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 notebook tabs that are activated by clicking on the tab names.  All of the tabs share a common set of 'Ok' and 'Cancel' buttons at the bottom of the form.  Clicking on 'Ok' means that changes made to any of the tab fields will be put into effect.  Clicking on 'Cancel' discards any changes that were made.

Global Tab
Camera Tab
Guiding Tab
Algorithms Tab
Other Devices Tab

Global Tab



The controls on the 'Global' tab are well-described by their respective tool-tips, but they are summarized here for completeness:
Since the images are saved in an industry-standard format, there are many astronomy-related applications that can display or analyze them, many of which are free.  Most of the image-capture and image-processing applications can do that along with other, more specific tools that can perform detailed measurements on the stars and the optical quality of the field of view.  You can just do a web search to find a list of applications that support the FITs format for whatever platform you're using.  If you simply want to look at the images to check focus or see the general quality of the images being returned from your camera, you can use PHD2 for that.  With PHD2 in an idle state - neither looping nor guiding - just drag-and-drop one of your saved FITs image onto the main window. The display will then update to show the image you just dropped.  There's no need for PHD2 to be connected to any of the hardware.  You can adjust the gamma slider, select a star (manually or automatically), and use the Star Profile tool to view the HFD and profile of the selected star.

Camera Tab



The controls on the 'Camera' tab are used as follows:
Use of Binning
Some of the guide cameras available in PHD2 support hardware-level binning, and this may be helpful in situations where you are guiding at long focal lengths or have a guide camera with very small pixels. These scenarios often result in having to use faint guide stars, and the guider images may be substantially over-sampled.  Over-sampling provides no real benefit, and the projection of a faint star disk onto many small pixels can result in a low signal-to-noise ratio (SNR).  By binning the image, you can reduce the impact of camera read noise and thus improve the SNR; and if you are over-sampled,  you won't degrade the accuracy of computing the guide star location.  Choosing a binning factor greater than one will have the following effects:
  1. Star images will have a higher SNR and will be easier to detect above the background noise level.  This is only beneficial if you are limited to a choice among faint stars (i.e. with SNR values near the threshold of 3).
  2. The amount of data downloaded from the camera will be reduced by the square of the binning factor.  This can be helpful if you are using a camera that makes heavy use of USB resources even if star brightness and SNR are already reasonable with un-binned images.  Of course, using sub-frames can achieve the same result once a star has been selected.
  3. The resolution (image scale) of your guider image will be reduced by the binning factor.  This is not likely to be a problem if the un-binned image scale is below 1 arc-sec/pixel, but your guiding results may suffer if the un-binned image scale is well above 1 arc-sec/pixel.  You may need to experiment because the results will also depend on the image scale of your main camera system.
Each binning level requires its own dark frames and bad-pixel map - they are not interchangeable, nor can a transform be done automatically.  If you foresee the need to switch back and forth between binning settings, you should create separate profiles for each binning value.  Then build a dark library and a bad-pixel map for each of those profiles.  When you want to change binning factors, just switch to the profile that has the setting you want, and a dark library and/or bad-pixel map will be available.   If you want to check that the camera is binning correctly, you can use the Stats window to confirm the frame size and current on-camera bin settings.

Guiding Tab



The guiding tab shows the  parameters used for calibration, star-tracking, and guiding behavior shared by all of the guide algorithms..

Guide Star Tracking
Calibration
Shared Guiding ParametersDetailed Calibration Parameters



To review or change calibration parameters, be sure the topmost four 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 show 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.  Note: changing the guide speed setting here never changes the guide speed setting in the mount - that can only be done via the mount driver or the hand-controller.   If this field is already filled in, changing it will have no useful effect. 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 .  You can also change the total calibration distance if you have some reason to think the default value of 25px is inadequate.  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 'Guiding' dialog.  Clicking 'Ok' will also populate the focal length and camera pixel size fields in the 'Guiding' 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.  Again, PHD2 never changes the guide speed setting in your mount regardless of what may be entered in the 'Guide Speed' field.

The goal of the calculator is to recommend a step-size that is likely to result in an accurate calibration without wasting too much time.  There is no great precision involved in any of this, and there is nothing magic about the choice of 12 steps as a target.  As long as you are getting successful calibrations without alert messages, the calibration parameters can be left alone.  If you've used the new-profile-wizard to build your equipment profile, there's rarely any reason to use the 'Advanced" button and the Calibration Parameters dialog.

Algorithms Tab



The algorithms tab can be used to select the guiding algorithms you want to use and to fine-tune the parameters associated with them.  The parameters displayed 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.  

Declination Backlash Compensation

Most commonly-used mounts have some amount of backlash in declination.  This causes a delay whenever there is a change in direction of the Dec guide commands.  During this interval, the declination gears aren't fully engaged and the axis doesn't move in response to the guide commands.  Many mounts have settings for backlash compensation but these should not be used for guiding - they are typically intended for visual use where high precision isn't required.  Because the actual amount of compensation needed at any given time may depend on the pointing position and the mechanical load on the system, a fixed value will usually result in oscillations that never stabilize.  The backlash compensation implemented by PHD2 is adaptive, meaning that the compensation amount is adjusted up or down depending on the measured results.  Before enabling this feature, you should run the Guiding Assistant and measure the declination backlash - the time delay required to fully reverse direction in declination.  Keep in mind, the higher the guide speed setting in the mount (e.g. 0.9x sidereal), the smaller this delay will be.  If the measured amount is 3 seconds or less, the Guiding Assistant will recommend trying backlash compensation.  If you apply that recommendation, the backlash compensation settings will be handled for you automatically.  The UI controls for backlash compensation include settings for 'minimum' and 'maximum' compensation amounts.  These effectively limit the range of the adjustments that are made to the starting compensation value.  If you're experienced with your mount's behavior, you can adjust these settings manually to be sure that the compensation stays within a range that you know works well.  Otherwise, you should just leave these at their default values.  The backlash compensation algorithm will generally work well if the backlash is less than a few seconds and the mount doesn't have other significant mechanical problems.  You should expect a short period of  instability when guiding starts because the initial state of the Dec gear system is unknown - just let it stabilize before you actually start imaging.  If you see recurring periods of Dec oscillation or the axis won't settle down, disable the compensation feature and submit your debug log file to the PHD2 support forum.  It's important to note that the correct amount of Dec backlash compensation will often change depending on the scope pointing position.  This can be caused by uneven gear wear on the Dec axis or by differences in the gravitational load being applied to the axis.

