Developers guide

16
2.5 CCD
The CCD, or Charge Coupled Device, is a digital camera, similar to those commonly found in the
marketplace today. The telescope is equipped with a Wright Instruments CCD camera with a
resolution of 1152 x 770 pixels (digitised at 16 bits/pixel). The CCD is controlled using a Linux
character device driver developed by Michael Ashley.
2.5.1 Focus control
Focus is achieved by adjusting the distance of the CCD
from the primary mirror inside the telescope. This is
achieved by using a stepper motor, controlled via the
focus motor interface, shown in Figure 2-3. Software
manipulates this interface through the /dev/focus device
special file.
Two limit switches (upper and lower) are also installed to
prevent damage to the focus mechanism.
2.6 Telescope movement
Astronomers use the equatorial coordinate system to describe the position of an object in the sky
according to its right ascension and declination. This is discussed in more detail in Appendix E.1.
In order to simplify the calculation of these equatorial coordinates, the mount itself has been
aligned with the earth’s North – South axes. A viewer standing in the control room looking toward
the telescope is facing directly South.
The mount itself can be rotated on two axes:
• Right ascension (RA) aligned with the North – South axis
• Declination (Dec) aligned with the East – West axis
There are two pancake servo motors, one for each axis. The RA motor, shown below in Figure
2-4, uses a harmonic gearbox and a friction drive to provide rotation, resulting in a very accurate
control of the axis.
Unlike the RA axis, the declination motor (Figure 2-5) rotates its axis via a worm gear. The anti
backlash motor, discussed in 2.6.1, is required to eliminate backlash..
Figure 2-3 Focus motor interface