Using PHD2 Guiding

There are five basic steps to start guiding.
  1. Open the Connect Equipment dialog by pressing the USB-icon button and select the configuration profile you created with the New Profile Wizard.  Connect to the camera and mount.
  2. Pick an exposure duration from the drop-down list.
  3. Hit the loop button and look at the available stars, adjusting focus if necessary. Move the mount or adjust the exposure duration until you can consistently see usable stars.
  4. Click the 'Auto-Select Star' icon to choose the best guide stars available.
  5. Press the PHD2 Guide button.
Details of these operations will be described in the sections below.

Equipment Connection

In order to begin guiding, PHD2 must first connect to your hardware: the guide camera, the mount, and, optionally, an 'aux' mount, an adaptive optics (AO) device, and a rotator.  When you click on the USB icon, you'll see a dialog that looks something like this although the equipment details will differ.  The various choices in this dialog are discussed below.



Equipment Profiles

At the top of the 'Connect Equipment' dialog are controls for managing equipment profiles.  All of the guider settings in PHD2, default or otherwise, are automatically stored as part of an equipment profile. This includes calibration data and the last three Guiding Assistant reports.  The only significant data not stored in the profiles are the dark libraries and bad-pixel maps which are stored in the file system.  If you have only one guiding setup - you use the same camera and guide scope combination each time - you will only need one profile.  But you may have multiple equipment configurations - for example, an off-axis-guiding arrangement for a long focal length scope and a separate guide scope/camera configuration for a shorter focal length imaging scope.  The PHD2 guide settings for those configurations will be different, so you should use separate equipment profiles.  The controls at the top of the 'Connect Equipment' dialog let you choose the profile you want to use and to create/edit/remove profiles as you see fit.  When you select a profile and connect to its associated equipment, all of the settings last used with that profile are automatically reloaded.  Once you've established the profiles you need - perhaps only the default one - you can simply click on the 'Connect All' button and you're ready to move ahead.   If you already have a suitable default equipment profile and you simply want to connect to the equipment just as before, you can do a <shift>-click on the main screen 'USB' button and PHD2 will automatically re-connect to your hardware.  Your profile information will be retained when you install new releases of PHD2 so long as you don't first un-install.  Un-installing PHD2 is almost never necessary and will simply create complications - it is not a useful trouble-shooting step.  More information about managing profiles can be found here: -  Managing Equipment Profiles

New-Profile-Wizard

The best way to create a new profile is to use the "Wizard" capability, and for new users, this is a critical first step.  The wizard takes you through a sequence of windows that explain the various settings and help you decide how to set them.  It will also calculate baseline algorithm settings that are likely to work reasonably well for your set-up.   Creating a profile this way is faster and less error-prone than doing it by hand in the 'Connect Equipment' dialog.  When you run PHD2 for the first time on your system, this wizard will be automatically launched.  Subsequently, you can use the new-profile wizard by clicking on the 'Manage Profiles' field in the 'Connect Equipment' dialog, then  choosing 'New using wizard...'.  

The wizard asks a number of questions that are important for getting your profile built correctly.  The explanatory text in each pane of the wizard should make clear what is being asked and what needs to be done.  But here are some additional tips to help you through the process:

