Advanced Settings

Global Tab

• 'Language' - determines the language used in the PHD2 user interface, subject to available localization.  Changing this requires a program restart.
• 'Reset Configuration' - restores all settings to their initial values as if PHD2 had been freshly installed.
• 'Reset Don't Show Again messages' - restores the display of alert messages if you have previously chosen to not show them.
• Software Update
• Automatically check for updates - allow PHD2 to check for software updates when the program starts up. If no internet connection is available, checking will be deferred until the next time PHD2 is run.
• Only check for major releases - don't include development builds when checking for software updates.
See the Software Update section for more information about PHD2 software updates.
• 'Log File Location' - specifies a directory where PHD2 guide logs, debug logs, and any diagnostic image files will be stored. The default location on Windows is the "My Documents" folder associated with the logged-in user..
• Dither Settings
• 'Random mode' - tells PHD2 to use a random-number generator to compute both the size and the direction of the dither, subject to any constraints imposed by RA-only mode or by the Dec guiding mode being set to 'Off'.
• 'Spiral mode' - tells PHD2 to dither with fixed-size amounts in a clockwise spiral pattern.  This can be a good choice when the imaging camera has significant fixed-pattern noise or the mount has a troublesome amount of Dec backlash.
• 'Dither RA only' - tells PHD2 to dither only on the RA axis.
• 'Dither scale' - an optional multiplier used to adjust the maximum-dither amount specified by the image application. See Dithering Operations 
• 'Enable diagnostic image logging' - used primarily for product support and diagnosis of problems dealing with the guide camera or PHD2 star-recognition and measurement.  Guide frame images are captured and logged in a FITs format subject to the filter/trigger controls in the group-box.  Images are saved in sub-folders of the PHD2 logging directory with the date and time encoded as part of the sub-folder name. For example, a folder named 'PHD2_CameraFrames_2022-03-03-184542' refers to a logging event that began on March 3, 2022 at 18:45:42 local time. Individual guide frames are saved with filenames that indicate the time the image was captured and the reason the frame was saved.  Since the guide frames are saved in a FITs format, the header will include other useful information such as exposure time.  Because the logging function is primarily used for trouble-shooting, the image sub-folders are automatically removed after 30 days.  If you wish to keep the images for your own purposes, you should either rename the sub-folders or copy/move them to a different directory.  When logging is triggered by one of the "events" - e.g. lost star or large errors - a group of images (an image set) will be saved, centered in time on the image that triggered the event. This provides a record  of guider images that will show what the guide star and guide frame looked like both before and after the unusual condition occurred.  The various triggering and filtering controls are described below and are also shown in the tooltips for the controls:
• 'All lost star frames' - logs the image set for any lost-star events, regardless of the reason for the lost star (low SNR, mass-change, etc.)
• 'All auto-select star frames' - logs the image set for any frames used for auto-selection of the star, regardless of outcome.  Note that any failed attempts to auto-select a star will always result in a logged image, regardless of choices made in the user interface.
• 'When relative error exceeds' - logs the image set when the star deflection on the current frame exceeds the running-average error by the factor chosen in the adjacent spin control.  For example, if the average (RMS) error is 0.5 pixels and the current frame's error is 1.5 pixels, the relative error is 3.
• 'When absolute error exceeds' - logs the image set when the star deflection exceeds the number of pixels specified in the adjacent spin control.
• 'Until this count is reached'  - logs images until the count matches the value of the adjacent spin control.  The counter is reset to zero when the limit is reached.

