Advanced Settings

Advanced Settings

Advanced settings are accessed by clicking on the 'Brain button', a feature well-known to users of the original PHD.  PHD2 has a considerably larger set of parameters that can be adjusted to optimize your guiding performance.  Although these are called "advanced" settings, they are not particularly difficult to understand, and you shouldn't hesitate to explore them.  All of the fields on these forms include "tool tips", small message windows that describe each field in some detail.  Simply "hover" the cursor over the field to see the tool-tip.  In many cases, this will provide all the information you need.   Because there are many more settings available, the Advanced Dialog in PHD2 is organized into notebook tabs that are activated by clicking on the tab names.  All of the tabs share a common set of 'Ok' and 'Cancel' buttons at the bottom of the form.  Clicking on 'Ok' means that changes made to any of the tab fields will be put into effect.  Clicking on 'Cancel' discards any changes that were made.

Global Tab
Camera Tab
Guiding Tab
Algorithms Tab
Other Devices Tab

Global Tab

The controls on the 'Global' tab are well-described by their respective tool-tips, but they are summarized here for completeness:
  • '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 Ask Again messages' - restores the display of alert messages if you have previously chosen to not show them
  • 'Image logging format' - specifies the file format if star-image logging is enabled. If you are doing this for the purpose of documenting a problem, use Raw FITS
  • 'Dither RA only' - for imaging apps that use the PHD2 server interface, specifies that dithering should be done 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 
  • 'Log File Location' - specifies a file directory where PHD2 guide and debug log files will be stored.

Camera Tab

The controls on the 'Camera' tab are used as follows:
  • '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.
  • 'Auto Exposure' - these are the settings that control Auto exposure time.
    • 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 min exposure time is set too low, you are likely to chase seeing effects and thereby get poor guiding results. Users of AO units will usually set this to a lower value, since rapid small corrections are often desirable with an AO.
    • Max Exposure - the maximum exposure time. Before a guide star is selected, PHD2 will set the exposure time to the maximum value. Once a guide star is selected, PHD2 will then incrementally decrease the exposure time until the desired SNR is reached.
    • Target SNR - this is the average SNR value that PHD2 will attempt to achieve by adjusting the exposure time. SNR can fluctuate 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 reject frames when SNR drops below 3.0. The default value of 6.0 should provide enough of a cushion to prevent fluctuations from causing the SNR to go below 3.0.  As mentioned in the 'Basic Use' section, SNR is similar but not identical to the signal-to-noise ratio used in photometry.
  • 'Pixel size' - The guide camera pixel size in microns. This is the second of two parameters needed by PHD2 to compute the guider image scale and thus report guider statistics in units of arc-seconds.  The other parameter required for this is the guide scope focal length, located on the 'Guiding' tab. 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 the user interface 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.
  • 'Camera gain' - Sets the gain level for the many 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.
  • '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, and display an alert message in the main window.  Since a hardware problem is likely 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 the SBIG driver that is packaged with Sequence Generator Pro.  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.  See below for a more detailed discussion.  This control will appear only if the camera is capable of on-chip binning and only if the camera is connected to PHD2.
  • 'Use subframes' - For cameras that support this feature, PHD2 will download only a 100x100 subframe of each guide exposure. This is very 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..
Use of Binning
Some of the guide cameras available in PHD2 support hardware-level binning, and this may be helpful in situations where you are guiding at long focal lengths or have a guide camera with very small pixels. These scenarios often result in having to use faint guide stars, and the guider images may be substantially over-sampled.  Over-sampling provides no real benefit, and the projection of a faint star disk onto many small pixels can result in a low signal-to-noise ratio (SNR).  By binning the image, you can reduce the impact of camera read noise and thus improve the SNR; and if you are over-sampled,  you won't degrade the accuracy of computing the guide star location.  Choosing a binning factor greater than one will have the following effects:
  1. Star images will have a higher SNR and will be easier to detect above the background noise level.  This is only beneficial if you are limited to a choice among faint stars (i.e. with SNR values near the threshold of 3).
  2. The amount of data downloaded from the camera will be reduced by the square of the binning factor.  This can be helpful if you are using a camera that makes heavy use of USB resources even if star brightness and SNR are already reasonable with un-binned images.  Of course, using sub-frames can achieve the same result once a star has been selected.
  3. The resolution (image scale) of your guider image will be reduced by the binning factor.  This is not likely to be a problem if the un-binned image scale is below 1 arc-sec/pixel, but your guiding results may suffer if the un-binned image scale is well above 1 arc-sec/pixel.  You may need to experiment because the results will also depend on the image scale of your main camera system.
Each binning level requires its own dark frames and bad-pixel map - they are not interchangeable, nor can a transform be done automatically.  If you foresee the need to switch back and forth between binning settings, you should create separate profiles for each binning value.  Then build a dark library and a bad-pixel map for each of those profiles.  When you want to change binning factors, just switch to the profile that has the setting you want, and a dark library and/or bad-pixel map will be available.   If you want to check that the camera is binning correctly, you can use the Stats window to confirm the firame size and current on-camera bin settings.

