Advanced settings are accessed by clicking on
the 'Brain button' in the main display.
PHD2 has a large set of parameters that can be
adjusted to optimize your guiding performance. Although these
called "advanced" settings, they are not particularly difficult to
understand, and you shouldn't hesitate to explore them. All
the fields on these forms include "tool tips", small message windows
that describe each field in some detail. Simply
cursor over the field to see the tool-tip. In many cases,
will provide all the information you need. Because there are so many parameters available, the Advanced Dialog in PHD2
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.
Other Devices Tab
The controls on the
'Global' tab are well-described by their respective tool-tips, but they
are summarized here for completeness:
- 'Language' - determines the language
used in the PHD2 user interface, subject to available localization.
Changing this requires a program restart
- 'Reset Configuration' - restores all settings to their
initial values as if PHD2 had been freshly installed
- 'Reset Don't Show Again messages' - restores the display of alert messages if you have previously chosen to not show them
- Software Update
See the Software Update section for more information about PHD2 software updates.
- Automatically check for updates - allow PHD2
to check for software updates when the program starts up. If no
internet connection is available, checking will be deferred until the
next time PHD2 is run.
- Only check for major releases - indicates whether to include development builds when checking for software updates.
File Location' - specifies a directory where PHD2 guide logs,
debug logs, and any diagnostic image files will be stored. The default
location on Windows is the "My Documents" folder associated with the
- Dither Settings
mode' - tells PHD2 to use a random-number generator to compute both the
size and the direction of the dither, subject to any constraints
imposed by RA-only mode or by the Dec guiding mode being set to 'None'.
- 'Spiral mode' - tells PHD2 to dither with fixed-size amounts in a clockwise spiral pattern.
This can be a good choice when the imaging camera has
significant fixed-pattern noise or the mount has a troublesome amount
of Dec backlash.
RA only' - tells PHD2 to dither only on the RA axis.
- 'Dither scale' - an optional multiplier used to adjust the maximum-dither amount specified by the image application. See Dithering Operations
diagnostic image logging' - used primarily for product support and
diagnosis of problems dealing with the guide camera or PHD2 star-recognition and
measurement. Guide frame images
are captured and logged in a FITs format subject to the filter/trigger
controls in the group-box. Images are saved in sub-folders of the
PHD2 logging directory with the date and time encoded as part of the
sub-folder name. Individual guide frames are saved with filenames
that indicate the time the image was captured and the reason the frame
was saved. Since the guide frames are saved in a FITs format, the
header will include other useful information such as exposure time.
Because the logging function is primarily used for
trouble-shooting, the image sub-folders are automatically removed after
30 days. If you wish to keep the images for your own purposes,
you should either rename the sub-folders or copy/move them to a
different directory. When logging is triggered by one of the
"events" - e.g. lost star or large errors - a group of images
(an image set) will be saved, centered in time on the image that
triggered the event. This provides a record of guider
images that will show what the guide star and guide frame looked like
both before and after the unusual condition occurred. The various
triggering and filtering controls are described below and are also shown in the tooltips for the controls:
lost star frames' - logs the image set for any lost-star events,
regardless of the reason for the lost star (low SNR, mass-change, etc.)
auto-select star frames' - logs the image set for any frames used for
auto-selection of the star, regardless of outcome. Note that any
failed attempts to auto-select a star will always result in a logged
image, regardless of choices made in the user interface.
relative error exceeds' - logs the image set when the star deflection
on the current frame exceeds the running-average error by the
factor chosen in the adjacent spin control. For example, if the
average (RMS) error is 0.5 pixels and the current frame's error is 1.5
pixels, the relative error is 3.
absolute error exceeds' - logs the image set when the star
deflection exceeds the number of pixels specified in the adjacent spin
this count is reached' - logs images until the count matches the
value of the adjacent spin control. The counter is reset to zero
when the limit is reached.
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
The controls on the 'Camera' tab are used as follows:
Use of Binning
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
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
very short and you don't want to overload either the mount or the
link with very high traffic rates.
- 'Auto Exposure' - these are the settings that control Auto exposure time.