Camera Tab

• 'Noise reduction' - specifies the algorithm to use for handling noisy guide camera images - those for which dark frames are not sufficient.  Choices include None, 2x2 mean, and 3x3 median. Both 2x2 mean and 3x3 median will reduce the noise considerably. 3x3 median is especially effective at removing hot pixels and neither will significantly affect guiding accuracy.  However,  creating a bad-pixel map is likely to be a better solution with less impact on your ability to detect faint stars.
• 'Time lapse' - imposes a fixed delay between guide exposures.  This can be useful if the guide exposures are very short and you don't want to overload either the mount or the camera link with very high traffic rates.
• Variable Exposure Delay - very precise/encoder-equipped mounts often  benefit from conservative auto-guiding.  This can include use of medium-long exposure times (4+ seconds) and an additional delay between guide camera exposures.  The goal is to create a slow guiding "cadence" that is only reacting to tracking changes that aren't already being handled by the mount encoders or by its pointing model.  Details of the approach can be found here:  Variable Delay Guiding  The variable exposure delay controls provide a way to achieve these goals while avoiding unnecessary delays when the system is engaged in actions other than "steady-state" guiding for imaging/science operations.
• 'Use Variable Exposure Delays - enables the feature
• Short delay (sec) - sets the exposure delay when  PHD2 is looping, star-selecting, calibrating, dithering and settling, or running the Guiding Assistant.  For some mounts, this value should be non-zero to give the mount encoders enough time to complete their position adjustments.
• Long delay (sec) - sets the exposure delay when PHD2 is guiding and not in any of the other states named above.  This delay actually defines the time between the completion of the last guide command, if any, and the start of the next camera exposure.
• For both the long and short delay scenarios, the interval between successive guide camera exposures (and guide commands) will be exposure_time + guide_pulse_times + delay_value.
• 'Auto Exposure' - these are the settings that control Auto exposure time:, useful for automated imaging or AO guiding:
• Min Exposure - the minimum exposure time.  PHD2 will not set the exposure time less than this value, even if the guide star SNR is higher than the target SNR value. If the minimum exposure time is set too low, you are likely to chase seeing effects and thereby get poor guiding results. Users of adaptive optics units might set this to a lower value since rapid small corrections are more manageable with an AO.  Lower values, down to 0.5-sec, can also be tried if multiple guide stars are normally available in the field of view.
• Max Exposure - the maximum exposure time. Before a guide star is selected, PHD2 will set the exposure time to the maximum value. After selection is completed, PHD2 will then incrementally decrease the exposure time until the desired SNR is reached.  The setting should be sufficiently small to allow PHD2 to adequately handle the tracking errors in your mount.
• Target SNR - this is the average SNR value that PHD2 will attempt to achieve by adjusting the exposure time. SNR often fluctuates from frame to frame even with a fixed exposure duration, so be sure to account for that when choosing a target SNR value. PHD2 will always reject frames when SNR drops below 3.0. The default value of 6.0 may provide enough of a cushion to prevent fluctuations from causing the SNR to go below 3.0 - but double-digit values are recommended.  As mentioned in the 'Basic Use' section, SNR is similar but not identical to the signal-to-noise ratio used in photometry.
• 'Star Saturation Detection' -  when identifying candidate guide stars, PHD2 tries to avoid saturated stars in order to get the most accurate calculations of their positions.  You can assist this process by specifying how the assessment of saturation should be done:
• Saturation by ADU value - this is the default option because it relies on the brightness levels measured by the guide camera sensor.  Most guide camera drivers return image data in either 8-bit or 16-bit formats even if the camera electronics natively work in 12-bit or 14-bit modes.  An 8-bit camera will saturate at a brightness level of 255 while a 16-bit camera will saturate at levels above 65000.  If you are uncertain about your camera's behavior, you can use the Star Profile tool to measure the brightest pixel level when taking exposures of a very bright star. This will quickly tell you if the camera is returning values in the range of 0-255 or 0-65K and you can set the saturation value accordingly.
• Saturation via star-profile - this relies on a less accurate judgment of saturation based on whether the star profile has a flat top.  It is not recommended for normal applications.
• 'Pixel size' - The guide camera pixel size in microns. This is needed by PHD2 to compute the guider image scale and thus report guider statistics in units of arc-seconds.  Refer to your camera documentation to determine the correct value for pixel size.  If your camera has non-square pixels, just choose one of the dimensions or input the average of the two.  The pixel size has no effect on guiding accuracy, so a small amount of imprecision in this parameter won't cause any problems.  If you're using the binning setting in this dialog to control camera binning, the pixel size should be the native, un-binned size.  Note:  this control may be disabled if the camera and camera driver can report the pixel size to PHD2 (most can).  In that case, the value displayed in the disabled control represents the device-reported pixel size - it is what it is.  If you're also specifying a binning factor at the camera driver level rather than in PHD2, the reported pixel size may change as a result.  It is generally better to use PHD2 to set binning (see below).
• 'Camera gain' - Sets the gain level for the cameras that support this feature.  Reducing this parameter can help to reduce the noise level or may allow use of a bright star without saturation.  The range of values and their specific effects are dependent on the individual camera, so this PHD2 parameter is treated as a percentage of the range between the minimum and maximum gain values supported by the camera.  For example, if the camera uses absolute gain values in the range of 40 to 80, a PHD2 gain value of 50% would translate to a camera gain of 60.  Adjusting the gain level is not usually required once the configuration has been stabilized, and doing so will require replacement of dark libraries and bad-pixel maps.
• 'Disconnect nonresponsive camera after (seconds) - Camera malfunctions will sometimes occur, often because of faulty USB connections.  In many cases, the camera will not return the requested image data, and PHD2 will appear to "hang."  This parameter determines how long PHD2 should wait for a response after the expected exposure time has expired.  For example, a timeout value of 5 seconds in conjunction with an exposure time of 2 seconds will tell PHD2 to wait up to 7 seconds for a response.  If the data are not received within that period, PHD2 will attempt to halt the operation, disconnect the camera, display an alert message in the main window, then try to reconnect the camera.  Since a hardware/power problem is nearly always the underlying issue, this recovery attempt won't always succeed.  You should be generous with these timeout values to avoid spurious recovery actions.  Also, if you are using a guide camera that shares electronics with the main imaging camera, you should set this timeout to a large value, well above the maximum expected time for a full-frame download from the main imager. This is a consideration for users of some of the SBIG dual-chip cameras.  Regardless of whether PHD2 is able to handle the situation gracefully, the underlying problem is almost certainly in the hardware or the camera driver and will need to be resolved before guiding is continued.
• Binning - for those cameras that support on-chip (hardware) binning, you can specify the binning that will be used while taking guide exposures.  Detailed information is available here: Use of Binning.  This control will appear only if the camera is capable of on-chip binning and only if the camera is connected to PHD2.  You should always use a separate PHD2 profile for each different binning setting because it affects many downstream guiding parameters, too many to try to adjust manually.
• 'Use subframes' - For cameras that support this feature, PHD2 will download only a subframe of each guide exposure. The subframe shows the region around the star bounded by the tracking region limits. This is useful for cameras with slow download times, allowing them to be used more effectively for guiding.  This feature applies to both calibration and guiding.  During initial looping without a selected star, the full frame is downloaded, but once a star is selected, only this small subframe is downloaded.   If you are using subframes but want to see the full frame to select a different star, just shift-click anywhere in the image display window or shift-click on the 'Auto-select star' button.  Using subframes eliminates the option for multi-star guiding, only a single star can be used with the subframe option.
• Cooler controls - a few guide cameras support sensor cooling.  These controls let you specify whether the cooler is on and what the cooler set-point should be.