Uni-directional Declination Guiding

As discussed elsewhere, some mounts have too much declination backlash to support guiding in both north and south directions.  This situation can be mitigated by configuring PHD2 to guide in only one of the directions, what we call uni-directional Dec guiding.  This can be a manageable situation because declination guiding is only intended to correct for slow drift - errors caused by polar misalignment and to a lesser extent, mechanical flexure.  Ironically, you might want to de-tune your polar alignment a bit to make it easier to see the drift direction and to reduce the likelihood that seeing will interfere with uni-directional guiding.  Remember that polar mis-alignment, within reason, doesn't usually degrade guiding performance.  Instead, it may introduce field rotation if you're imaging near the pole and have a large camera sensor.  A good first step would be to polar align to within a few arc-minutes of the pole before setting up for uni-directional guiding.  You can always go back later and check for field rotation.  Just take a sample image with your main camera at the highest declination you would expect for imaging - perhaps 70 degrees north.  If you don't see field rotation there, you can leave the polar alignment where it is.  With any amount of polar mis-alignment, the direction of  Dec corrections will change at some point in the sky.  (Technically, it will reverse directions at two points in the sky but one of those is usually below the horizon.)  The sky location for the reversal depends entirely on how you are mis-aligned on the pole - the relative amounts of azimuth and altitude alignment errors.  You may even have a situation where the reversal point is near enough to the horizon that you don't encounter it during normal imaging.

To set up for uni-directional guiding, you can follow these steps:
  1. Move to a field with a good guide star and open the Guiding Graph window.  Disable Dec guiding entirely by setting the Dec guide mode to 'off', then start guiding.  Now watch the graph until you can see a clear trend in the way the guide star is drifting either north or south.  Once you see this, reset the Dec guide mode to issue corrections in the right direction.  For example, if the star is drifting north, set the Guide mode to 'south.'
  2. Try using the 'LowPass' or 'LowPass2' guiding algorithms for declination and start with a fairly low aggressiveness factor, say 50%.  If the aggressiveness is too high, the correction may push the star to the "wrong" side of the lock position, where it will remain until the slow drift rate moves it back.  It's better to issue a few consecutive small corrections rather than one larger one in order to minimze this type of over-shoot.
  3. Watch the guiding graph to be sure the corrections are being issued in the right direction and the star isn't just steadily drifting off-target.  Over the course of minutes or hours, you may notice the amount of drift is decreasing.  This means you are slowly approaching the point of declination reversal and you should be prepared to change the Dec guide mode accordingly.
  4. If you are dithering, set the dithering parameters to "RA-only" to avoid disrupting the Dec guiding.

Other Devices Tab



If you are using either an adaptive optics or rotator device, the "Other Devices" tab will be shown.  The upper section deals with the AO device if one is being used.  You can use these parameters 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.  The 'AO Travel' field specifies the number of steps the AO can make on each axis.  The default value works well for most SX AO devices but in some cases it may be too large.  If you encounter problems during AO calibration where the AO hits its limits, you can adjust the 'AO Travel' amount downward.  

The 'Bump on dither' option tells PHD2 to bump the mount when a dither command is received and thus move the guide star back closer to the center position of the AO.  The option to enable or disable AO guide commands operates independently from the 'Enable mount guiding' checkbox in the Guiding tab.  So you can independently enable/disable either the guide commands to the tip/tilt device or the 'bump' guide commands to the mount.  The same principle holds for the 'Clear AO calibration' option - that will force a recalibration of the AO without affecting calibration of the mount.  The 'Mount Backlash Compensation' controls let you apply PHD2 Dec backlash compensation when mount bumps are done.  This can help speed up large dither operations, but it's only appropriate if the mount has a limited amount of Dec backlash. You should use the Guiding Assistant to measure your Dec backlash and see what it recommends with regard to using Dec backlash compensation.

When an AO is in use, the 'Algorithms' tab will only show choices for controlling the tip/tilt optical element in the AO device itself.



Since the AO is not trying to move a heavy piece of equipment, you can afford to be more aggressive in your guide algorithm choices.  The default algorithms for an AO are 'None', which means there will be no damping or history-based calculations 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.  If you use a different algorithm, you should probably start with a high level of aggressiveness there as well, perhaps 100%.   The other, shared guiding parameters normally displayed on the 'Algorithms' tab will not be shown for the AO because they aren't used to control the device.

The rotator device has only one parameter, which lets you match the behavior of the device to the ASCOM notion of positive and negative angles.  The "Reversed" checkbox can be used for optical systems that reverse the image, usually 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."  Experimentation is likely to be the quickest way to determine if the box should be checked.