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. 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 "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 'Clear Calibration' checkbox. Or, you can simply do a 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.