Guiding Tab

• 'Search region' - specifies the size of the "tracking rectangle", in units of pixels.  You may need to increase this value if your mount does not perform well or, more commonly, if it's not well-aligned on the celestial pole.  You may also want to increase it temporarily while using the Guiding Assistant so that backlash measurement can be done without losing the guide star.  Just remember that an overly large search region also increases the likelihood that multiple stars will live within its boundaries, which could lead to guiding problems.  The value is actually a "radius" amount, so a search region of 15 pixels translates to a square of 30x30 pixels.
• 'Star mass detection' - tells PHD2 to monitor the brightness and size of the guide star compared to the sky background.   This feature has been deprecated and can generally be left un-checked (default value) for most configurations.
• 'Star mass tolerance' - if the 'Enable' box is checked, PHD2 will trigger a 'lost star' error if the measured brightness and size vary by more than this percentage.  This might be useful if you have two stars inside the tracking rectangle and you want to be sure PHD2 doesn't mistakenly switch stars.  It can also prevent errors caused by thin clouds, high camera noise, or alpha particle artifacts; but it may be unreliable if you are guiding on a faint star.  If you are getting too many 'lost star - mass change'' errors when the star is plainly visible on the display, try increasing the value of this setting or disable it entirely. .
• 'Minimum star HFD' - specifies the minimum half-flux-diameter (roughly the 'size') of a suitable guide star.  This is probably the best way to prevent PHD2 from mis-identifying clumps of hot pixels as usable guide stars.  You can determine a suitable value for your system by manually selecting some small stars that you know are not just hot pixels, then use the star-profile tool to see the HFD values of those stars.  You'll want to specify a minimum HFD value that allows selection of legitimate faint stars but not hot pixels.  The default value of 1.5px should work for most configurations but you can adjust it if needed - for example, if the guide camera pixels are large and too many candidate guide stars are being rejected because of their apparent small sizes.
• 'Minimum star SNR for AutoFind': specifies the minimum SNR value that will be accepted by the auto-select function for non-saturated stars.  This should only be adjusted by experts because a value that is too large can result in no stars being selected.  It can be used  in situations where you would rather use a saturated star instead of a faint star that you know from experience will prove to be unusable.  However, many of these situations can be avoided by getting better guide camera focus, refreshing the dark library, or slightly increasing the exposure times.
• 'Use multiple stars' - determines whether PHD2 will use more than one star for guiding.  See Multi-star Guiding for details.
• 'Auto-selection frame downsample' - can be used in unusual situations where PHD2 guide star auto-selection is not recognizing misshapen stars.  If this happens, you can try values greater than 1 to improve the chances of recognizing such stars.  For most users, this property should be left at its default value of 'Auto'.
• 'Beep on lost star' - specifies whether PHD2 will produce an audible 'beep' sound when the guide star is lost during guiding.