Guiding Tab

The guiding tab shows the  parameters used for calibration, star-tracking, and guiding behavior shared by all of the guide algorithms..

Guide Star Tracking
  • '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.
  • 'Star mass detection' - tells PHD2 to monitor the brightness and size of the guide star compared to the sky background. 
  • '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' errors when the star is plainly visible on the display, try increasing the value of this setting.  Resetting the 'Enable' checkbox or setting the threshold to 100 will disable the warnings entirely.
  • '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.  The other parameter required for this is the guide camera pixel size, located on the 'Camera' tab.
  • 'Calibration step-size' - specifes 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 good to calibrate within about 30 degrees of the celestial equator (declination = 0), and to use a calibration step size that will result in 8-14 steps in each direction.   The 'calculate...' button to the right of this control will launch a dialog that can help you compute an appropriate value (see below) 
  • '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, this option will be set automatically whenever you connect to the gear.  Conversely, if PHD2 has no scope pointing information available, this option will be automatically reset.  Since you can force a re-calibration whenever you want, there is little reason to manually change the value of this checkbox..
  • '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 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 - you might have rotated the guide camera or slewed to a target close to the celestial pole, etc.  You can also 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 connection ('Mount' or 'Aux'), it will automatically adjust the RA guide rate based on the current declination.  This box should normally be left checked except in unusual cases.  For example, SiTech mount controllers evidently apply a compensation automatically, in which case the box should be left un-checked..
Shared Guiding Parameters
  • '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.  This is only an optimization, so the use of this checkbox is completely optional.  If you find that calibration fails because the star is lost during the fast re-center, you should disable this option.  That sort of problem probably indicates you have a large polar alignment error or excessive periodic error in RA.
  • 'Reverse Dec output after meridian flip' - tells PHD2 how to adjust the calibration data after a meridian flip.  Some mounts track their 'side of pier' state and automatically reverse the direction of the declination motor.  Most 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 it's probably easiest to just run a 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 run-away in declination, check the box and repeat the entire procedure, including calibration.
  • 'Enable mount guide output' - this is normally checked because it tells PHD2 to send guide commands to the mount.  But there are some circumstances where you might want to disable this, usually because you want to observe the uncorrected behavior of the mount.  For example, you can disable guider output in order to see the general shape and amplitude of your mount's periodic error or to check the amount of drift from polar mis-alignment.
  • 'Stop guiding when mount slews' - if guiding through an ASCOM interface, PHD2 can detect that a slew operation is underway and will stop issuing guide commands..
Calibration Step Calculator


To use the calculator, be sure the topmost three edit controls are correctly filled in.  If you have already specified the focal length and the camera pixel size in the 'Global' and 'Camera' tabs respectively, those fields will already be populated in this form.  If you are using an ASCOM connection to your mount, the fields for "Guide speed" and "Calibration declination" will also have the correct values.  Otherwise, you'll need to supply them yourself.  The guide speed is specified as a multiple of sidereal speed - most mounts will use something like 1X or 0.5X sidereal, but you can choose something else.  You can leave the 'calibration steps' field at the default value of 12, which is likely to result in a good calibration.  Use of a significantly smaller value raises the likelihood that seeing errors or small mount errors will cause calibration errors .  As you change the values in these fields,  PHD2 will recalculate your current image scale and a recommended value for the calibration step-size.  If you then click on 'Ok', that value will be inserted into the calibration step-size field of the 'Guiding' dialog.  Clicking 'Ok' will also populate the focal length and camera pixel size fields in the 'Guiding' and 'Camera' tabs, so any changes you made in the calculator will be reflected there as well.  However, this will not be done if you click on 'Cancel' in the calculator dialog.  Note that PHD2 never changes the guide speed setting in your mount regardless of what may be entered in the 'Guide Speed' field.

