Short of an autofocuser, the easiest way to focus a CCD camera is using the built-in features of the camera control software.  There is generally no need for other focusing devices such as Hartmann masks, etc.

Focusing is usually divided into two stages.  The section below describes focus settings, finding a focus star, and then the two stages of actually focusing the telescope.  The first stage uses the entire CCD chip to get close to focus.  The second stage uses a smaller portion of the chip (subframe) to speed up the process and utilizes the CCD imaging software to reach an accurate focus.

Exposure Settings

Select the Focus button on the CCDOPS Toolbar.

Above:  The CCDOPS Toolbar.

The Focus settings window pops up, allowing you to select focus parameters.

The following section describes each parameter and appropriate settings.

Exposure Time

The appropriate exposure time will depend on the telescope, CCD camera, and brightness of the star being used for focusing.  Typically, 1 second is a good starting point.  This can be changed later once you start the fine focusing stage.

Frame Size

Start out by selecting Full.  This will be changed in a later step to speed up the focusing procedure.  For now you will want to see the entire chip.  By setting Resolution to Auto in the Setup menu (previous section) the full frame will be binned for a faster download.

Update Mode

Automatic update will take continuous images until interrupted.  The only time you might not want to use this feature is if your camera has a very slow readout time (like an old parallel-port large-format CCD), but any modern CCD will read out so fast as to make continuous focusing preferable.

Exposure Delay

Number of seconds delay between exposures.  This can stay at 0.

Turbo Mode

Using an ST-7/8/9/10/2000 camera, setting Turbo Mode to On forces the shutter to stay open decreasing time between exposures.  Usually this is left off since there is little delay anyway.  For other cameras, leave this Off.

Finding a Focus Star

It is easiest to start by focusing on a relatively bright star.  Do not choose a star that is too bright.  Begin by aiming to a moderately bright star (about 2nd or 3rd magnitude).  Start the Focus mode to begin taking exposures.

You should see a star (likely out of focus) somewhere in the image.  Use either the telescope's hand controller or the N-S-E-W buttons in the Focus window (if your telescope is connected to the computer) to center the star in the field of view.

Tip:  Note the direction the star moves in the field when pushing each button.  Often two of the buttons will move the "correct" direction and two will be reversed.

First Stage

Start bringing the star toward focus by making the image smaller.  The first time you turn the focus knob you have a 50/50 chance of going the right way.  You might make note of which way you need to turn the knob to focus the CCD relative to visual focus for future reference.

Continue focusing until the star appears as a small dot.  At this point pause the Focus mode if you are using the Auto Update mode.

Above:  A visually focused star image.  Using the software, further improvements can be made.

Second Stage

Use the Frame pull-down menu in the Focus window to select Planet.  Click Resume.  At this point the CCD will take one more full frame image and display it with a box, as below.

Above:  CCDOPS displays a subframe box in the main image.

Click and drag the small boxes in the corners of the box to shrink it so it just surrounds the star.

Above:  Selecting the focus subframe from the full-frame image.

When you click Resume, only the small part of the CCD chip within the box is read out making the download time far shorter.  The binning will also be 1x1 now, instead of 3x3.

Above:  Focus subframe.

You will now use the Peak value in the Focus window to determine the best focus.  The closer the telescope is to focus, the more concentrated is the light from a star.  The more concentrated the light, the higher the value of the brightest pixel in the image, the Peak value.  By maximizing the Peak value, you accurately focus the telescope.

The peak value is read out after each download, so be sure to wait for a new image each time you turn the focus knob.

Tip:  Telescope vibration and seeing conditions affect the Peak value.  Let a few exposures go by for each adjustment you make, especially once you get close to focus.  Touching the telescope to focus it can smear the star image, lowering the Peak value.  The number will jump around somewhat due to atmospheric turbulence, so mentally average a few sequential Peak values to get a better idea of the actual value.

You will see the values (on average) increase as the telescope approaches best focus.  Once you have passed focus, you will see the numbers begin to drop off again.  Reverse focus direction to return to the highest Peak value.

Above:  A Peak value of 5201 was determined to be the point of best focus.

Next, Finding Targets...

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