Advanced Settings

Advanced Settings

Advanced settings are accessed by clicking on the 'Brain button' in the main display.  PHD2 has a large 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 so many parameters 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 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 - indicates whether to 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 'None'.
    • '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.  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.
Since the images are saved in an industry-standard format, there are many astronomy-related applications that can display or analyze them, many of which are free.  Most of the image-capture and image-processing applications can do that along with other, more specific tools that can perform detailed measurements on the stars and the optical quality of the field of view.  You can just do a web search to find a list of applications that support the FITs format for whatever platform you're using.  If you simply want to look at the images to check focus or see the general quality of the images being returned from your camera, you can use PHD2 for that.  With PHD2 in an idle state - neither looping nor guiding - just drag-and-drop one of your saved FITs image onto the main window. The display will then update to show the image you just dropped.  There's no need for PHD2 to be connected to any of the hardware.  You can adjust the gamma slider, select a star (manually or automatically), and use the Star Profile tool to view the HFD and profile of the selected star.

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 might set this to a lower value since rapid small corrections are more manageable 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 but a higher value is desirable.  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.  Note:  this control may be disabled if the camera and camera driver can report the pixel size to PHD2.  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 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.  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.
  • '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 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 especially helpful if you are limited to using faint stars (i.e. with SNR values below about 10).
  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 above 3 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 want the flexibility of switching 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 frame 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.   You may need to disable this feature or set the tolerance to a higher number if you're using a fine image scale and are getting too many lost-star events because of  this feature.
  • '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.
  • 'Minimum star HFD' - specifies the minimum half-flux-diameter (roughtly 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.  If you encounter situations where PHD2 is trying to guide on a hot pixel, be sure to set this parameter.
  • '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 should 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.  The other parameter required for this is the guide camera pixel size, located on the 'Camera' tab.  If these two parameters aren't set properly, some of the default guiding parameters may not be set correctly, and we will be handicapped in trying to assess your guiding performance and help you resolve problems.  If you use the new-profile-wizard, these values will probably be correct.
  • '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 'Advanced...' button to the right of this control will launch a calculator 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, you may want to set this option.  Conversely, if PHD2 has no scope pointing information available, this option will normally 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, you can simply clear the mount calibration (see below).  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 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 changing the mount guide speed, for example.  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 may apply a compensation automatically, in which case the box should be left un-checked.  Don't confuse this option with 'Declination backlash compensation', which is an entirely different feature.
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 and by the size of your tracking region.  This is only an optimization, so 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.  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.
  • '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.  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' - a diagnostic option for working with ASCOM mount drivers that may report the slewing status incorrectly.  The option should be left checked unless you know the driver for your mount has this problem (highly unlikely).   Also,  it will generally not detect slews that are initiated by the hand-controller.  In any case, PHD2 will continue to try to track the guide star, so slews will typically result in lost-star notifications - beeping and flashing of the PHD2 image window.
Advanced Calibration Calculator

To review or change calibration parameters, be sure the topmost four 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 (or via the new-profile-wizard), 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 usually show the correct values unless the driver doesn't report them correctly.  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.  Note: changing the guide speed setting here never changes the guide speed setting in the mount - that can only be done via the mount driver or the hand-controller.   If this field is already filled in, changing it will have no useful effect. 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 .  You can also change the total calibration distance if you have some reason to think the default value of 25px is inadequate. You should be careful with this because "bigger" is not always "better".  With larger required calibration distances, the calibration is more likely to be affected by large uncorrected periodic error in RA or large drift in Dec.   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.  Again, PHD2 never changes the guide speed setting in your mount regardless of what may be entered in the 'Guide Speed' field.

The goal of the calculator is to recommend a step-size that is likely to result in an accurate calibration without wasting too much time, and the automatically computed values will usually work well.  As long as you are getting successful calibrations without alert messages, the calibration parameters should be left alone.  If you've used the new-profile-wizard to build your equipment profile, there's rarely any reason to use the 'Advanced" button and the Calibration Calculator dialog.

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.) .'  However, there is rarely a good reason for changing it if you have protected yourself against hot pixels (Minimum star HFD).
  • 'Use backlash comp' - 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 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. 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 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, 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 probably repeat that procedure.

Declination Backlash Compensation

Most commonly-used mounts have some amount of backlash in declination.  This causes a delay whenever there is a change in direction of the Dec guide commands.  During this interval, the declination gears aren't fully engaged and the axis doesn't move in response to the guide commands.  Many mounts have settings for backlash compensation but these should not be used for guiding - they are typically intended for visual use only.  Because the actual amount of compensation needed at any given time may depend on the pointing position and the mechanical load on the system, a fixed value will usually result in oscillations that never stabilize.  The backlash compensation implemented by PHD2 is adaptive, meaning that the compensation amount is adjusted up or down depending on the measured results.  Before enabling this feature, you should run the Guiding Assistant and measure the declination backlash - the time delay required to fully reverse direction in declination.  Keep in mind, the higher the guide speed setting in the mount (e.g. 0.9x sidereal), the smaller this delay will be.  If the measured amount is 3 seconds or less, the Guiding Assistant will recommend trying backlash compensation.  If you apply that recommendation, the backlash compensation settings will be handled for you automatically.  The UI controls for backlash compensation include settings for 'minimum' and 'maximum' compensation amounts.  These effectively limit the range of the adjustments that are made to the starting compensation value.  If you're experienced with your mount's behavior, you can adjust these settings manually to be sure that the compensation stays within a range that you know works well.  Otherwise, you should just leave these at their default values.  The backlash compensation algorithm will generally work well if the backlash is less than a few seconds and the mount doesn't have other significant mechanical problems.  You should expect a short period of  instability when guiding starts because the initial state of the Dec gear system is unknown - just let it stabilize before you actually start imaging.  If you see recurring periods of Dec oscillation or the axis won't settle down, disable the compensation feature and submit your debug log file to the PHD2 support forum.  It's important to note that the correct amount of Dec backlash compensation will often change depending on the scope pointing position.  This can be caused by uneven gear wear on the Dec axis or by differences in the gravitational load being applied to the axis.

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 5-10 arc-min 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 run the Guiding Assistant for several minutes.  Look at the guiding graph and note whether the guide star is drifting north or south.  Once you see this, reset the Dec guide mode to issue corrections in the appropriate direction.  For example, if the star is drifting north, set the Guide mode to 'south.'
  2. Try using the 'LowPass2' guiding algorithm 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, you may want to 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 these 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 elsewhere, 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 'AO Travel' field specifies the number of steps the AO can make on each axis.  The default value works well for most SX AO devices but in some cases it may be too large.  If you encounter problems during AO calibration where the AO hits its limits, you can adjust the 'AO Travel' amount downward.  

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.  The 'Mount Backlash Compensation' controls let you apply PHD2 Dec backlash compensation when mount bumps are done.  This can help speed up large dither operations, but it's only appropriate if the mount has a limited amount of Dec backlash. You should use the Guiding Assistant to measure your Dec backlash and see what it recommends with regard to using Dec backlash compensation.

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 'Hysteresis'', which gives you an easy way to control damping and aggressiveness.  If you use a different algorithm, you should probably start with a high level of aggressiveness there as well.   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 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