Advanced Settings

Advanced settings are accessed by clicking on the 'Brain button', a feature well-known to users of the original PHD. PHD2 has a considerably larger set of parameters that can be adjusted to optimize your guiding performance. Although these are called "advanced" settings, they are not particularly difficult to understand, and you shouldn't hesitate to explore them. All of the fields on these forms include "tool tips", small message windows that describe each field in some detail. Simply "hover" the cursor over the field to see the tool-tip. In many cases, this will provide all the information you need. Because there are many more settings available, the Advanced Dialog in PHD2 is organized into tabs, and the documentation will follow that model as well.

Global Tab
Guiding Tab
Camera Tab
Mount Tab
AO Tab
Rotator Tab

Global Tab

Most of the fields on the global tab deal with overall program functions and behavior.

General Parameters

The remaining controls on the 'Global' tab are well-described by their respective tool-tips, but they are summarized here for completeness:

'Reset Configuration' - restores all settings to their initial values as if PHD2 had been freshly installed
'Image format' - specifies the file format if star-image logging is enabled
'Dither RA only' - for companion apps that use the PHD2 server interface, specifies that dithering should be done only on the RA axis
'Dither scale' - controls the magnitude of the dithering operation
'Noise reduction' - specifies the algorithm to use for handling noisy guide camera images - those for which dark frames are not sufficient. Choices include None, 2x2 mean, and 3x3 median. Both 2x2 mean and 3x3 median will reduce the noise considerably. 3x3 median is especially effective at removing hot pixels and neither will significantly affect guiding accuracy. However, creating a bad-pixel map is likely to be a better solution with less impact on your ability to detect faint stars.
'Time lapse' - imposes a fixed delay between guide exposures. This can be useful if the guide exposures are very short and you don't want to overload either the mount or the camera link with very high traffic rates.
'Focal length' - the focal length of the guide scope (millimeters). This provides one of two parameters needed by PHD2 to compute the image scale and thus report guiding performance in units of arc-seconds. The other parameter required for this is the guide camera pixel size, located on the 'Camera' tab.
'Language' - determines the language used in the PHD2 user interface, subject to available localization. Changing this will usually require a program restart
'Auto restore calibration' - tells PHD2 to automatically reload the most recent calibration data as soon as the equipment is connected. Be sure to read the Auto calibration section to understand the implications and potential risks of doing this.
'Auto Exposure' - these are the settings that control Auto exposure time.

Min Exposure - the minimum exposure time. PHD2 will not set the exposure time less than this value, even if the guide star SNR is higher than the target SNR value. If the min exposure time is set too low, you could end up sending excessive small corrections to the mount which may not give the best guiding results. Users with AO units would want to set this to a lower value, since rapid small corrections are often desirable with an AO.
Max Exposure - the maximum exposure time. Before a guide star is selected, PHD2 will set the exposure time to the maximum value. Once a guide star is selected, PHD2 will then incrementally decrease the exposure time until the desired SNR is reached.
Target SNR - this is the average SNR value that PHD2 will attempt to achieve by adjusting the exposure time. SNR can fluctuate from frame to frame even with a fixed exposure duration, so be sure to account for that when choosing a target SNR value. PHD2 will reject frames when SNR drops below 3.0. The default value of 6.0 should provide enough of a cushion to prevent fluctuations from causing the SNR to go below 3.0. As mentioned in the 'Basic Use' section, SNR is similar but not identical to the signal-to-noise ratio used in photometry.

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.

Mount Tab

The mount tab is the most complex area of Advanced Settings largely because it has so many parameters and UI controls. Most of these settings are closely tied to the various guiding algorithms, and the contents of the dialog will change significantly if you change the algorithm selections. For that reason, all the parameters related to guide algorithms will be treated together, in a separate section.

The remaining controls, the ones that are independent of the guiding algorithm selections, are described below.

'Clear calibration' - tells PHD2 you want to clear the calibration data currently being used and re-calibrate before guiding is restarted. You might do this for a variety of reasons - you might have moved the guide camera, you might have done a meridian flip and want to recalibrate on the "new" side of the pier, etc. You can also accomplish the same result by doing a Shift-Click on the guiding icon on the main page, which will force a re-calibration.
'Enable guide output' - this is normally checked because it tells PHD2 to send guide commands to the mount. But there are some circumstances where you might want to disable this, usually because you want to observe the uncorrected behavior of the mount. For example, you can disable guider output in order to see the general shape and amplitude of your mount's periodic error or to check the amount of drift from polar mis-alignment.
'Declination Backlash Comp' - this controls whether PHD2 will apply a compensation factor when the direction of declination guiding needs to be reversed. Measurement of backlash and calculation of a good starting value for the compensation factor is done in the Guiding Assistant. The size of the additional guide correction (compensation value) is shown in the 'Amount' field adjacent to the checkbox. This amount may be adjusted downward by PHD2 if necessary to avoid over-shooting and oscillation. Since PHD2 has the ability to detect and adapt to over-corrections, the backlash compensation available here should work better than the fixed backlash compensation available in many mount controllers. If you use the PHD2 backlash compensation, you should disable any backlash compensation in the mount. See the help section on the Guiding Assistant for more details.
'Calibration step-size' - specifes the duration of the guide pulse that PHD2 will use during calibration. Its use is described in the 'Auto Calibration' section of the 'Basic Use' help page. You can adjust the value depending on whether the guide star is moving too quickly or too slowly during calibration. As a general guideline, it is good to calibrate within about 30 degrees of the celestial equator (declination = 0), and to use a calibration step size that will result in 8-14 steps in each direction. The 'calculate...' button to the right of this control will launch a dialog that can help you compute an appropriate value:

