Using PHD2 Guiding

There are five basic steps to start guiding.
  1. Press the USB-icon button and connect to your guide 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 as needed to find a suitable guide star.
  4. Click on a non-saturated star that's not very near an edge for use as the guide star.
  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, or a rotator.  When you click on the camera icon, you'll see a dialog that looks like this:



Camera Selection

The Camera drop-down list shows all the camera types currently supported by PHD2.  With the exception of ASCOM-compatible cameras, the camera drivers are bundled with  PHD2  and will reside on your system after the PHD2 installation.  ASCOM-compatible camera drivers are available from the ASCOM web site or from the camera manufacturer, and the drop-down list will show only those ASCOM cameras that are already installed on your system, if any.

The list of supported cameras at the time of writing is shown below - but you can contact the open-phd-guiding forum to inquire about your specific camera or mount.

Supported cameras, December 2014:

Windows:
Mac:

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.  At the time of this writing (October 2015), the only 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.

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 telescope driver that sends guide commands to the mount over a serial cable (or more commonly,  a USB/Serial connection)
  2. Use the ST-4 compatible guide port interface on the mount with a specialized cable and an intermdiate 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.  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 mount, especially the declination and side-of-pier, which can be used as factors in guider calibration.

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 have selected an ST-4 style of guiding in the 'mount' section, that interface cannot be used to query the pointing position of the telescope.  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 only 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.

Adaptive Optics and Rotator Selections

With PHD2, you now have the option of controlling the Starlight Xpress adaptive optics unit and/or any of several ASCOM-compatible camera rotators  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.  PHD2 does not control a rotator, but it will read the current angle setting from the rotator and adjust the guiding calibration if needed.

Simulators

All of the PHD2 devices - camera, mount, AO - include built-in simulators.  You can use these simulators to explore how PHD2 works and to decide how you want to use the program.  There's no reason to waste valuable dark-sky time learning to use PHD2!  Virtually all of  PHD2's features, including full calibration and all the graphical display options, will work properly when device simulators are used.  You will even see fairly realistic guiding performance to give you some idea of what to expect in the field.  To get started using the simulators, choose 'Simulator' for the camera type and 'On-camera' for the Mount type.

That said, the simulators are not useful for trouble-shooting any problems you encounter with your real mount.  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.

Equipment Profile

At the top of the 'Connect Equipment' dialog are some additional controls for managing equipment profiles.  All of the guider settings in PHD2, default or otherwise, are automatically stored as part of an equipment profile.  If you have only one guiding setup - you use the same camera and guide scope combination each time - you will only need one profile; and you can just use the default 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 are likely to be different, so you would want to 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.

The recommended way to create a new profile is to use the "Wizard" capability.  The wizard takes you through a sequence of dialogs that explain the various settings and help you decide how to set them.  It will also calculate a recommended set of guiding algorithm settings that are likely to get reasonable results right away.   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...'.  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.

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 'Camera' button and PHD2 will automatically re-connect to your hardware.

Exposure Time and Star Selection

The guide star can be selected (clicked on) while "looping" is active - in fact, this is the recommended method.  It can also be selected after looping has been stopped, but this opens the possibility that the star might have moved since the last exposure.  No great precision is required in clicking on the star - PHD2 will find the star nearest to the cursor.  After you do this, a green box will appear to frame the star.  If you pick a star that is too bright, a message box will tell you the star is saturated, and you should either use a different star or decrease the exposure duration.  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 benefit from 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 4-6 seconds, the effects of seeing are smoothed out.  The camera is essentially averaging out the larger, high-frequency seeing movements, so the guide algorithms have less difficulty distinguishing "seeing jitter" from actual guide star displacements that need to be corrected.
As a starting point, try using exposure durations in the range of one to five seconds.  Rather than choosing the star yourself with a mouse-click, you can let PHD2 Auto-select the guide star by using the Alt-S keyboard shortcut after stars are visible in the main display.  If you are just starting with this equipment set-up, you'll probably need to focus the guide camera - doing so is important for good guiding.  You can use 
the Star Profile tool to help with that process.

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 constant signal-to-noise ratio (SNR) value. 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 can be especially beneficial for AO users who want to minimize exposure time without losing the guide star. The settings to control Auto-exposure are on the Global Tab of the Advanced Dialog.

Automatic Calibration

Conventional Mounts

Two things need to be measured by PHD2 as part of guider calibration:
  1. The angle of the camera relative to the telescope axes 
  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 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 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.