Exposure - the minimum exposure time - PHD2 will not set the exposure
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 are likely to chase seeing
effects and thereby get poor guiding results. Users of AO
units might set this to a lower value since rapid small
corrections are more manageable 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
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
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 but a higher value is desirable. As mentioned in the 'Basic Use' section, SNR is
similar but not identical to the signal-to-noise ratio used in
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. The other
parameter required for this is the guide scope focal length, located on
'Guiding' tab. Refer to your camera documentation to determine
correct value for pixel size. If your camera has non-square
pixels, just choose one of the dimensions or input the average of the
two. The pixel size has no effect on guiding accuracy, so a small
amount of imprecision in the user interface won't cause any
problems. If you're using the binning setting in this dialog to
control camera binning, the pixel size should be the native, un-binned
this control may be disabled if the camera and camera driver can report
the pixel size to PHD2. In that case, the value displayed in the
disabled control represents the device-reported pixel size - it is what
it is. If you're also specifying a binning factor at the camera
driver level rather than in PHD2, the reported pixel size may change as
a result. It is generally better to use PHD2 to set binning (see
gain' - Sets the gain level for the many cameras
that support this feature. Reducing this parameter can help to
reduce the noise level or may allow use of a bright star without
saturation. The range of values and their specific effects are
dependent on the individual camera, so this PHD2 parameter is treated
as a percentage of the range between the minimum and maximum gain
values supported by the camera. For example, if the camera uses absolute
gain values in the range of 40 to 80, a PHD2 gain value of 50% would
translate to a camera gain of 60.
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. You should 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. This is a consideration for 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.
- for those cameras that support on-chip (hardware) binning, you can
specify the binning that will be used while taking guide exposures.
See below for a more detailed discussion. This control will
appear only if the camera is capable of on-chip binning and only if the
camera is connected to PHD2.
- 'Use subframes' - For cameras that support this
will download only a subframe of each guide
exposure. This is very useful for cameras with slow download
times, allowing them to be used more effectively for guiding.
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 shift-click
anywhere in the image display window or shift-click on the 'Auto-select
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
binning level requires its own dark frames and bad-pixel map -
they are not interchangeable, nor can a transform be done
automatically. If you want the flexibility of switching 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
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.
- Star images will have a higher SNR and
will be easier to detect above the background noise level. This
is especially helpful if you are limited to using faint stars
(i.e. with SNR values below about 10).
- 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
- 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 above 3
arc-sec/pixel. You may need to experiment because the results
will also depend on the image scale of your main camera system.
guiding tab shows the parameters used for calibration,
star-tracking, and guiding behavior shared by all of the guide algorithms..
Guide Star Tracking
region' - specifies the size of the "tracking rectangle", in units of
pixels. You may need to increase this value if your mount
not perform well or, more commonly, if it's not well-aligned on the
pole. You may also want to increase it temporarily while using
the Guiding Assistant so that backlash measurement can be done without
losing the guide star. Just remember that an overly large search
region also increases the likelihood that multiple stars will live
within its boundaries, which could lead to guiding problems.
- 'Star mass detection' - tells PHD2
to monitor the
brightness and size of the guide star compared to the sky background.
You may need to disable this feature or set the tolerance to a
higher number if you're using a fine image scale and are getting too
many lost-star events because of this feature.
- 'Star mass tolerance' - if the 'Enable' box
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 PHD2
doesn't mistakenly switch stars.
It can also
prevent errors caused by thin clouds, high camera noise, or alpha
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.
Resetting the 'Enable' checkbox or setting the threshold to 100 will disable the warnings
star HFD' - specifies the minimum half-flux-diameter (roughtly the
'size') of a suitable guide star. This is
probably the best way to prevent PHD2 from
mis-identifying clumps of hot pixels as usable guide stars. You
determine a suitable value for your system by manually selecting
some small stars that you know are not just hot pixels, then use
the star-profile tool to see the HFD values of those stars.
