Visualization Tools

Visualization Tools

PHD2 provides numerous visualization and display tools to help you see how your guider is performing.  All of these tools are accessed under the 'View' pull-down menu and are described below.

Overlays

The simplest display tools are grid overlays superimposed over the main guider display window.  These are quite straightforward and include the following choices:
  • Bullseye target
  • Fine grid
  • Coarse grid
  • RA/Dec - this shows how the telescope axes are aligned relative to the axes of the camera sensor
  • Spectrograph slit/slit position - for spectroscopy users, this will overlay a spectrograph slit graphic on the main display window.  The size, position, and angle of the graphic can be adusted to match the optical configuration.
  • None
You can just click on the various overlay options under the 'View' menu and choose one that suits you.

Graphical Display

The graphical display window is one of the more powerful tools for judging guiding performance, and you will probably learn to rely on it.  A typical example is shown below:




The major portion of the window shows the detailed displacements of the guide star for each guide exposure, plotted left-to-right.  Normally, one line shows displacements in right ascension while the second line shows declination displacements.  However, you can use the 'Settings' button to the left of the graph to switch to camera (X/Y) axes if you prefer.  You can also use the 'Settings' button to switch between display units of arc-seconds vs. camera pixels or to change the colors of the two graph lines.  The range of the vertical axis is controlled by the second button from the top, labelled y:+/-4" in this example.  The range of the horizontal axis - the number of guide exposures being plotted -  is controlled by the topmost button, labelled x:50 in this example.  This scale also controls the sample size used for calculating the statistics you see in the lower left part of the graph window.  These values show the root-mean-square (RMS or standard deviation) of the motions in each axis along with the total for both axes.  These are probably your  best estimators of guiding performance because they can be directly compared to star sizes and seeing conditions.  The 'RA Osc'  value shows the odds that the current RA move is in the opposite direction as the last RA move.  If you are too aggressive in your guiding and over-shooting the mark each time, this number will trend toward 1.0.  If you were perfect and not over- or under-shooting and your mount had no periodic error, the score would be 0.5  Taking periodic error into account, the ideal value would be closer to 0.3 or 0.4.  If this score gets very low (e.g. 0.1), you may want to increase the RA aggressivness or decrease the hysteresis.  If it gets quite high (e.g. 0.8), you may want adjust aggressivness/hysteresis in the opposite direction. There are two other checkboxes to the left that can help you evaluate guider performance.  Clicking on the 'Corrections' box results in an overlay showing when guide commands are actually sent to the mount, along with their direction and magnitude.  In this example, these are shown as the vertical red and green lines appearing at irregular intervals along the horizontal axis.  This shows you how "busy" the guiding is - under optimal conditions, you should expect to see extended intervals when no guide commands are sent at all.  The other checkbox, labelled 'Trendlines', will superimpose trend lines in both axes to show if there is a consistent overall drift in the star position.  This is primarily useful for drift aligning where the declination trendline is used extensively.  But the RA trendline can show if your mount is tracking systematically slow or fast (or is seeing the effects of flexure) and can help if you are trying to set up custom tracking rates.  If dithering commands are issued, usually by an external imaging application, a 'dithering' label will be superimposed on the graph in the appropriate time interval.  This tells you the star displacements being graphed are being influenced by the dithering operation.  If you are using an ASCOM connection for either the 'mount' or 'aux-mount', PHD2 will also show the directions (GuideNorth, GuideEast) associated with the guide commands, as shown in the example above.  This can be helpful if you are looking at overall drift and want to determine how to set uni-directional guiding for declination.  The up/down convention used in this graph has nothing to do with the camera orientation or N-S-E-W movements in the field of view.

The recommended way to look at guiding performance is to use units of arc-seconds rather than pixels.  Doing this allows an equipment-independent way of evaluating performance because it transcends questions of focal length and image scale.  To do this, you need to provide PHD2 with sufficient information to determine your guider image scale: namely, the focal length of the guide scope and the size of the guide camera pixels.  These parameters are set in the 'Brain' dialog, on the 'Global' and 'Camera' tabs, respectively.  If they are not specified, PHD2 will use default values of 1.0, and the guiding performance numbers will effectively be reported in units of pixels.

At the bottom of the graph window are active controls for adjusting guiding parameters "on the fly".  The guiding algorithm selections you've made will control which controls are shown.  These controls have the same effect as those in the 'Brain' dialog, and they eliminate the need to stop guiding and navigate to another window to adjust guiding parameters.