1. Connection Options: As you make selections for the various devices, you will usually see a prompt asking if the device is already connected and ready to communicate with PHD2.  If you say 'yes', PHD2 will try to connect and then fill in some of the data fields with information read from the device.  Saying 'no' simply means you'll have to enter the data by hand.  If PHD2 tries to connect with the device and fails, you'll still be able to proceed by just entering the data manually.   Device-connection in the wizard is basically a convenience feature that makes it easier to fill in the fields with accurate values.  You won't see the prompt if PHD2 already knows the device can't return useful information - for example, if the mount choice is 'on-camera.'
2. Camera connection pane: unbinned pixel size.  If you said 'yes' to the connection prompt,  this information will usually be filled in automatically and the control will be disabled.   If you said 'no' or if the device doesn't report its pixel-size, you'll need to enter the value by hand.  You should be able to get the unbinned pixel size from the camera spec sheet or the manufacturer's web site.  If the pixels aren't square, just specify the larger dimension or the average value if you prefer.  This won't have any effect on your actual guiding results, but it will allow PHD2 to know the image-scale for your set-up.  This is used for setting baseline guiding parameters, doing sanity-checks on calibrations, reporting guiding performance, and getting support on our forum..
3. Camera connection pane: binning level.  If your guide camera supports binning (some do not), you can specify what level of binning you want to use for this equipment profile.  If you want to use the same equipment set-up with different binning levels, it's best to create separate profiles for each binning value.  If your guide camera has very small pixels and you have also specified a long focal length, you may see a 'warning' icon next to the Pixel Scale field.  That is telling you the specified image scale is probably too small and you should bin the camera if possible.
4. Camera connection pane: guide scope focal length.  This seems to be a common place for mistakes, so it's worth being careful and getting it right.  The correct value is not the aperture of the guide scope, it is the focal length.  For example,  if you're guiding with a 50mm finder scope, the focal length will not be 50mm - it will probably be something closer to 150-175mm.   A 60-80mm refractor guide scope will probably have a focal length in the range of 240-500mm, not 60-80mm.  Similarly, if you're guiding with an OAG on your main imaging scope, the focal length will be that of the main scope.  In some cases, you may be using a small threaded focal reducer on the guide camera, so that must also be taken into account.  Like the pixel-size entry, the focal length doesn't demand a great deal of precision, but you should get as close as you can.  Otherwise, the performance numbers may not reflect your actual results and the baseline guiding parameters may be sub-optimal.
5. Mount connection pane:  mount guide speed.  This is another area that seems to cause confusion.  The guide speed is a parameter set in the mount or in the mount driver, it's not something controlled by PHD2.  PHD2 never sets the mount guide speed, it only reads it.  It is usually expressed as a multiple of the sidereal rate and is typically in the range of 0.5x - 1x sidereal.  Despite what you may read elsewhere, it's usually best to use guide speeds in this range rather than much lower speeds.   Higher guide speeds can help to clear backlash more quickly and may help to overcome stiction problems.  If you say 'yes' to the connection option prompt, PHD2 will attempt to read the current guide speed from the mount.  If this fails for some reason, you'll need to enter the guide speed manually.  PHD2 uses this value to automatically set the calibration step-size and to aid in checking calibration results; but the guide speed information is not important for the actual guiding.  If you're using different guide speeds on the RA and Dec axes, enter the larger value.   If you really can't determine what the guide speed settings are in the mount, leave the setting at the default value of 0.5X.  This pane also has a checkbox regarding the presence of high-precision encoders in the mount.  These devices are sometimes included on expensive, high-precision mounts, and you are likely to know if you have them.  Most users will leave this box unchecked.

In the last pane of the wizard dialog, you're given two options: 1) To automatically restore the PHD2 calibration whenever the profile is loaded and 2) to immediately build a dark library when the new profile is saved and the wizard completes.  The first option is only presented if your profile uses an ASCOM or INDI mount connection.  It is a convenience item especially for users with permanent setups.  You can also use this option if you can insure that the guide camera orientation in its adapter tube remains the same from one night to the next.  If you're in doubt or don't want to bother with how the guide camera is oriented, leave the box unchecked.  You should always select the option to build a dark library unless you already have a compatible dark library that you're going to import from a different profile.  If you are changing cameras and want to keep the dark libraries and bad-pixel maps associated with the old camera, you should create a separate profile for the new camera.  When a camera selection is changed in an existing profile, the previously built dark library and bad-pixel map data will no longer be usable.   That also applies to using the same camera with different binning values. Setups using different binning factors should be kept in separate profiles because the dark library and bad-pixel maps depend on the binning factor.

Camera Selection

The Camera drop-down list shows all the camera types natively supported by PHD2 in addition to all the ASCOM cameras you have already installed.  In all cases, the OS-level drivers for the camera must be installed correctly in order for PHD2 to connect to the device.  If the camera uses an ASCOM interface, you'll also need to install the corresponding ASCOM driver for the camera.  If you don't see your ASCOM-compatible camera shown in the drop-down list, you probably don't have the ASCOM driver installed. Neither the ASCOM nor OS-level drivers are included with PHD2, so they must be located, downloaded, and installed separately.  For non-ASCOM cameras, the PHD2 distribution does include the additional application libraries (SDKs)  needed by PHD2 to use the camera.  In some cases, the camera may have both ASCOM and native interfaces and you're free to choose whichever you prefer.  In some cases, the two different versions may operate the camera in a slightly different way or expose different sets of camera properties.  The camera vendors update their drivers and SDKs frequently so you need to keep track of this and keep your camera drivers current.  In some cases, a release update to PHD2 may require the latest camera driver software so you should also be aware of that.