• 'Focal length' - the focal length of the guide scope (millimeters). This provides one of two parameters needed by PHD2 to compute the image scale and thus report guiding performance in units of arc-seconds. If the focal length value is wrong, some of the default guiding parameters will not be set correctly, and we won't be able to help you assess performance and resolve problems.  You should use the new-profile-wizard to specify focal length as part of defining your equiipment configuration.
• 'Calibration step-size' - specifies the duration of the guide pulse that PHD2 will use during calibration.  Its use is described in the 'Auto Calibration' section of the 'Basic Use' help page.  You can adjust the value depending on whether the guide star is moving too quickly or too slowly during calibration.  As a general guideline, it is best to calibrate within about 20 degrees of the celestial equator (declination = 0), well above the east and west horizons; and to use a calibration step size that will result in 8-14 steps in each direction.   The 'Advanced...' button to the right of this control will launch a calculator dialog that can help you compute an appropriate value (see below) .  As with other parameters, this value is computed for you as part of running the new-profile-wizard so there is usually no need to adjust it.
• 'Auto restore calibration' - tells PHD2 to automatically reload the most recent calibration data as soon as the equipment is connected.  If you're using an ASCOM (or Indi) mount connection or have an 'aux-mount' connection, you probably want to set this option.  Conversely, if PHD2 has no scope pointing information available, this option should be reset.  The new-profile wizard will ask you how you want to set this option.  Note that auto-restore is remembered for each separate equipment profile, and it only has an effect when you load the profile and connect to the equipment.  If you want to force a recalibration before an individual guiding session begins, there are multiple ways to do that - doing a Shift-Click on the PHD2/guiding 'Guide' button is the easiest.  If you have set the auto-restore option, you must be sure that the guide camera is not rotated with respect to the guide scope or the sky.
• 'Assume Dec orthogonal to RA' - Normally, the calibration process independently computes the camera angles for both right ascension and declination.  There is no need for great precision on these values, and the default behavior normally works well.  However, if your mount has very high periodic error, very large Dec backlash, or you are dealing with very bad seeing conditions, you may want to force the RA and Declination angles to be perpendicular.  If you choose that option, PHD2 will compute the camera angle for RA, then assert a declination angle that is orthogonal to it.
• 'Clear mount calibration' - tells PHD2  you want to clear the calibration data currently being used for the mount and re-calibrate before guiding is restarted.  You might do this for a variety of reasons - rotating the guide camera or seeing that guiding results are substantially worse than normal, for example.  You can accomplish the same result by doing a Shift-Click on the PHD2 guiding icon on the main page, which will force a re-calibration.
• 'Use Declination Compensation' - if PHD2 can get pointing information from the mount via an ASCOM/Indi connection ('Mount' or 'Aux'), it will automatically adjust the RA guide rate based on the current declination.  This box should be left checked except in very unusual cases where the mount controller is making the adjustment. Don't confuse this option with 'Declination backlash compensation', which is an entirely different feature.