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 angle. 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.  Starting with the 2.5 release, PHD2 takes a more aggressive approach to clearing this backlash, watching for a clear pattern of movement in a single direction with no reversals.  Even so, these pulses may still not clear all the declination backlash in your mount, particularly if you are guiding at a long focal length.   In that case, the computed declination rate may be too low, a situation that is discussed further in the Tools and Utilities section.  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.

In most cases, calibration will complete automatically without any user involvement.  Obviously, this assumes that the mount and all the cable connections are working correctly.  If they are not, the calibration is likely to fail with a message that the "star did not move enough";  and you'll need to do some trouble-shooting.  If the star has basically not moved at all in one or more directions, you should look first at the cable connections and mount behavior. The "manual guide" function under the 'Tools' menu can help with this as can the trouble-shooting section of this document.   But if the star has moved by some amount in each direction, you may need to adjust a calibration setting called the calibration step-size.  PHD2 uses this parameter during calibration as a fixed-size duration of movement.  The default value has been chosen to cover a wide range of typical guide scope set-ups, but you may need to adjust it for your situation.  For example, an off-axis-guider on a long focal length scope will probably need a setting that's different from the default value.   To adjust it, go into the "brain dialog" (Advanced Settings) and click on the 'Mount' tab.  On that tab, you'll find a setting for 'Calibration step(ms)', and that's where you change the value.  There's even a 'Calculate' button that will help you choose an optimal value for the parameter.  Once you've set this value, it will be remembered as part of your equipment profile and you probably won't have to adjust it again.

You may also see a calibration failure if you're using a star too close to the celestial pole.  In those locations, fixed-length movements in right ascension often move the star only a very small distance.  In that case, move to a star location closer to the equator, ideally somewhere in the range of -30 to +30 degrees declination, and re-run the calibration.  But if this is not feasible, you can adjust the calibration step-size upward until calibration succeeds.  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.

Like all other guide settings, the calibration data is automatically saved as part of your current equipment profile.  If nothing has changed in your configuration from one session to the next - even over an extended time period - you can restore the previous calibration data and start guiding immediately.  This function is located under the 'Tools/Modify Calibration' menu and is labelled 'Restore Calibration Data.'  Remember, the data are saved whenever a full calibration is completed or when you use the 'flip calibration data' command under the 'Tools/Modify Calibration' menu (see below).  If you're using a German equatorial mount and the ASCOM interface or you have an 'aux' mount connected, the saved calibration data will include the side of pier where you were last guiding; so PHD2 will adjust automatically for your current pointing position.  But if you are not using mount interfaces that can return pointing information, you'll need to remember which side of the pier was being used the last time you were guiding.  Clicking on 'Restore Calibration Data' will open a window that shows the results from your last calibration, essentially identical to what is shown using 'Review Calibration Data.'  You can quickly check the information in this window to be sure the calibration you are restoring is sufficiently accurate and relevant to your current set-up.  If it looks good, click on 'Restore' to re-use it.

If you're using an ASCOM mount (or 'aux' mount) connection, PHD2 will also include the calibration and target declinations as part of its calculations.  In that case,  guiding moves in right ascension will be automatically adjusted to account for the declination of the current guide star, as opposed to the star you used for calibration.  This is really just a refinement and isn't critical in most situations, so absence of pointing information is not a big problem.  However, if you are concerned about it and are imaging near the celestial pole, you should consider calibrating using a star that's also at high declination.  If you do a calibration at a declination > 60 degrees, PHD2 will not adjust the guide rates if you move to a different location.  So calibration at high declination values is really for the purpose of imaging in the same region and should not be used as a baseline calibration for general use.  Finally, if you have a rotator configured as part of your equipment profile, PHD2 will use its angle position property to adjust the calibration for changes in camera angle, thus eliminating another need to re-calibrate.

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 'alert' message at the top of the main window that describes the calibration result that looks questionable.  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 proceed using calibration results that are significantly in error.

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 mirror 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 commanda 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, select your star, 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
Obviously, you'll need to identify the source of the problem and fix it.  However, it's important to understand that PHD2 will not start moving the telescope around in an attempt to relocate the guide star.  It will simply continue to take exposures and look for the guide star to reappear within the bounds of the current tracking rectangle.  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 are likely to 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 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 data" and then resume guiding immediately.  

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 'Mount' tab, and clicking the 'Clear 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.  But PHD2 provides many tools for measuring and displaying your performance, as described in the Visualization 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, you may want to increase the values for these settings or otherwise solve the underlying problem.