Guiding Tab
There are relatively few parameters found on the 'Guiding' tab. Their use is as follows:
'Always Scale Images' - tells PHD2 to always rescale the guider image to fit the display window. PHD2 does this automatically in almost all cases, so you'll rarely need to do this.
'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. This is only an optimization, so the use of this checkbox is completely optional.
'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.
'Star mass change detection' - tells PHD2 to monitor the brightness and size of the guide star compared to the sky background.
'Star mass tolerance' - if 'star mass change detection' 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 that PHD2 doesn't mistakenly "switch stars." It can also prevent errors caused by thin clouds, high camera noise, or alpha particle "hits"; 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. Setting the value to 100 effectively disables the warnings entirely.
Camera Tab
The controls on the 'Camera' tab are used as follows:
'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 and can mean the difference between the camera being too slow to effectively guide and being quite responsive. When this feature is enabled, only a small subframe (100x100 pixels) will be downloaded once a star is selected. 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 click anywhere outside the subframe.
'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. Refer to your camera documentation to determine the correct value.
'Camera gain' - Sets the gain level for the many cameras that support this feature. If you really want to use a bright star and a longer exposure duration or if your camera makes a very noisy image, try reducing this parameter.
'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. It is advisable to 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. At this time, this affects 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.