It is not practical to provide an exhaustive list of cameras that are supported by PHD2.  In many cases, camera vendors extend their product lines by updating their lower-level drivers without having to change the application libraries used by PHD2.  In those cases, we aren't aware of the changes unless a user reports problems.  The list shown below should be interpreted as follows:
1.  If the camera vendor is completely absent, it is unlikely that the camera is supported, or it may only be supported using a web-cam interface
2.  If the camera model is shown in the list, it is supported unless there are unresolved problems with the vendor's drivers
3.  If the specific camera model is absent but earlier models are shown, it is likely the camera is supported
4.  If the camera uses an ASCOM interface, it is supported

Since the PHD2 download is free, the simplest course of action is to  install it and see if your camera is shown in the PHD2 drop-down list.  Alternatively, you can check for camera support info in the Wiki on the PHD2 Google forum:  https://github.com/OpenPHDGuiding/phd2/wiki/CameraSupport

Finally, you can always post a message on the open-phd-guiding forum asking if anyone has experience with the camera.  Access to the forum requires a user log-in so you should establish an account in order to get help with any guiding problems you encounter.

Baseline list of supported cameras:

Windows:
Mac:
USB cables and computer ports are typically used to communicate with the guide cameras.  These components can be the source of problems for a variety of reasons. Inexpensive, poor quality USB cables - even the ones supplied with the cameras - often have very small conductors and are therefore prone to being damaged.  The connectors and cables are generally not intended for outdoor use although that's where we're using them.  Finally, Microsoft Windows releases starting with version 10 will, by default, try to conserve power by suspending USB ports when the OS senses periods of inactivity.  All of these things can interfere with imaging and guiding operations and you should be aware of them and devise strategies for avoiding them.  If you encounter these problems, you should  follow the instructions in the Trouble-shooting section - Camera Connection Problems

Support for SBIG Dual-chip Cameras

Many cameras from the Santa Barbara Instrument Group (SBIG) have two sensors - a primary one for imaging and a second, smaller one for guiding.  While the two sensors are physically separate, they share electronics inside the camera and more importantly, share a single USB data link to the computer.  This means that  downloading of data from the two sensors must be coordinated - you can't retrieve a guider image while an image from the main sensor is being downloaded.  Beyond that, Windows will only allow one application at a time to connect to the camera over the single USB link.  These are physical and architectural restrictions that can't be circumvented by PHD2.  However, it is possible for the camera-controlling (image capture) application to implement an interface for PHD2 to get data from the guide chip - essentially, a "side door" mechanism that won't violate any of the above rules.  With this arrangement, the image capture application is acting as a traffic cop to coordinate access to the two camera sensors.  One imaging application that does this is Sequence Generator Pro (SGP).  If you use SGP as your main imaging application, you can also use their "SGP API Guider" module, which allows PHD2 to access the guide chip on the SBIG camera.

ASCOM Camera Properties

If you choose an ASCOM camera, you'll also be able to access the ASCOM setup dialog for that camera by clicking on the properties button immediately to the left of the 'Connect' button:



Depending on the camera, this may provide access to properties that are not controlled by PHD2.

Multiple Cameras of the Same Type

If your computer is connected to multiple cameras from the same manufacturer, you'll usually need to specify which camera should be used by PHD2.  You can do that by clicking on the 'fork' button to the right of the camera drop-down list:



Clicking this button will show a list of the available cameras and you can choose the specific one you want.  PHD2 will remember the choice and save it as part of your equipment profile, so you should only need to do this once.  An alternative is to simply connect one of the cameras, the intended guide camera, at the time you build your equipment profile.

Some camera manufacturers don't do a good job of staying connected to the right camera and may switch from the guide camera to the imaging camera without any notification.  If you see PHD2 alert messages warning you that the camera sensor geometry has changed and you need to re-build your dark library, the likely source of the problem is that PHD2 has been connected to the imaging camera.  These problems are caused by the camera drivers, there's nothing PHD2 can do to prevent them.

