5. Refining the focus.

To change the exposure time and resize the sub-area, you need to quit focus mode. The easiest way to do this is to select "Focus" from the "Camera" menu, or simply click on the "Focus" button in the icon bar. Then re-enter focus mode by selecting "Focus" again, either from the "Camera" menu or the icon bar. You will be asked if you want to save the current image; do so if you wish. The focus dialog box will appear again. The only thing you should need to change here is the exposure time. Because the CCD is a linear detector (it will collect twice as much light if you double the exposure time), you can choose a new exposure time based on your previous peak values. If your peak values were around 30,000 in one second, for example, then a half second should give you peak values close to 15,000. When choosing a new exposure time make sure that the peak value will remain above 1000. A general exposure time at this point might be 0.1 seconds.

Click "Ok." This part should be familiar. Once the image appears, center the transparent box on the white circle (the star image) and resize the box so that there is one circle diameter between the edge of the circle and the border of the box. Then click on the "Resume" button.

Now you can begin turning the focus knob again.

Three final pieces of advice that will help you to reach the optimum focus follow:

First, as the star comes to focus and the image of the star decreases in diameter, it will be hard to tell visually if any improvement is being made by further focusing. To keep the visual indication of focus, magnify the star image by choosing either 2:1 or 4:1 (4:1 is best) from the "Magnification" pull down menu in the "Contrast" window.

Second, as the peak value increases, you will notice that it tends to undergo rather extreme variations. This is due to the fact that as the star image gets smaller and most of the light is being collected into fewer and fewer pixels, a slight shift of the star in position on the chip will cause light to be divided amongst pixels resulting in a lower peak value. A fluctuating peak value means you are getting close to focus. In order to keep using the peak value as an indicator of focus quality, note the peak value of two or three image updates (between turns of the focus knob) and then roughly average those values to find a general peak value.

Third, if you should happen to go past optimum focus, the image of the star will start to get larger again and the peak value will decrease. It is best to approach focus by turning the knob counterclockwise, so if you are already turning the knob counterclockwise try not to go past focus. If you are turning clockwise, however, going a bit past the focus and coming back is recommended.

In the end, deciding when you have reached the optimum focus is up to you, but practice will help.

Finally, if you want to test your focus, try imaging a globular cluster.

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