4. Begin focusing.
You can now start turning the telescope's focus knob. Use the following guidelines to determine which direction you should be turning the knob:
- Turn the focus knob this way if, before you attached the CCD camera to the telescope, you had focused the telescope for your eye using just the low power eyepiece.
- Turn the focus knob this way if, before you attached the CCD camera to the telescope, you had focused the telescope for your eye using both the right angle and the low power eyepiece.
With a large donut, it is safe to start with either full or half turns of the knob. It's then a good idea to wait for at least two image updates before turning the knob again so you can be sure of the last turn's effect on the image. When the donut becomes a white circle you will want to turn the focus knob only a quarter or an eighth of a turn between image updates. As you approach optimum focus you will want to turn it even less. You will gain a better feel for how much to turn the focus knob with practice.
After you have turned the focus knob once or twice verify that you are turning the knob in the right direction by making sure that the donut's whitish ring is moving into the image rather than out of it (the donut is getting smaller). If it disappears from the image completely then you are turning the focus knob the wrong way.
As you continue to focus, you will notice that besides the donut becoming smaller, the "Value" reading under the "Peak" section of the "Contrast" window will increase. This peak value gives you another indication of focus quality, besides the visual one. Your goal in terms of the peak value is to make it as large as possible. Once the donut becomes a circle it is possible that the peak value will become so large that it will reach the saturation value of 65,535. If this happens you will need to decrease the exposure time. At the same time you can resize the sub-area that you originally selected. Even if you do not reach saturation, it is still a good idea to reduce the exposure time and resize the selected sub-area once the star image has become much smaller than the original sub-area. Doing so will make it easier to reach the point of optimum focus.