Mount Selection

The Mount drop-down list displays options for connecting to your mount.  There are generally two ways to do this:
  1. Use an ASCOM-compatible (or INDI) telescope driver that sends guide commands to the mount over a serial cable (or more commonly,  a USB/Serial connection).  This is the recommended approach for the reasons explained below.
  2. Use the ST-4 compatible guide port interface on the mount with a specialized cable and an intermediate device like a camera or a Shoestring box
The ASCOM interface relies on third-party drivers to communicate with the mount.  These drivers are available from the ASCOM web site (ASCOM Standards) or from the mount manufacturer - they are not distributed with PHD2. So the drop-down list will be populated by only those ASCOM drivers you already have installed on your system. If you can't find your mount in the list, you either haven't installed its driver correctly or it only supports ST-4 guiding.  The ASCOM driver must support the 'PulseGuide' interface, which has been a requirement for ASCOM compliance for many years and is widely supported.  With this type of mount control, guide commands are sent from PHD2 to the mount over the serial interface.  The high-level PHD2 guide commands (e.g. "Move west 500 mSec") are translated by the mount firmware into the appropriate motor control signals to execute the command.  With the ASCOM interface, PHD2 can also obtain the pointing position of the telescope, especially the declination and side-of-pier, which can be used as factors in guider calibration and provide greatly improved ease-of-use.

The "Guide-port" interfaces use a specialized, hardware-level control port available on most mounts.  To use this type of interface, there must be another device in the link between PHD2 and the mount:

  1. Any of the guide cameras which have an ST-4 compatible "on-camera" guider interface. Use the 'on camera' mount choice for these setups.
  2. Any of the Shoestring GP-xxx devices
  3. A supported AO device with a guide port interface
With this style of interface,  PHD2 guide commands like "Move west 500 mSec" are translated by the intermediate device (camera, Shoestring box, AO) into electrical signals necessary to drive the mount motor for the correct length of time.  

Aux Mount Selection

If you are forced to use the ST-4 style of guiding in the 'mount' section, PHD2 will not be able to use that interface to determine the scope's pointing position.  As a consequence,  guider calibration won't be automatically adjusted for declination, nor will it be automatically flipped when the side-of-pier changes.  You can restore these features by specifying an "aux" mount connection that will be used to get the telescope pointing information.  An example is shown below:



For Windows users, the "aux" mount can use any of the ASCOM-compatible mount drivers, while Linux users can take advantage of INDI drivers.  The "aux" mount choice will be used only if the primary mount interface cannot return pointing information - it will otherwise be ignored.  Note: some mounts (e.g. Celestron and iOptron) have a separate hardware port also labeled 'Aux' - DO NOT USE THIS  for guiding - it is completely unrelated to the 'Aux' connection in PHD2.  The last entry in the list of 'Aux mount' connections is labeled "Ask for coordinates."  This can provide a rudimentary aux-mount facility if you can't use an ASCOM or INDI connection to your mount.  If you need to pursue this option, you can read about the details here: Ask-for-coordinates.

Most ASCOM mount drivers use a serial port interface, so the driver expects to use one of the Windows 'COM' ports.  Since most personal computers no longer have serial port connectors, you can use one of the USB ports and a USB-serial adapter. Newer mounts may include such an adapter or have one built in. The software that comes with the USB-serial adapter will create a software COM port, and that's the one you'll use with the ASCOM driver. The first time you connect to the mount with ASCOM (either as 'mount' or 'aux-mount'), you'll need to tell the driver which COM port to use.  That's part of the driver's setup dialog, and you can quickly open that window by clicking on the 'properties' icon just to the left of the 'Connect' button in the above image.  Once you've done this, the COM port will be saved as part of the equipment profile

Benefits of Using ASCOM (or INDI) connections

If you're running on a Windows platform, you'll probably be better off using an ASCOM connection for guiding your mount.  On other operating systems, your best choice is likely to be an INDI connection if one is available.  This advice may be contrary to some old-school experience or folklore on the Web and probably isn't what you'll hear from the manufacturer of the guide camera.  But the benefits of doing so with PHD2 are substantial, and you should use this alternative unless you have specific and credible information against it.  Here are some of the primary benefits:
  1. A drastic reduction in the number of calibrations you'll need to perform.  Changing targets will not require another calibration because PHD2 can know where the scope is pointing and automatically make adjustments to the guider calibration.  Most users get a good calibration and then re-use it until they make hardware changes of some kind.
  2. Automatic adjustment for meridian flips - no need to remember to manually flip the calibration data.
  3. Automatic adjustment of RA calibration to handle targets in different parts of the sky (declination compensation)
  4. Elimination of the ST-4 guide cable as a point of failure - this is a surprisingly common problem because the cables can be damaged or confused with similar-looking cables (e.g. telephone cables)
  5. Elimination of a moving cable that can snag, drag, or bind as the scope is moved around.
  6. Improved ability for PHD2 to sanity-check calibration results and warn of possible problems before you waste hours of imaging time.
  7. Better diagnostic and trouble-shooting information, which is particularly helpful if you need to ask for assistance
  8. Availability of scope-slewing options during drift alignment which can further speed the process of polar alignment
A common misconception, sometimes seen on Web forums, is that ST-4 guiding is hardware-based and thus more accurate or efficient.  For any of the modern mounts you're likely to encounter, this is no longer true - there will always be software running at each end of the cable, just like ASCOM guiding.  The bottom line is this: if you have an ASCOM or Indi driver available for your mount, you should probably use it.