• 'Fast re-center after calibration or dither' - during calibration or dithering, the mount may be moved a significant distance from the initial "lock" position.  If you click this checkbox, PHD2 will move the mount back to the lock position as quickly as possible, using the largest guide commands permitted by the 'Max Duration' settings of your guide algorithms and by the size of your tracking region.  This is only a way to speed up the calibration process,  the use of this checkbox is completely optional.  If you find that calibration often fails because the star is lost during the fast re-center, you should disable this option.  That sort of problem may indicate that you have a large polar alignment error or excessive periodic error in RA.  You can run the Guiding Assistant to help see what's causing the problem.
• 'Reverse Dec output after meridian flip' -  tells PHD2 how to adjust the calibration data after a meridian flip.  Some newer mounts track their 'side of pier' state and automatically reverse the direction of the declination motor after a meridian flip.  Older mounts do not do this.  In either case, PHD2 needs to know if the mount will automatically change its behavior based on side-of-pier.  You may have difficulty finding information about how your mount behaves in this respect, so PHD2 provides the Meridian flip calibration tool to determine the correct setting automatically. It's also easy to figure out the setting manually with this quick experiment:
  • With the checkbox disabled, calibrate on one side of the pier, then move the mount to the other side.
  • If you are guiding via ASCOM or Indi or are using an 'aux mount' connection, just start guiding.
  • If you're guiding only via ST-4 and PHD2 has no scope pointing information, first select 'Flip Calibration' under the 'Tools' menu, and then start guiding.
  • In either case, if the guiding works normally, leave the box un-checked; but
  • if you see guiding run-away in declination, check the box and repeat the entire procedure, including calibration.  You must repeat the procedure, simply checking the box doesn't "fix" anything.
   Note: this procedure must be following in its entirety until you determine the correct setting.  Changing the 'Reverse Dec' setting only determines what will happen in the future, at the next calibration. 
• 'Enable mount guide output' - this is normally checked because it tells PHD2 to send guide commands to the mount. There are some circumstances where you might want to disable this, usually because you want to observe the uncorrected behavior of the mount.  In most cases, you will be better off running the Guiding Assistant for these purposes, which automatically handles this setting for you.
• 'Stop guiding when mount slews' - prevents PHD2 from trying to continue guiding when the mount is being slewed, often by an imaging automation app.  It should normally be left checked..

Algorithms Tab

• 'Max RA duration' - specifies the maximum allowed guide pulse duration for right ascension.  You might reduce this below the default value if you want to avoid chasing a large deflection that could be caused by a spurious event (e.g. wind gust, hot pixel, etc.) .'  However, there is rarely any reason for changing it if you have protected yourself against hot pixels (Minimum star HFD).
• 'Enable' PHD2 Backlash Compensation - this controls whether PHD2 will apply a compensation factor when the direction of declination guiding needs to be reversed.  See section below.
• 'Max Dec. duration' - specifies the maximum allowed guide pulse duration for declination (same as above but for declination).
• 'Declination guide mode' - gives you additional control over declination guiding.  Declination guiding is not like RA guiding because the tracking errors are not caused by imperfections in your mount's drive system - the Dec motor is idle during normal sidereal tracking, running only in response to short Dec guide commands.  Instead, tracking errors in declination are caused by polar alignment error, mechanical flexure, wind, binding or dragging of cables, or unwanted movement of the guiding assembly.  Absent these mechanical problems, declination tracking should be smooth and mostly uni-directional assuming there is no over-shoot from an earlier correction.  The default value of 'auto' tells PHD2 that some reversals in direction are acceptable, subject to the behavior of the various guiding algorithms.  However, if your mount has severe declination backlash, you may want to prevent direction reversal altogether. You could then select either 'north' or 'south' to restrict corrections to only that direction (uni-directional Dec guiding).  Keep in mind, however, that an over-shoot in correction with one of these modes might leave the star positioned off-target for an extended period of time. So you'll probably want to use conservative aggressiveness values for either 'North' or 'South' modes and you may want to intentionally degrade the polar alignment to help keep the drift rate larger than the deflections caused by seeing.  Finally, a choice of 'off'' here disables declination guiding altogether, an appropriate choice for simple tracking mounts that don't support Dec guiding.
• 'Reset' - resets the guiding parameters for the selected RA or Dec algorithm to their default values.  Min-move settings will be set using the same algorithm employed in the new-profile-wizard.  If you previously used the Guiding Assistant to adjust the min-move settings, you should repeat that procedure.

Declination Backlash Compensation

Other Devices Tab