Algorithms Tab

The algorithms tab can be used to select the guiding algorithms you want to use and to fine-tune the parameters associated with them.  The parameters displayed will change significantly if you change the algorithm selections.  For that reason, all the parameters related to guide algorithms will be treated together, in a separate section.

The remaining controls, the ones that are independent of the guiding algorithm selections, are described below.  
  • '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.) .'
  • 'Use backlash comp' - this controls whether PHD2 will apply a compensation factor when the direction of declination guiding needs to be reversed.  Measurement of backlash and calculation of a good starting value for the compensation factor is done in the Guiding Assistant.  The size of the additional guide correction (compensation value) is shown in the 'Amount' field adjacent to the checkbox.  This amount may be adjusted upward or downward by PHD2 if necessary to tune the guiding results.  In either case, the adjustments are made conservatively in order to avoid making guiding unstable.  Since PHD2 has the ability to detect and adapt to over-corrections, the backlash compensation available here should work better than the fixed backlash compensation available in many mount controllers.  If you use the PHD2 backlash compensation, you should disable any  backlash compensation in the mount.  See the help section on the Guiding Assistant for more details.  The 'Max Dec duration' parameter may be adjusted automatically to avoid truncating the backlash compensation pulse.  That said, you shouldn't expect backlash compensation to work well if your mount has many seconds of backlash.
  • '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 errors are not caused by imperfections in your mount's gears.  Instead, deflections in declination are primarily the result of imperfect polar alignment or flexure.  The result is an error that 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.  If so, you can select either 'north' or 'south' to restrict corrections to only that direction.  Keep in mind, however, that an over-shoot in correction with one of these modes will leave the star positoned off-target for an extended period of time. So you'll probably want to use conservative parameters for aggressiveness if you are disallowing direction reversals.  Finally, a choice of 'off'' here disables declination guiding altogether.
  • '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 probably repeat that procedure.

Uni-directional Declination Guiding

As discussed elsewhere, some mounts have too much declination backlash to support guiding in both north and south directions.  This situation can be mitigated by configuring PHD2 to guide in only one of the directions, what we call uni-directional Dec guiding.  This can be a manageable situation because declination guiding is only intended to correct for slow drift - errors caused by polar misalignment and to a lesser extent, mechanical flexure.  Ironically, you might want to de-tune your polar alignment a bit to make it easier to see the drift direction and to reduce the likelihood that seeing will interfere with uni-directional guiding.  Remember that polar mis-alignment, within reason, doesn't usually degrade guiding performance.  Instead, it may introduce field rotation if you're imaging near the pole and have a large camera sensor.  A good first step would be to polar align to within a few arc-minutes of the pole before setting up for uni-directional guiding.  You can always go back later and check for field rotation.  Just take a sample image with your main camera at the highest declination you would expect for imaging - perhaps 70 degrees north.  If you don't see field rotation there, you can leave the polar alignment where it is.  With any amount of polar mis-alignment, the direction of  Dec corrections will change at some point in the sky.  (Technically, it will reverse directions at two points in the sky but one of those is usually below the horizon.)  The sky location for the reversal depends entirely on how you are mis-aligned on the pole - the relative amounts of azimuth and altitude alignment errors.  You may even have a situation where the reversal point is near enough to the horizon that you don't encounter it during normal imaging.