Adaptive Optics and Rotator Selections

With PHD2, you have the option of controlling the Starlight Xpress adaptive optics unit and/or any ASCOM-compatible camera rotator.  These can be specified by clicking on the 'More Equipment..." button in the above dialog:



If you don't have these devices, just leave the selections at 'None.'  If these devices are connected, you'll see additional tabs in the 'Advanced Settings' dialog that provide access to various device-related properties. Adaptive optics (AO) devices generally require some study before use and you should familiarize yourself with their operation before trying to use them.  Some background information can be found here: Using an AO .  PHD2 does not control a rotator, but it will read the current angle setting from the rotator and adjust the guiding calibration if needed.  Rotators are used to control the orientation of the imaging camera with respect to the sky - perhaps to keep the orientation the same on opposite sides of the pier or to create a favorable composition of the objects in the field of view.  If the rotator is part of an off-axis-guider assembly, its rotation will affect the PHD2 calibration.  In this situation, PHD2 should be connected to the rotator so the calibration can be adjusted automatically.

Simulators

All of the PHD2 devices - camera, mount, AO, rotator - include built-in simulators.  Don't confuse these with any of the ASCOM simulators which may be installed on your system - those will have 'ASCOM' in their names.  Although you can connect to the ASCOM simulators, they don't provide the necessary feedback to PHD2 for guiding and calibration.  As a result, they're only useful for limited types of testing and experimentation.  But you can use the built-in simulators to explore how PHD2 works and to decide how you want to use the program.  Virtually all of  PHD2's features, including full calibration and all the graphical display options, will work properly when the built-in device simulators are used.  You'll even see fairly realistic guiding performance to give you some idea of what to expect in the field.  To get started using the simulators, use the new-profile-wizard, choosing 'Simulator' for the camera type and 'On-camera' for the Mount type.

Remember, the simulators are not useful for trouble-shooting any problems you encounter with your real mount and they can't be used for testing your actual hardware.  Both the camera and the mount must be real devices in order to diagnose problems or otherwise get your gear calibrated and working.  In that sense, what you see when using the simulators is realistic but "fake" behavior.  The simulators can be useful in some cases for reproducing PHD2 application problems, but not for anything having to do with your actual guiding equipment.

Exposure Time and Star Selection

The guide stars are selected while "looping" is active.  The best approach is to let PHD2 Auto-select the guide stars for you by clicking on the 'Auto-Select Star' icon in the main window.  In nearly all cases, it will do a better job of star selection than you can do manually because it takes a number of things into account - for example,  lack of saturation, qualification as a "real" star, proximity to other stars and to the edge of the field, etc.  Using the 'Auto-Select' function is also the only way you can take advantage of multi-star guiding.  If you don't use the auto-select feature, you can select a guide star candidate by clicking on it but that will result in using only that single star for guiding.  After that is done, a green box will appear to frame the star.  If you pick a star that is too bright (saturated), the status bar will show a  red 'SAT' label  and you should choose a fainter star.  You should adjust the gamma slider to the left to see fainter stars. Most new users are fooled by this and often choose the brightest star they happen to see in the field of view.  But that choice is often a saturated star, not a good choice for auto-guiding. In other words, if you know the name of the star, it's almost certainly too bright to be used for effective guiding.  All of these pitfalls can be avoided by using the auto-select feature. The choice of exposure time will depend entirely on your equipment, sky conditions, and the available stars.  The exposure time you choose has several implications:
  1. It affects the signal strength (brightness) of the selected star - a brighter star will stand out better from the background and will generally produce better guiding results so long as it is not saturated.
  2. It also determines the frequency with which guide commands are sent to the mount - guide commands cannot be sent any more frequently than once for each exposure cycle.  Some mounts need frequent small guiding adjustments while others do not - you may need to experiment to understand what works best for your situation.  
  3. It has a strong effect on the sensitivity of the guide algorithms to seeing conditions.  As the exposure time is increased up through 2-4 seconds, the effects of seeing are smoothed out.  This is particularly noticeable if you are guiding with a long focal length setup.  Of course, the convenience of using longer exposures must be traded off against the need for the mount to get sufficiently frequent corrections.  For mounts that have too many high-frequency errors in RA, you may need to work with 1-sec or even 0.5-sec exposures in which case using multi-star guiding becomes even more important.  If you're not familiar with the concept of astronomical seeing, you can get more info here: Astronomical Seeing
As a starting point, try using exposure durations in the range of one to three seconds.  If you want to de-select a star without choosing another one, you can do a shift-click anywhere on the image display window.   If you are just starting with your equipment set-up, it's critical that you carefully  focus the guide camera.  You can use the Star Profile tool to help with that process or whatever other focusing tool you are comfortable with - a focusing mask, a camera app like SharpCap, etc.  Just trying to do it "by eye" is rarely going to produce good results.   If you're using a small guide scope, like a finder-scope, the focus may react strongly to small adjustments.  It's important to spend the time to get a good focus because poorly focused guide stars can lead to poor guiding results.  

