Using PHD2 Guiding

There are five basic steps to start guiding.
  1. Press the camera 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 adaptive optics (AO) device.  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 users should consult web resources to see updates (e.g. Stark Labs)

Supported cameras, December 2013:

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, which can be used as a factor 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-board" guider interface
  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.  

Adaptive Optics Selection

With PHD2, you now have the option of controlling the Starlight Express adaptive optics unit.  If you don't have such a device, just leave the AO selection at 'none.'   Support for other AO devices may be added in the future.

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.

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 s 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.

If you already have a suitable default equipment profile and you simply want to connect to the equipment "just like 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 two major 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.  
As a starting point, try using exposure durations in the range of one to five seconds.

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..  

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.

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' 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' menu (see below).  If you're using a German equatorial mount and the ASCOM interface, 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 an ASCOM interface, you'll need to remember which side of the pier was being used the last time you were guiding.

If you're using an ASCOM 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 mount connections through a guider port aren't significantly dis-advantaged.  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.

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" often 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.

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 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 the ASCOM interface, you will need to take action to adjust the guider calibration.  You can, of course, simply do another calibration on the "new" 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' 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 'Force 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.  The 'Force Calibration' checkbox will be cleared automatically, so you don't need to worry about resetting it.

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.