You'll want to specify a minimum HFD value that allows selection
of legitimate faint stars but not hot pixels. If you encounter
situations where PHD2 is trying to guide on a hot pixel, be sure to set
- 'Auto-selection frame downsample' - can
be used in unusual situations where PHD2 guide star auto-selection is
not recognizing misshapen stars. If this happens, you can try
values greater than 1 to improve the chances of recognizing such stars.
For most users, this property should be left at its default value
- 'Beep on lost star' - specifies whether PHD2 should produce an audible 'beep' sound when the guide star is lost during guiding.
Shared Guiding Parameters
- '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
'Camera' tab. If these two parameters aren't set properly, some
of the default guiding parameters may not be set correctly, and we will
be handicapped in trying to assess your guiding performance and help
you resolve problems. If you use the new-profile-wizard, these
values will probably be correct.
step-size' - specifes the duration of the guide pulse that
will use during calibration. Its use is described in the
Calibration' section of the 'Basic Use' help page. You can
the value depending on whether the guide star is moving too quickly
or too slowly during calibration. As a general guideline, it
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 'Advanced...' button
the right of this control will launch a calculator dialog that can help you
compute an appropriate value (see below)
- 'Auto restore calibration' - tells
PHD2 to automatically reload
the most recent calibration data as soon
as the equipment is connected. If
you're using an ASCOM (or Indi) mount connection or have an 'aux-mount'
connection, you may want to set this option.
Conversely, if PHD2 has no scope pointing
information available, this option will normally be reset.
The new-profile wizard will ask you how you want to set this
Note that auto-restore is remembered for each separate equipment
profile, and it only has an effect when you load the profile and
connect to the equipment. If you want to force a recalibration
before an individual guiding session begins, you can simply clear the
mount calibration (see below). If you have set the auto-restore
option, you must be sure that the guide camera is not rotated with
respect to the guide scope or the sky.
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
- 'Clear mount calibration'
- tells PHD2
you want to clear the calibration data currently being used for the mount and
guiding is restarted. You might do this for a variety of
- rotating the guide camera or changing the mount guide speed, for example.
You can also accomplish the same result by doing a Shift-Click on
the PHD2/guiding icon on the main page, which will force a re-calibration.
Declination Compensation' - if PHD2 can get pointing information from
the mount via an ASCOM connection ('Mount' or 'Aux'), it will
automatically adjust the RA guide rate based on the current
This box should normally be left checked except in unusual
cases. For example, SiTech mount controllers may apply a
compensation automatically, in which case the box should be left
un-checked. Don't confuse this option with 'Declination backlash
compensation', which is an entirely different feature.
Advanced Calibration Calculator
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'
your guide algorithms and by the size of your tracking region. This is only an optimization, so the use
of this checkbox is completely optional. If you find that
calibration often fails because the star is lost during the fast re-center,
you should disable this option. That sort of problem may
indicate that you have a large polar alignment error or excessive periodic
error in RA. You can run the Guiding Assistant to help see what's causing the problem.
- 'Reverse Dec output after meridian flip' -
tells PHD2 how to adjust the
calibration data after a meridian flip. Some
newer mounts track their 'side of pier' state and automatically reverse the
direction of the declination motor. Older 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 PHD2 provides the Meridian flip calibration tool to determine the correct setting automatically.
It's also easy to figure out the setting manually with this quick experiment:
- With the checkbox disabled, calibrate on one side of the pier, then move the mount to the other
- If you are guiding via ASCOM or Indi or are using an 'aux mount' connection, just start guiding.
you're guiding only via ST-4 and PHD2 has no scope pointing
information, first select 'Flip Calibration' under the 'Tools' menu,
and then start guiding.
- In either case, if the guiding works normally, leave the box un-checked; but
- if you see guiding run-away in
declination, check the box and repeat the entire procedure, including calibration.
- 'Enable mount guide output'
- this is normally checked because it tells PHD2
to send guide
commands to the mount. But there are some
where you might want to disable this, usually because you want to
observe the uncorrected behavior of the mount. For example,
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. In most cases, you will be better
off running the Guiding Assistant for these purposes, which
automatically handles this setting for you.
guiding when mount slews' - a diagnostic option for working with ASCOM
mount drivers that may report the slewing status incorrectly. The option
should be left checked unless you know the driver for your mount
has this problem (highly unlikely). Also, it will
generally not detect slews that are initiated by the
hand-controller. In any case, PHD2 will continue to try to track
the guide star, so slews will typically result in lost-star
notifications - beeping and flashing of the PHD2 image window.