The camera exposure control displays a wide range of pre-set exposure times.  Exposure times smaller than 0.5 second are intended for use with adaptive optics devices or in other special situations - they are not recommended for use with typical guide camera set-ups.  There is also a 'custom' exposure option at the bottom of the list that lets you specify a value not already displayed.  Again, this is intended for special applications, for example where an unusually long exposure time is needed.

There is also an Auto exposure time selection available. When exposure is set to Auto, PHD2 will attempt to adjust the exposure to keep the selected guide star at a consistent signal-to-noise ratio (SNR) value while working within the exposure limits you choose. This is a specialized measurement used by PHD2 to determine how well the star can be distinguished from the background - it is similar but not identical to the signal-to-noise ratio used in photometry.  The Auto setting is intended for use in automated imaging or for AO users who want to minimize the exposure time without losing the guide star.   Automated imaging  involves slewing and subsequent unattended guide star selection, and failure to find a bright-enough star will cause the automated session to fail.  In such cases, a "long" exposure time may be preferable to finding no guide star at all.  The settings to control Auto-exposure are on the Camera Tab of the Advanced Settings dialog.  

Multi-Star Guiding

Most guiding configurations can benefit from guiding on multiple stars rather than just one.  This results in using a weighted average centroid position of multiple stars rather than just the centroid of a single star.  Multi-star guiding is enabled using a check-box on the Guiding Tab of the Advanced Settings dialog - one that is "checked" by default.  When this option is enabled, the Auto-select function will identify up to 12 stars in the field of view that have adequate sizes and SNR values.  No more than 9 of these will be used at any one time, but the remainder will be used to replace secondary stars that are lost or rejected for some reason.  The "primary" star is the single best candidate, the same one that will be selected if multi-star guiding is disabled.  As guiding proceeds, some of the secondary stars may be rejected because they are too dim or have drifted outside the field of view.  This is of no concern, the algorithm handles the secondary list automatically.  When multi-star guiding is active, the PHD2 image display will show the usual rectangle around the primary star plus circles around the secondary stars.  All other UI features associated with a guide star - brightness properties, SNR, FWHM, etc - relate to the primary star, not the entire list.  Multi-star guiding can be enabled or disabled while guiding is active.  However, enabling the feature will force another 'auto-select' procedure. The multi-star algorithm uses the secondary stars to refine the measurements of guide star movement and lower their volatility, so there is essentially no way for it to degrade guiding performance compared to single-star guiding.  The benefit it provides to your overall guiding will depend on many factors including image scale, star and background sky brightness, star size, focus, tube currents, and camera noise.  Because of the way the algorithm is implemented, your best option will be to try it and decide for yourself.  Note: if you don't use the auto-select function and instead choose a guide star manually, multi-star guiding will be inactive.

