD, and the user selects a guide star using the mouse. Guide stars that are bright, but not
saturating, and isolated from other stars are preferred. Once the star is selected, the computer
starts guiding the telescope. When the telescope corrections settle down (usually once the
backlash is all taken up in the declination drive) the user starts the exposure by striking the
space bar. The computer then integrates for the prescribed time while guiding the telescope,
and downloads the image for display.
A calibration star should be chosen that is relatively bright and isolated. The calibration
software can get confused if another star of comparable brightness moves onto the tracking
CCD during a move. The unit will self-guide on much fainter stars. Tests at SBIG indicate that
the probability of finding a usable guide star on the tracking CCD is about 95% at F/6.3, in
regions of the sky away from the Milky Way. If a guide star is not found the telescope position
should be adjusted, or the camera head rotated by a multiple of 90 degrees to find a guide star.
We recommend that the user first try rotating the camera 180 degrees. Rotating the camera will
require recalibration of the tracking function. [Note: CCDSoftV5 software allows SBIG
cameras to calibrate and track in any orientation, similar to the STV video autoguider].