Stats



If you want to monitor guiding performance without necessarily having the graph window open, you can click on  the "Stats" menu item.  That will display the salient statistics with controls for clearing the data or changing the number of guide exposures used to compute the statistics.  This window is also useful for confirming camera binning, monitoring the guide camera temperature, and getting a quick calculation of your guide camera's field of view.

Star Profile and Target Displays



The star profile display shows the cross-section of the guide star along with measurements for its full-width-half-maximum (FWHM) and half-flux-diameter (HFD).  HFD is generaly a more stable measure of the star size since it doesn't require curve fitting or any assumption about the overall shape of the star image.  That's why automated focusing applications like FocusMax use it.  If you see substantial fluctuations in this parameter or wildly varying star profiles, it may be an indication that the star is too faint or the exposure time is too short.  This tool can also help with focusing the guide camera, a procedure that can be a bit tedious if you're using an off-axis-guider at a fairly long focal length.  For that purpose, the HFD number is shown in a large font so you can see it from a distance while focusing your guide scope/camera.  Just un-dock the Star Profile window and expand it until you can see the HFD number easily.  If you are starting well out-of-focus, you'll probably see only a few fuzzy stars in the frame, so just choose the smallest one that is clearly visible.  Use exposure times of at least 2 seconds if possible so you don't chase the seeing.  At the same time, don't let the star become saturated, showing a distinctive flat top.  Now adjust the focus so the HFD gets consistently smaller - but stop as soon as HFD reverses direction or seems to plateau.  At that point, the star may be saturated, so move to a dimmer star in the field.  Since you have already improved the focus, you can hopefully see a dimmer star.  Continue in this way until you've reached a focus point that shows a minimum level of HFD for the faintest star you can use.  At each point in the focusing process, you'll probably want to watch the HFD values for a few frames so you can mentally average out the effects of seeing.  Bad focus is a common issue for beginners, leading to problems in calibration or generally poor guiding results.  Use the Star Profile tool to be sure the star doesn't have a flat top (saturation) and shows a tapered shape like the example shown above.




The target display is another useful way to visualize overall guider performance.  The red 'X' shows the star displacement for the most recent guide exposure, while the blue dots show the recent history.  You can zoom in or out with the controls at the upper left of the window, as well as change the number of points shown in the history.   If you are using an ASCOM connection for either the 'mount' or 'aux-mount', PHD2 will also show the directions (SkyNorth, SkyEast) associated with the star movement, as shown in the example above.  This can be helpful if you are looking at overall drift and want to determine how to set uni-directional guiding for declination.  The up/down convention used in this graph has nothing to do with the camera orientation or N-S-E-W movements in the field of view.

Adaptive Optics (AO) Graph

AO Graph Image

The AO graph is equivalent to the 'target' display, but shows the history of corrections relative to the axes of the adaptive optics device.  The red rectangle indicates the outer edges of the AO device, while the interior yellow rectangle shows the "bump" region.  If the star moves outside the yellow rectangle, PHD2 will send a sequence of move commands to the mount - the "bump" - to smoothly place the guide star back near the center position.  When this occurs, green and blue lines will show the incremental bump and the remaining bump respectively.  The white dot on the display shows the current AO position, and the green circle (red when a bump is in progress) shows the averaged AO position.  The button in the upper left controls how many points will be plotted in the history.

Dockable/Moveable Graphical Windows

When the various performance windows are initially displayed, they are "docked" in the main window.  This means they are sized in a particular way and are aligned with two edges of the window - they are entirely contained within the bounds of the main PHD2 window.  However, you can move them around and resize them by clicking and dragging on the title bar of the window you want to examine.  This will often let you get a better view of the details being shown in the graphs.  They can be re-docked by dragging the title bar to the general region in which you want them docked - bottom, right, etc.  With just a bit of practice, it's easy to place them where they are most convenient.

There is also a menu item under the 'View" pulldown menu labeled 'Restore window positions.'  Clicking on this menu item will automatically restore all of the dockable/moveable windows to their default, docked positions.  This can be useful , for example, if you are switching between screens with different resolutions and one or more of the dockable windows has been "lost."  This function also restores the main PHD2 window to its default size, with a position near the upper lefthand corner of the screen.

News & Resources

February 12, 2017 - PHD2 v2.6.3 Released

October 28, 2016 - PHD2 Best Practices

June 12, 2016 - PHD2_Broker package available

December 29, 2015 - Tutorial: Analyzing PHD2 Guiding Results

December 20, 2015 - Tutorial: Off-axis guiding on comets with PHD2