Focus Tab

The Focus Tab can control one or two focusers, and can also perform SharpStar™ autofocus.

Note: The ASCOM focuser standard does not include an explicit capability of controlling more than one focuser. This means that you must use a different driver for each focuser. Therefore it is likely that you will have to use two different brands of focusers, if you wish to have simultaneous independent focuser controls.

When operating two focusers, Focuser 1 is expected to be attached to Camera 1, and Focuser 2 is expected to be attached to Camera 2. You can switch the controls between the two focusers using the Focuser 1 / Focuser 2 selector.

Focuser Status shows the current position, temperature (if available), type of focuser, and the latest Half-Flux Diameter and Full-Width Half Maximum measurements captured using the controls on this Tab. A log window below the status shows the progress of the autofocus sequence.

The Incremental controls allow you to adjust the focuser in adjustable steps. Enter a number and click either Move In or Move Out. The actual distance moved for a given increment number depends on the focuser mechanism.

If you are running an absolute positioning focuser, e.g. one with a highly repeatable stepper or servo motor, then the Absolute function is also available. Enter the desired focus position and click Move To.

For focusers with built-in temperature compensation, you can turn on Temp. Tracking to enable temperature compensation. Some focusers have more than one profile; the profile used will be as selected in the ASCOM Setup dialog (Setup tab, Focuser Options menu, Setup). Most focusers will not allow focus operations while tracking is on, so when you make a focus adjustment, MaxIm DL will automatically turn off temperature compensation briefly, move the focuser, then turn temperature compensation back on again.

The Snapshot button will trigger a camera exposure, select the brightest star in the image for SharpStar autofocusing, and display its FWHM and HFD measurements in the Focuser Status window. If desired, you can select the autofocus star manually by clicking on it in the image.

If you need to abort an exposure, click the Abort button.

You can adjust the camera exposure using the Exposure button. This button is also used to configure the exposures for Autofocus.

This dialog allows you to set an optional Delay after Move, which allows you to delay the camera exposure after a focuser move. Such a delay is rarely needed; however, if your camera hogs the CPU during download, a small delay may make it easier to abort an autofocus sequence.

The Exposure Time can be set in seconds. You should pick an exposure time that will not cause the star to saturate even when close to focus.

The default binning is 1x1. When using the SharpStar Autofocus feature, you can change this using Binning and Allow Binning Override for Coarse Focus and/or Fine Focus.  As an example, you may wish to use 2x2 binning for coarse focus to speed up the process, while using 1x1 binning for fine focus for best accuracy.  In that case set Binning to 2 and turn on the override for Coarse Focus.  If you are using a one-shot color camera, you can use 2x2 binning for both modes in order to remove the Bayer matrix.

Subframe Width and Subframe Height are used during autofocus to speed up camera download; if you have "mirror flop" you should set this large enough to ensure that the star will not leave the frame after a single focus movement.

Readout Mode allows you to select from the available modes provided by the camera driver.

You can Apply Auto-dark calibration, which is recommended if your camera has a dark frame shutter.


SharpStar Autofocus

For an introduction to this command please see Autofocus Tutorial. Troubleshooting information is available in the Autofocus Troubleshooting section.

Autofocus requires a bit of setup. We recommend that you review the Autofocus Tutorial before starting. If you have trouble, please also refer to the Autofocus Troubleshooting section.

SharpStar autofocus uses the Half Flux Diameter (HFD) measurement technique, invented by Larry Weber and Steve Brady. HFD is a highly accurate and repeatable method of measuring the star diameter. Half Flux Diameter is the diameter that encompasses half of the total light received from the star. It can handle extremely large ”donuts” from centrally obstructed systems without trouble.For more information on HFD focus methods, and an excellent autofocus utility program designed for use with MaxIm DL, please see

Autofocus requires a compatible digital focuser with absolute positioning, i.e. it uses a stepper or servo motor. If you are using an open-loop DC motor (relative focuser) then we recommend using FocusMax.

It is extremely important before starting autofocus to click the Settings button and set up parameters for your telescope. If this is not done, the autofocus will almost certainly fail. Once the settings have been set up properly for your equipment, they do not need to be adjusted again.

The Telescope Focal Ratio is essential. It determines the depth of focus, and therefore the step size that is used when hunting for best focus. The focal ratio is often found in the telescope manual; it can also be calculated by dividing the focal length by the aperture.

Also essential is the Focuser Step Size. Some digital focusers know what this is inherently; if you click the Default button it will automatically load the value from the driver.  Many focusers do not have this information; they are just motors hooked up to focusers from other manufacturers. In this case, take out a ruler! Move the focuser 1000 steps, measure the distance it moves, and divide that by 1000. Enter this value into the Focuser Step Size. Be sure to select the proper units for the focuser step size, either Microinches (millionths of an inch) or Microns (millionths of a meter) as appropriate.

Focuser Max Step limits the range of focuser movement.  The autofocus algorithm will never attempt to move beyond this position.

Tip: if you cannot see the focuser move, as for a Schmidt-Cassegrain telescope, focus the telescope with an eyepiece, move the focus out 1000 steps, and then pull the eyepiece out of the focuser until the image is sharp. Measure the distance the eyepiece is moved. If the motion is small, move more steps and divide by the total number.

You also need to set the Target ½ Flux Dia. The Half Flux Diameter is a measure of how large the star images are; obviously we want them to be as small as possible. The target needs to be set somewhat higher than the optimum; typically by 2 pixels. This is required because the measurements are made on either side of focus, not right at focus. Typical values are 5 to 7, but it depends on how small your star images are at best focus.

The Autoselection Exclusion Margin control reduces the size of the initial "finder" image from which SharpStar selects the focus star. Setting this to 25%, for example, discards one quarter of the image from each edge, leaving a central subframe half the width and height of the sensor. This is useful to avoid focusing on stars in the corners of a large sensor.

You should also set up the Exposure for the camera; see above for a description of the fields.

Autofocus Tips

If you using a one-shot color camera, you need to eliminate the Bayer matrix encoding.  One option is to use 2x2 binning (enable for both Coarse Focus and Fine Focus).  Alternatively, if your camera driver has a Readout Mode that decodes the Bayer matrix to form a color image, you can use that option.  

The software moves the focuser in steps based upon the focal ratio and focuser step size you have entered in the Autofocus Settings dialog box. You want the fine focus mode measurements to be about 1 to 1.5 half flux diameter units apart (except near focus, where the value changes more slowly). Watch the log during autofocus operation and verify that this is the case.

If the Focal Ratio is set too high, or the Focuser Step Size is set too small, the focuser will move too quickly. It will either not find focus at all in coarse focus mode, or will scan through focus too quickly in fine focus mode. If it scans too quickly, it may miss best focus; if you see the half flux diameter exceed 20 in fine focus mode, chances are that the focuser is moving too quickly. In this case, either reduce the Focal Ratio setting or increase the Step Size setting. This will slow down the focus movement.

If the Focal Ratio is set too low, or the Focuser Step Size is set too big, the focuser may move too slowly. In this case, it will take an excessively long time to find focus. If far too many steps are taken the autofocus algorithm will abort. In this case, either increase the Focal Ratio setting or decrease the Step Size setting. This will speed up the focus movement.

If the focuser passes through focus and keeps on going forever, check that the Target Half Flux is at least two pixels higher than the minimum value seen in the log. If this is not the case, the software will never detect that it has passed best focus.