To set up for uni-directional guiding, you can follow these steps:
  1. Move to a field with a good guide star and open the Guiding Graph window.  Disable Dec guiding entirely by setting the Dec guide mode to 'off', then start guiding.  Now watch the graph until you can see a clear trend in the way the guide star is drifting either north or south.  Once you see this, reset the Dec guide mode to issue corrections in the right direction.  For example, if the star is drifting north, set the Guide mode to 'south.'
  2. Try using the 'LowPass' or 'LowPass2' guiding algorithms for declination and start with a fairly low aggressiveness factor, say 50%.  If the aggressiveness is too high, the correction may push the star to the "wrong" side of the lock position, where it will remain until the slow drift rate moves it back.  It's better to issue a few consecutive small corrections rather than one larger one in order to minimze this type of over-shoot.
  3. Watch the guiding graph to be sure the corrections are being issued in the right direction and the star isn't just steadily drifting off-target.  Over the course of minutes or hours, you may notice the amount of drift is decreasing.  This means you are slowly approaching the point of declination reversal and you should be prepared to change the Dec guide mode accordingly.
  4. If you are dithering, set the dithering parameters to "RA-only" to avoid disrupting the Dec guiding.

Other Devices Tab

If you are using either an adaptive optics or rotator device, the "Other Devices" tab will be shown.  The upper section deals with the AO device if one is being used.  You can use the first four parameters to control the calibration process and the manner in which 'bump' operations are done.  The 'calibration step' field tells PHD2 the amount to move the tip/tilt element in each of the up/down/left/right directions, in units of AO steps, during calibration.  The guide star position is measured at the beginning and end of each leg of the calibration, and the 'samples to average' parameter tells PHD2 how many samples to take at each of these points.  Averaging images is important because the seeing will always cause the guide star to "bounce around" a bit.  As discussed earlier, the AO unit can make corrections only within a limited range of guide star movement.  You will want to initiate mount 'bump' corrections before these limits are actually reached, and the 'bump percentage' field is used for that purpose.  To move the mount, the full bump correction is accomplished in steps - the 'bump step' field controls the size of these increments.  If the bump operation has begun and the guide star remains outside the "bump percentage" area, PHD2 will increase the bump size until the guide star is back within that range.  Additional movement from that point to the center position will continue at the specified "bump step size".  This complexity is required in order to maintain good guiding, with no elongated stars, even as the mount is being bumped.  During the bump operation, the AO is continuing to make corrections, so the long "mount bump" is continuously offset by adjustments in the AO.

The 'Bump on dither' option tells PHD2 to bump the mount when a dither command is received and thus move the guide star back closer to the center position of the AO.  The option to enable or disable AO guide commands operates independently from the 'Enable mount guiding' checkbox in the Guiding tab.  So you can independently enable/disable either the guide commands to the tip/tilt device or the 'bump' guide commands to the mount.  The same principle holds for the 'Clear AO calibration' option - that will force a recalibration of the AO without affecting calibration of the mount.

When an AO is in use, the 'Algorithms' tab will only show choices for controlling the tip/tilt optical element in the AO device itself.

Since the AO is not trying to move a heavy piece of equipment, you can afford to be more aggressive in your guide algorithm choices.  The default algorithms for an AO are 'None', which means there will be no damping or history-based calculations applied at all.  In that case, each correction will be based only on the most recent guide frame and will make a 100% correction of the most recent deflection.  If you use a different algorithm, you should probably start with a high level of aggressiveness there as well, perhaps 100%.   The other, shared guiding parameters normally displayed on the 'Algorithms' tab will not be shown for the AO because they aren't used to control the device.

The rotator device has only one parameter, which lets you match the behavior of the device to the ASCOM notion of positive and negative angles.  The "Reversed" checkbox can be used for optical systems that reverse the image, usually because they have an odd number of mirrors.  The direction and amount of rotation is used to adjust the calibration data, so PHD2 follows the ASCOM standard:  "the rotator position is expressed as an angle from 0 up to but not including 360 degrees, counter-clockwise against the sky."  Experimentation is likely to be the quickest way to determine if the box should be checked.

News & Resources

February 12, 2017 - PHD2 v2.6.3 Released

October 28, 2016 - PHD2 Best Practices

June 12, 2016 - PHD2_Broker package available

December 29, 2015 - Tutorial: Analyzing PHD2 Guiding Results

December 20, 2015 - Tutorial: Off-axis guiding on comets with PHD2