Glossary of Technical Terms


Although PHD2 strives for ease-of-use, the underlying task of guiding a telescope is actually quite complicated.  If you're trying to resolve equipment-related problems or fine-tune your guiding, you may brush up against unfamiliar topics and terminology from statistics, astronomy, and mechanical engineering.  Some of these terms appear in the PHD2 user interfaces as well as in the support forums and discussion groups, so it's worthwhile to have a basic knowledge of what they mean.  Some of the terms you are likely to encounter are discussed below.

RMS - is a shorthand for "root-mean-square", a statistical quantity used to characterize a set of data.  In the context of PHD2, it is the same as standard deviation.  It is a useful quantity to describe the spread of the data points around some average value.  If we concentrate on the task of guiding, we're interested in knowing how much we see the guide star moving around on the RA and Dec axes.  Let's consider a sequence of guide star displacements that look like this:   1.0, -1.2, 1.5, -1.3, 1.0 -1.6, 1.4, -2.1, 1.2, -2.4, 1.2, -2.1 -1.1, 2.1, 0, 2.4, -1.2, 1.6, -1.5, 1.3, -1.0, 1.2 -1.6, 1.2.   How should we characterize how much movement we're seeing around the zero-point?  We could take a simple approach and compute the average of these deflections - and the result is zero!  That tells us nothing at all, obviously 0 is not a good characterization.  The reason for this result is that we happened to get equal displacements in both the + and - directions, so the displacements are cancelling each other out. Hence the reason for using a slightly more complicated RMS (standard deviation) calculation, which factors out the signs of the displacements.  The RMS value for this sequence is 1.5, which tells us a lot more.  Over a sufficiently long period (somewhat longer than the 25 points in this example), the underlying mathematics tells us we would expect 68% of the excursions to be smaller than +- 1.5 px, and 95% of the excursions to fall within +/- 2*RMS (3.0) arc-sec.  Obviously, this tells us a lot more about the guiding performance, and it's why RMS is used in the PHD2 visualization tools.

Backlash - is a typical shortcoming of most telescope mounts that use gears to drive the two axes (most but not all amateur mounts use gears).  It is caused by a variable amount of looseness or slack anywhere two gears are meshed together.  Some amount of slack is necessary or the gears wouldn't be able to reverse direction at all, they would simply lock up.  This mandatory (hopefully small) amount of slack in the gear mesh means that a reversal in direction will briefly push the drive gear into a small dead-zone where the gears are no longer meshed at all.  At that point, the drive motor will continue to turn but the mount axis won't move.  This will continue until the motor has turned enough to re-engage the gears in the reverse direction.  High-quality geared mounts have a very small amount of backlash, so small that it typically doesn't affect guiding.  But lower-end mounts may have a large amount of backlash, and direction reversals may experience long lags before the axis starts moving in the desired direction.  Backlash is not a problem in RA so long as the guide speed is less than or equal to 1x sidereal.  In those cases, the RA motor never reverses direction during guiding, so the backlash is not seen.  However, backlash is typically an issue for declination because the drive motor and gears are usually stationary except for intermittent commands to move north or south to track the star.  Direction reversals in Dec will quite often trigger backlash delays.   The Guiding Assistant in PHD2 can measure this, and you will quite often see discussions about backlash in the support forums.  As a general rule, backlash is best addressed by re-meshiing or otherwise improving the gear train on your declination axis.  Do NOT use backlash compensation or correction settings in the mount firmware, those will almost inevitably result in unstable guiding.

Periodic Error - is another typical characteristic of geared mounts, this time affecting only the RA axis.  Periodic error is usually caused by small irregularities in the worm that drives the RA gear train.  It is periodic because whatever irregularities are present will appear each time the worm makes a full cycle (the worm period).  The irregularities make their presence known by small RA tracking errors that appear on a consistent and predictable basis.  Periodic error can be more complicated than this when defects are present elsewhere in the RA gear mechanism, but the principle is the same.  Mounts that are advertised as being imaging-capable should have a way to correct for this by applying proactive corrections as the worm reaches various points in its cycle.  This correction mechanism is called periodic error correction, usually abbreviated as PEC.  The PEC is usually programmed into the mount firmware through use of a separate application (e.g. PecPrep).   Since the PE corrections are applied by the mount firmware proactively, they do not interfere with PHD2 guiding.  In fact, they help it substantially - there will usually be a smaller range of motion for which PHD2 needs to correct..

Image Scale - is a fairly simple property of the imaging or guiding system, although not something most users want to calculate.  It is expressed in units of arc-sec/pixel and basically describes how angular distances in the sky (arc-seconds) are translated into linear distances on the camera sensor (pixels).  To apply this concept to guiding, consider a telescope/mount system that has some amount of tracking error. The tracking errors create very small angular movements in pointing, so they are measured and expressed in units of arc-seconds.  For example, the mount might have a periodic error of 10 arc-secs, so a guide star would appear to move by 10 arc-seconds during the worm period.  But how much movement will show up on the camera sensor?  This is what will determine how the guiding software reacts.  If the guiding is being done with a long focal length scope, the image scale will be (perversely) small because  each camera pixel is looking at a smaller angular distance in the sky.  A given angular deflection (arc-secs) will result in a comparatively large linear movement on the sensor.  If the same guide camera is used on a shorter focal length scope, the image scale is larger and the star deflections will appear smaller - each pixel is looking at a larger region of the sky.  PHD2 computes the image scale for you if you correctly enter the guide scope focal length and guide camera pixel sizes.  Once this is known, the guiding performance shown with tools like the guiding graph can be displayed in units of arc-seconds. These are the units you want to use to measure, improve, and discuss your guiding performance because they address the real, physical behavior of your guiding system and aren't dependent on the specifics of your optical configuration.

SNR - is the acronym used for signal-to-noise-ratio.  This is a specialized measurement used by PHD2 to determine how well the star can be distinguished from the background. It is similar but not identical to the signal-to-noise ratio used in photometry.  PHD2's calculation of an accurate position for the guide star can be affected by the SNR, but the effects typically become unimportant when the SNR is at 15 or higher.  At that point, errors in computing the star position will be much smaller than the other measurement uncertainties inherent to guiding.

Saturation - occurs when the brightest parts of the guide star image exceed the maximum capacity of the sensors in your guide camera.  When this happens, the star profile no longer has a sharp central peak because the pixel values are clipped (truncated) at the maximum value for the sensor.  The star profile then has a flat top, and the computation of the star position is degraded.  You can see this by using the Star Profile tool, and you should try to avoid this situation whenever possible.

Star Mass - is an internal PHD2 metric that indicates the overall brightness and apparent size of the guide star.  It can be a rough indicator of star-dimming events such as passing clouds or fog, and it is used primarily in the "star mass detection" control on the Guide tab of the Advanced Dialog.