Automatic Calibration

Conventional Mounts

Two things need to be measured by PHD2 as part of guider calibration:
  1. The angular orientation of the camera sensor relative to the right ascension and declination axes of the mount and the sky
  2. The length of the guide pulse needed to move the telescope by a specific amount
PHD2 handles these measurements automatically by sending guide pulses to the mount and watching how far and in which direction the star moves between guide camera images.  This process begins after you have selected a star and then clicked on the PHD2/Guide icon button.  Yellow cross-hairs will appear over the original location of your guide star and PHD2 will start to move the mount in various directions, tracking how the star moves as a function of what move commands were sent to the mount.  The status bar will display the commands as they are sent to the mount, along with the incremental movements of the guide star relative to its starting position.  PHD2 will do this on both axes, first moving east and west, then north and south.  PHD2 typically wants to move the star up to 25 pixels in each direction in order to get an accurate calibration.  Once this is complete, the crosshairs will turn green and guiding will start automatically.  Because this is a measurement process that is subject to various kinds of mount and atmospheric effects, the most accurate results will be gotten when the scope is pointing within 20 degrees of Dec = 0 (near the celestial equator) and at least 60 degrees above the nearest east/west horizon (i.e. within 2 hours of the celestial meridian).  Calibrations can be done in other pointing positions if required by conditions at your site but they will be subject to somewhat more measurement uncertainty.  You cannot do calibrations pointing very close to the north or south celestial poles, and the mount must be tracking at the sidereal rate.  If you encounter difficulties completing a calibration, you should immediately retry using the Calibration Assistant .

Although PHD2 moves the guide star in all four directions, only the west and north movements are actually used to compute the guide rates and camera angles. The east and south moves are used only to restore the star roughly to its starting position.  Before the north moves are begun, you will see a sequence of pulses that are intended to clear backlash.  PHD2 takes only a moderate approach to clearing this backlash, watching for a clear pattern of movement in a single direction with no reversals.  If your mount has a large amount of Dec backlash or you're guiding at a long focal length in poor seeing conditions, this phase of calibration may not remove all the backlash.  Your best option to avoid this problem is to use the Calibration Assistant. You may also see that the south pulses leave the guide star well-short of its starting position - this is another visual clue that you have significant declination backlash in your mount but it does not invalidate the calibration.  If you see evidence of sizable backlash, you can run the Guiding Assistant tool and measure it directly.

In most cases, calibration will complete automatically without any user involvement, particularly if you are using the Calibration Assistant.  If the mount doesn't move sufficiently in the west or north directions, you will get an alert saying the calibration has failed. Failures of this type are pretty uncommon but they can occur if some part of the hardware is simply not working (e.g. a bad guide cable) or if you haven't followed all the recommendations in the Calibration Assistant.

If you're using an ASCOM (or Indi) connection for either the 'mount' or 'aux-mount', a good calibration can be re-used for a long time, and that is the preferred way to operate.  These  connection options allow PHD2 to know where the telescope is pointing, so a calibration done at one point in the sky will be automatically adjusted as you slew to different targets.  The old rule of having to re-calibrate whenever you slewed the scope or switched the side-of-pier is a thing of the past so long as PHD2 has pointing information.  With this type of set-up, you would only re-calibrate if you rotate the position of the guide camera by more than a few degrees or make other major changes to the hardware configuration.  If you chose 'auto-restore calibration' when you built your profile or chose that option in the Guiding tab of the Advanced Settings dialog, you can simply connect to your gear, auto-select your guide stars, then begin guiding immediately.  Beginners will probably have more success if they do a fresh calibration at the start of each night's session.  Finally, if you're using an instrument rotator as part of your equipment profile, PHD2 can use the 'Rotator' connection to adjust the calibration data based on the angular position of the guide camera - one less reason for re-doing a calibration.

You can always review the results of your last calibration by using the 'Tools' menu and clicking on 'Review Calibration Data'  That will open a dialog that shows a graphical representation of the mount's movements along with the values that were computed for guiding your mount.  This window is described elsewhere in the Calibration Details section of the help file.  As a quick quality check, you can open this window and confirm that 1) the RA and Dec lines are roughly perpendicular and 2)  the plotted points are roughly linear with no significant curves, bends, clumping of points, or reversals in direction.   If you do see these kinds of odd patterns in the graph, you should probably re-do the calibration.  Even with high-end mounts, calibrations can occasionally go awry because of environmental conditions, especially wind and bad seeing.  

After a calibration is completed, PHD2 will "sanity check" the results to be sure the calculations at least look reasonable.  If they don't, you will see an advisory message at the top of the main window that describes the situation.  You can choose to ignore the alert or click on 'Details' to get more information.  It is generally advisable to pay attention to these alerts because there is no point in trying to guide using a significantly bad calibration.

