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 describe focus settings, finding a focus star, and then the two stages of 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 tab in the MaxIm CCD window.

You will set all of the necessary focus parameters in this window.

The following section describes each parameter and appropriate settings.

CCD/Guider

This allows you to select either the main CCD chip or the guiding CCD if you are using an SBIG self-guiding camera or separate autoguider.  For focusing purposes this is set to CCD.

Seconds

This is where you set the exposure time for the focus images.  The appropriate exposure time will depend on the brightness of the star, the telescope and CCD being used, and the binning mode.  Typically, 1 second is a good starting point.

Binning

Binning the CCD chip 3x3 increases the sensitivity and cuts the download time by a factor of 9.  Quick exposures and downloads make focusing simpler, so setting Binning to 3 is highly recommended for the initial focusing stage.

Delay

During continuous focusing, the process can only be interrupted during an exposure (not during a download).  If the exposure is very short--less than about 0.5 seconds--it is very difficult to hit the Stop button during that brief span.  Setting a delay between exposures allows more time to stop focusing.  Usually a 1 second delay is sufficient.

Dark

Checking this box will begin the focusing sequence by taking a dark frame which is then subtracted from the focus exposures.  This is recommended, and as long as the temperature does not change significantly during the focus procedure, only a single dark will be taken at the very beginning of the sequence.

Continuous

This mode is recommended.  With short exposures and quick downloads this provides almost real-time imaging of the focus star, allowing easy adjustments to be made.  If you have an older large-format CCD with a parallel-port connection which has slow downloads even in binned mode, you may find it easier to use the manual update mode (leave Continuous unchecked) to begin with.  You will still use continuous mode in the second stage of focusing.

Subframe

This feature is covered in the second stage of focusing section below.  For now, leave the default settings.

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).  Click Start Focus 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 Nudge buttons in the Telescope Control window (if the telescope is linked to the computer) to center the star in the field of view.

Above:  Telescope Control window.  The Nudge buttons at right can be used to center an object in the CCD.

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, especially when using a German equatorial mount.

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 stop the focus mode if you are using the Continuous mode. 

Above:  A visually focused star image.

Second Stage

In the image window, draw a small box (maybe 50x50 pixels or so) around the star.  You will see the numbers change in the Subframe portion of the Focus window.  You may also select a portion of the image by entering numbers into the Subframe boxes, but drawing the subframe box in the image window is far easier.

Above:  A subframe selected around the star.

You should select a Binning of 1 at this point.  This will increase the resolution and accuracy of the focus procedure.  The subframe will allow fast downloads even with slower camera connections.  Be sure Continuous is checked.

Select the Inspect tab of the MaxIm CCD window.  This allows you to see the profile of the star being imaged along with information about the star which you will use to achieve an accurate focus.

Above:  An out-of-focus star in the Inspect window.

The important pieces of information for focusing are described below.

Max Pixel

Gives the value of the brightest pixel in the image.  The closer the telescope is to focus, the higher this value will be.

FWHM

Stands for Full Width at Half Maximum.  This is the width of the star image at half the peak value of the star, a common measure of focus, seeing, and resolution.  Ideally the minimum value is achieved for best focus.

Half Flux Dia.

The Half Flux Diameter is similar to FWHM and is used primarily by autofocusing routines.  It describes the diameter which encompasses half the light from the focus star.  It should also be minimized, but Max Pixel and FWHM are the numbers to really watch.

You will now use the Max Pixel and FWHM values in the Inspect window to determine the best focus.  Click the Start Focus button to continue focusing the telescope.

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 Max Pixel value.  By maximizing the Max Pixel value and minimizing the FWHM 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 Max Pixel and FWHM values.  Let a few exposures go by for each adjustment you make, especially once you get close to focus.  Touching the telescope to focus can smear the star image, lowering the Max Pixel value and raising the FWHM value.  The number will jump around somewhat due to atmospheric turbulence, so mentally average a few sequential values to get a better idea of the actual numbers.

You will see the values (on average) increase as the telescope approaches best focus.  Once you see the number begin to drop off again, reverse focus direction to return to the highest Max Pixel value (lowest FWHM).

Above:  A Max Pixel value of 55661 and a FWHM of 2.54 were determined to indicate the point of best focus.

Tip:  Use the Large View button to increase the display of the Focus Statistics in big font that is readable while standing at the telescope and focusing.

Next, Finding Targets...

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