To review or change calibration parameters,
be sure the topmost four edit controls are
correctly filled in. If you have already specified the focal
the camera pixel size in the 'Global' and 'Camera' tabs respectively (or via the new-profile-wizard),
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 usually show the correct values unless the driver doesn't report them correctly.
Otherwise, you'll need to supply them yourself. The guide
speed is specified as a multiple of sidereal speed - most mounts will
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.
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. You should be careful with this because
"bigger" is not always "better". With larger required calibration
distances, the calibration is more likely to be affected by large
uncorrected periodic error in RA or large drift in Dec. As you
change the values
in these fields, PHD2
will recalculate your current image scale and a recommended value for
calibration step-size. If you then click on 'Ok', that value
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.
goal of the calculator is to recommend a step-size that is likely to
result in an accurate calibration without wasting too much time, and
the automatically computed values will usually work well. As long
as you are getting successful calibrations without alert messages, the
calibration parameters should 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 Calculator
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
change the algorithm selections. For that reason, all the
parameters related to guide algorithms will be treated together, in a
The remaining controls, the ones that are independent of the guiding
algorithm selections, are described below.
- 'Max RA duration' - specifies the
guide pulse duration for right ascension. You might reduce
below the default value if you want to avoid chasing a large
deflection that could be caused by a spurious event (e.g. wind gust,
hot pixel, etc.) .' However, there is rarely a good reason for
changing it if you have protected yourself against hot pixels (Minimum
backlash comp' - this
controls whether PHD2 will apply a compensation factor when the
direction of declination guiding needs to be reversed.
See section below.
- 'Max Dec. duration' - specifies the
maximum allowed guide pulse duration for declination (same as above but
- 'Declination guide mode' - gives you additional control over declination
guiding. Declination guiding is not like RA guiding because the
errors are not caused
by imperfections in your mount's gears. Instead, deflections in
declination are primarily the result of imperfect polar alignment or flexure.
The result is an error that should be smooth and mostly uni-directional, assuming there is no over-shoot from an
earlier correction. The default value of 'auto' tells PHD2
some reversals in direction are acceptable, subject to the behavior of
the various guiding algorithms. However, if your mount has severe
declination backlash, you may want to prevent direction reversal
altogether. You could then select either 'north' or 'south'
restrict corrections to only that direction (uni-directional Dec
guiding). Keep in mind,
however, that an over-shoot in correction with one of these modes
might leave the star positoned off-target for an extended period of
time. So you'll probably want to use conservative parameters for
aggressiveness if you are disallowing direction reversals.
Finally, a choice of 'off'' here disables declination guiding
altogether, an appropriate choice for simple tracking mounts that don't
support Dec guiding.
- resets the guiding parameters for the selected RA or Dec algorithm to
their default values. Min-move settings will be set using the
same algorithm employed in the new-profile-wizard. If you
previously used the Guiding Assistant to adjust the min-move settings,
you should probably repeat that procedure.
Declination Backlash CompensationMost
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 only.
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 GuidingAs
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
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,
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 5-10 arc-min of the pole before setting up for
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
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:
to a field with a good guide star and run the Guiding Assistant for
several minutes. Look at the guiding graph and note whether the
guide star is drifting north or
south. Once you see this, reset the Dec guide mode to issue
corrections in the appropriate direction. For example, if the
drifting north, set the Guide mode to 'south.'
- Try using the 'LowPass2' guiding algorithm 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.
- 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
- If you are dithering, you may want to set the dithering parameters to "RA-only" to avoid disrupting the Dec guiding.
Other Devices Tab
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 elsewhere, 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
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.
'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
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
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 'Hysteresis'', which
gives you an easy way to control damping and
you use a different algorithm, you should
probably start with a high level of aggressiveness there as well.
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.