To use the calculator, be sure the topmost three edit controls are correctly filled in. If you have already specified the focal length and the camera pixel size in the 'Global' and 'Camera' tabs respectively, those fields will already be populated in this form. If you are using an ASCOM connection to your mount, the fields for "Guide speed" and "Calibration declination" will also have the correct values. Otherwise, you'll need to supply them yourself. The guide speed is specified as a multiple of sidereal speed - most mounts will use something like 1X or 0.5X sidereal, but you can choose something else. You can leave the 'calibration steps' field at the default value of 12, which is likely to result in a good calibration. Use of a significantly smaller value raises the likelihood that seeing errors or small mount errors will cause calibration errors . As you change the values in these fields, PHD2 will recalculate your current image scale and a recommended value for the calibration step-size. If you then click on 'Ok', that value will be inserted into the calibration step-size field of the 'Mount' dialog. Clicking 'Ok' will also populate the focal length and camera pixel size fields in the 'Global' and 'Camera' tabs, so any changes you made in the calculator will be reflected there as well. However, this will not be done if you click on 'Cancel' in the calculator dialog.

'Max Duration RA' - specifies the maximum allowed guide pulse duration for right ascension. You might reduce this below the default value if you want to avoid "chasing" a large deflection that could be caused by a spurious event (e.g. a sudden 'hot pixel') .'
'Max Duration Declination' - specifies the maximum allowed guide pulse duration for declination (same as above but for declination). This can be useful to avoid over-correction and subsequent direction reversal in declination guiding.
'Reverse Dec output after meridian flip' - tells PHD2 how to adjust the calibration data when you use the function to "flip" it. Some mounts track their 'side of pier' state and automatically reverse the direction of guide commands in declination. Others, perhaps most mounts, do not do this. In either case, PHD2 needs to know if the mount will automatically change its behavior based on side-of-pier. You may have difficulty finding information about how your mount behaves in this respect, so it's probably easiest to just run a quick experiment. With the checkbox disabled, calibrate on one side of the pier, then move the mount to the other side. Select 'Flip Calibration' under the 'Tools' menu, and start guiding. If the guiding works normally, leave the box un-checked; but if you see run-away in declination, check the box and repeat the experiment.
'Assume Dec orthogonal to RA' - Normally, the calibration process independently computes the camera angles for both right ascension and declination. There is no need for great precision on these values, and the default behavior normally works well. However, if your mount has very high periodic error or you are dealing with very bad seeing conditions, you may want to force the RA and Declination angles to be perpendicular. If you choose that option, PHD2 will compute the camera angle for RA, then assert a declination angle that is orthogonal to it.

All of the other controls in this dialog are related to the guiding algorithms you have chosen and are described here: Guiding Algorithms and Settings

AO Tab

Most of the edit controls at the top of the AO tab have the same meaning as their counterparts on the 'Mount' tab. The guiding algorithms apply to the control of the tip/tilt optical element in the AO device itself, not to the "bump" commands sent to the mount. Since the AO device is not trying to move a heavy piece of equipment, you can afford to be more aggressive in your guide algorithm parameters. If you use a hysteresis-based algorithm, for example, you should probably start with a high level of aggressiveness, perhaps 100%. Or you can choose the 'None' algorithm, which means there will be no damping or history-based algorithm applied at all. In that case, each correction will be based only on the most recent guide frame and will make a 100% correction of the most recent deflection.

You can use the four parameters at the bottom of the AO tab to control the calibration process and the manner in which 'bump' operations are done. The 'calibration step' field tells PHD2 the amount to move the tip/tilt element in each of the up/down/left/right directions, in units of AO steps, during calibration. The guide star position is measured at the beginning and end of each leg of the calibration, and the 'samples to average' parameter tells PHD2 how many samples to take at each of these points. Averaging images is important because the seeing will always cause the guide star to "bounce around" a bit. As discussed earlier, the AO unit can make corrections only within a limited range of guide star movement. You will want to initiate mount 'bump' corrections before these limits are actually reached, and the 'bump percentage' field is used for that purpose. To move the mount, the full bump correction is accomplished in steps - the 'bump step' field controls the size of these increments. If the bump operation has begun and the guide star remains outside the "bump percentage" area, PHD2 will increase the bump size until the guide star is back within that range. Additional movement from that point to the center position will continue at the specified "bump step size". This complexity is required in order to maintain good guiding, with no elongated stars, even as the mount is being bumped. During the bump operation, the AO is continuing to make corrections, so the long "mount bump" is continuously offset by adjustments in the AO.

Rotator Tab

Currently, the rotator tab has only one control, and this window is likely to change in the future. The "Reversed" checkbox can be used for optical systems that reverse the image because they have an odd number of mirrors. The direction and amount of rotation is used to adjust the calibration data, so PHD2 follows the ASCOM standard: "the rotator position is expressed as an angle from 0 up to but not including 360 degrees, counter-clockwise against the sky."