Adaptive Optics Devices

If you are using an adaptive optics device, there are actually two calibration processes that must complete.  The first handles calibration of the tip/tilt optical element in the AO and calculates the magnitude and direction of the adjustments as they relate to displacements of the guide star.  The second calibration is the one described above, dealing with guide commands that need to be sent to the mount.  Known as "bump" commands, these will be issued when the guide star has moved beyond the range of corrections that can be achieved with the AO alone.

Guiding

Once guiding has begun, diagnostic messages will be displayed in the status bar to show what guide commands are being sent to the mount. PHD2 will continue guiding until you click on the 'Stop' icon.  To resume guiding, simply start looping exposures again, auto-select your stars, and click on the 'Guide' button.  You will not need to repeat the calibration in order to resume guiding.

In some cases, PHD2 may lose the guide star and you'll be alerted by an audible beep and flashing orange crosshairs.  There are several reasons this might occur:
  1. Something may be obscuring the star - clouds, the observatory roof, a tree, etc.
  2. The star may have abruptly moved out of the tracking rectangle because something shifted in the mount/camera/cabling infrastructure - cable snags can cause this
  3. The star may have "faded" for some other reason, perhaps because it is overly faint, the camera is not well-focused, or the star size and brightness is fluctuating above and below the thresholds for acceptability.
  4. The star's brightness may be fluctuating enough to trigger the star-mass checking feature.  If this happens frequently, the feature should be disabled (Advanced Settings/Guiding tab)
Obviously, you'll need to identify the source of the problem and fix it.  It's important to understand that PHD2 will not start moving the telescope trying to relocate the guide star.  It will simply continue to take exposures and look for a guide star to reappear within the bounds of the current tracking rectangle.   If the lost star condition persists for a considerable time, your mount will probably drift signiificantly off-target.  In that event, a later recovery of a guide star in the tracking region is likely to be a different star and your imaging target won't be properly centered.  If you're doing unattended imaging, the imaging application should handle this situation by recentering the frame after star recovery has occurred - this is not something that can be done by PHD2.  Isolated or intermittent lost-star events do not usually cause poor guiding or ruined images.  But extended periods of lost-star events can allow the mount to go too long without guiding corrections and the image quality may suffer.

When you first start guiding, you may see an 'alert' dialog at the top of the window if no dark library or bad-pixel map is being used.  You can choose to ignore this warning and continue with guiding, but you will probably get better results if you spend the few minutes needed to construct a dark library for future use.

If you are using a German equatorial mount (GEM), you will usually have to do a "meridian flip" around the time your image target crosses the meridian.  This means you (or your imaging application) will move the telescope around to the opposite side of the pier and then resume imaging.  Doing this invalidates the original calibration, typically because the declination directions are now reversed.  If you are using an ASCOM (or 'aux' ) mount interface, your calibration will be adjusted automatically and you can simply resume guiding (assuming you haven't also rotated the camera or focuser).  If you aren't using an interface that returns pointing position, you will need to take action to adjust the guider calibration.  You can, of course, simply do another calibration on the current side of the pier, a process that will typically take only a couple of minutes.  Or, you can use the pull-down menu item under 'Tools/Modify Calibration' to "Flip Calibration Now" and then resume guiding immediately.  Note: 'flip calibration data' will have no effect if PHD2 is using an ASCOM or 'aux-mount' connection.

In some cases, you may want to force a re-calibration.  For example, you may have rotated the guide camera as part of resolving a cable problem.  You can do this by clicking on the 'Brain button',  moving to the 'Guiding' tab, and clicking the 'Clear mount calibration' checkbox.  Or, you can simply do a <shift>click on the  'Guide' button on the main screen and PHD2 will start a calibration run.  

Once you have started guiding, you will almost certainly want to know how things are going.  You can of course watch the star in the guide camera display but in many cases you won't be able to see all the small adjustments that are taking place.  PHD2 provides many tools for measuring and displaying your performance, as described in the Monitoring Tools section.  Several of the guiding algorithms have limit settings for the maximum guide correction that can be issued with a single command.  If these values are smaller than what is needed to correct the mount's position, you will see an alert dialog at the top of the main window advising you of the situation.  If this is a recurring problem and you haven't reduced the max-move parameters below their default values, the source of the problem probably lies with the mount.