Main Screen

Main Screen

The PHD2 main window is designed for ease of use and clarity.  Its intent is to support a quick and natural sequence of interactions to start and control guiding.  The basic steps for doing this are as follows:
  1. Connect to your guide camera and mount
  2. Start a sequence of guide exposures to see what stars are available in the field of view
  3. Let PHD2 auto-select a suitable star and calibrate the guider
  4. Continue guiding on the target star while using various display tools to see how things are going
  5. Stop and resume guiding as necessary

The majority of the screen is taken up by the display of the star field from your guide camera.  The display is automatically adjusted for size, brightness, and contrast so you can view the available stars.  However, these adjustments are done only for display purposes.  Internally, PHD2 operates on the raw, un-adjusted data in order to maximize guiding accuracy.  This display can also used to manually select a guide star by  clicking on it although it's better to use the 'Auto-select star' feature..  You can adjust the slider control to see even the faintest stars in the field.

Basic control

Near the bottom of the screen are the main controls.  PHD2 is largely controlled by these buttons and sliders, with additional pull-down menus at the top of the window for more detailed functions.  Moving from left to right in the window, the primary buttons are as follows:
  1. The USB connector icon - used to connect to your camera and mount devices
  2. The Loop icon - used to start a sequence of repeated exposures with the guide camera ("looping"), with each resultant image (guide frame) being displayed in the main window.  If guiding is subsequently started, clicking on the 'loop' icon again will pause guiding while continuing to take guide exposures.
  3. The Auto-Select Star icon - used to trigger an automatic selection of the best guide star candidate in the field of view.  This selection is done quantitatively, taking many things into account - star saturation, minimum star size, signal-to-noise ratio, proximity to other stars, proximity to the edge of the field, etc.  The technique will nearly always make a better choice than you can manually even though the selected star may appear quite faint.
  4. The PHD2/Guide icon - used to start calibration, if needed, and then to start guiding on the selected star.  
  5. The Stop icon - used to stop both guiding and looping
To the right of the stop icon is a pull-down list of exposure durations (0.01s - 15s).  You use this control to quickly set the guide camera's exposure duration.  If your camera does not support an exposure duration, PHD2 will do its best to emulate that duration.  For example, if you use a short-exposure webcam, your maximum true exposure duration might be only 1/30th of a second.  If you select one second as the desired exposure time, PHD2 will automatically acquire images for one second and stack them on the fly to create a composite image for guiding.

The next control to the right is a slider for adjusting screen stretch and contrast, essentially a "gamma" adjustment.  PHD2 automatically adjusts the display accounting for the darkest and brightest pixels in the image, and the slider is used to fine-tune the display to better see the stars in the field of view.  This may be useful, for example, if you are trying to focus the guide camera for the first time and need to see the large, out-of-focus star image. Moving the gamma slider only makes the display brighter or dimmer for your viewing. PHD2 always uses the raw pixels from the camera for guiding, and moving the gamma slider has no effect on guiding.  A display of "completely white" or "completely black" is usually an indication that no stars are available in the field of view.  The automatic gamma-stretching of the image is a very common source of confusion for users because they don't adjust the gamma slider properly to see faint stars on the display.  The stars that PHD2 auto-selects may appear to be overly faint and inferior to bright stars you see in the display.  These results are illusory because PHD2 can generally do a better job of selecting a guide star than you can accomplish by eye.  As a rule of thumb, if you've chosen a star whose name is known to you, it is almost certainly too bright to be a good choice as a guide star.

Next to the gamma slider is the "brain button."  This button brings up an Advanced Settings dialog for making detailed adjustments to PHD2's guiding operations.  An important design goal of the program is to minimize your need to change these parameters, but "the brain" is nothing to be feared - there are adjustments available there that can improve your guiding results and make your life easier.  Over a period of time, you should take a look at this dialog and learn what it can do for you.

The rightmost control in this row is a "camera properties" button.   Depending on the particular camera, this button may be enabled to provide access to a configuration dialog unique to the camera.  However, common camera properties such as gain and binning will normally be set in the 'Camera' tab of the PHD2 Advanced Settings dialog.  If the button is disabled, any available properties can be set in the PHD2 Advanced Settings dialog.  With the newer class of guide cameras, this button is often disabled because it adds no capabilities beyond what is available in the PHD2 Advanced Settings dialog.


The pull-down menus above the main guider display are used to access a variety of functions.  These are described in the Darks, Tools and Utilities, and Visualization sections of this help document.

Status Bar

The status bar at the bottom of the main window is used to display messages and status information that will help you keep track of guiding operations.  

Near the center of the status bar are fields showing the current state of the guide star.  If the SNR value drops below 10, its value will be shown in yellow as a warning that you may encounter some 'lost-star' events.  If the guide star is saturated, the field to the left of SNR will show 'Saturated' in a red typeface.

To the right of the star status fields are two text fields showing the latest RA and Dec guide commands.  These show the size of the guide pulse, the guide star displacement size in pixels, and an arrow showing the direction.  The arrows follow the usual compass conventions: Dec up/down corresponds to north/south, RA left/right corresponds to west/east.  All of this information is captured in the log files and displayed in the various graphical tools, and those are what you should use for visualizing your guide performance.  But these status fields may give you a quick visual clue when something is behaving unusally.

The rightmost panels in the status bar show icons that give you visual clues about the current state of PHD2:.  These icons are color-coded to give you a dashboard view of current status and have the following meanings:

'Dark' - red means neither a dark library nor a bad-pixel map is being used, green means one or the other is in-use.  If you're using a bad-pixel map, the text will say 'BPM' rather than 'Dark'

'Cal' - shows the state of calibration.  Red means the mount is currently uncalibrated, while yellow means there is a calibration but it isn't being adjusted automatically to account for scope pointing position.  This will happen when you aren't using either an ASCOM or 'aux' mount connection in PHD2.  If the icon is yellow, you will generally need to recalibrate when you move the scope to different declination positions.

"The Ball" - shows whether all the equipment in your profile has been successfully connected.  If the ball is yellow, some components are not connected, while green means everything is connected.

If you hover the mouse cursor over any of these status icons, you'll see details about the current state.

News & Resources

February 4, 2020 - PHD2 v2.6.9 Released

December 21, 2019 - PHD2 Best Practices

December 7, 2019 - macOS Catalina

April 26, 2018 - Polar Alignment tool video tutorials

June 12, 2016 - PHD2_Broker package available