Focusing is one of the most character building steps when imaging with a CCD camera. You will find, however, that a well focused image is definitely worth the trouble and time that it takes to achieve optimum focus.

The software that controls the camera and downloads data from the processor in the camera is called CCDOPS. These steps describe using CCDOPS to focus the optical system formed by an 8 inch Meade telescope and an attached ST-6 CCD camera.

Please use common sense and be careful when using the CCD cameras.

Remember that it’s best not to focus on an extremely bright star.

1. Some notes on using CCDOPS

One thing to keep in mind when using CCDOPS is that to access menus, buttons, or text fields which appear in a window, you must click on the window first. Clicking on a window makes that window the “active” window on the screen and then you can access any of the window’s features.

One feature of CCDOPS you might find useful is the icon bar. If this is not visible you can make it so by choosing “Show Icon Bar” from the CCDOPS “Edit” menu.

If you want to save any image while focusing simply choose “Save” from the “File” menu or click on the “Save” button in the icon bar.

2. The focus dialog box

From the “Camera” menu choose “Focus” (or click Focus on the icon bar). This will bring up the focus dialog box. The important fields in this box are “Frame Size” and “Exposure Time.”

For the “Frame Size” field, the most common setting when focusing is “Planet.”

The appropriate “Exposure Time” can be somewhat difficult to determine. It has to be long enough to produce an image of the out-of-focus star, but short enough so that focusing does not take too long. It is better to start with an exposure time long enough to locate the star rather than worrying about the overall focusing time. While variables such as the star’s magnitude, the focal length of the telescope, and the filter that you are using all play a role in determining the “right” exposure time, a general starting time (for any of the 8 inch scopes, a typical star, and a clear filter) is one or two seconds (units of the “Exposure Time” field are seconds, so for one second enter “1”).

The other fields in the focus dialog box should be fine as they are set but typical values are as follows:

  • Update Mode: Automatic
  • Exposure Delay: 0
  • Readout Type: Low

Choose “Ok” when you are finished entering this information.

3. Select a sub-area of the chip for real-time image updates

When the focus window appears, indicating that you are in focus mode, the status bar in the focus window should be active as a dark frame and a full frame light image are taken. After being digitized and downloaded, the image will appear in its own window. A separate “Contrast” window will also appear. A typical full frame acquisition time is about 30 seconds.

The image should contain a donut-shaped object, a large whitish ring with a darker center (“donut hole”). The donut is the out of focus star. As the star comes into focus, the donut will decrease in size until it becomes a small white circle. Optimum focus has been reached when this circle is as small as possible.

To reach optimum focus, you will use the near real-time image update provided in “Planet” mode. This will allow you to see, almost instantaneously, the effects (on the image of the star) of turning the telescope’s focus knob. This real time update is achieved by selecting a sub-area of the chip with the transparent box (outlined in white) in the image window. By choosing a smaller section of the chip, a full image frame does not need to be downloaded each time the image is updated. Image acquisition times will therefore be much faster than the regular 30 seconds. The smaller this selected sub-area is made, the quicker the download times will be.

Ideally, the sub-area that you choose can be small because a star only covers a small part of the chip. At this point, however, your donut is probably fairly large and a small box would not be helpful. You can use a medium sized box, however, by realizing that the star will come to focus at the center of the donut hole. Center the transparent box on the approximate center of the donut hole (move the box by clicking and dragging with the mouse). Then resize the box (click and drag on one of the two small squares in the top left hand and bottom right hand corners of the transparent box) so that it includes a small part of the donut’s whitish ring. This will allow you to verify, in the next step, that you are turning the focus knob in the correct direction.

After positioning the box, click on the “Resume” button in the “Focus” window. Images will now be continuously downloaded and updated.

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:

  • Counterclockwise: 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.
  • Clockwise: 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.

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.