Glossary of Technical Terms
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
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
-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
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.
- 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
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
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
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
because the drive motor and gears are usually stationary except
intermittent commands to move north or south to track the star.
Direction reversals in Dec will quite often trigger backlash
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.
- 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
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.
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
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