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To help with precise equatorial mount polar alignment is a
task for the new SIPS Polar Alignment tool, introduced in
SIPS v4.5. The align procedure is based on imaging of the pole with
the main camera. To function properly, several prerequisites need to
be satisfied:
Main imaging camera must be connected and online. If you want
to use the Guiding camera for pole align, temporarily
select it as the main Imaging camera. Astrometry of images, acquired with the main camera, must be
properly working. This means the Star search parameters
must be defined to find even the brightest stars within field of
view, astrometry catalog must be selected etc. The
Astrometry tool should be used to configure and test plate
solution of acquired images prior to polar alignment. Exposure time must be set so the image contains enough stars
to allow astrometry (acquiring just Polaris is not enough). The celestial pole must be visible from the observing site,
and the equatorial mount must be at last roughly aligned to the
pole, so the real pole remains in the field of view even if the
telescope is rotated in the R.A. axis. If the algorithm should count with the atmospheric refraction
(required for sub-arcminute alignment precision), the observing side
location must be defined in the Observatory Setup
dialog.
Equatorial mount pole align dialog box
The dialog box offers several steps, allowing to determine
actual position of the mount polar axis and its adjustment, so it
precisely points to the celestial pole.
Step 1: Introduction
The first screen of the mount alignment dialog box
summarizes these prerequisites. It also allows to choose if
the algorithm should count with refraction.
Hint: To properly calculate refraction, SIPS needs to know
the site geographic latitude. The Polar alignment dialog box
relies on the value specified in the Observatory Setup dialog
box, instead of yet another latitude definition controls. If
the Count with refraction check box remains disabled, the
observing site geographical location is not defined
properly. Step 2: Pole Shot One
When the mount points the telescope to the pole, SIPS needs
to perform an exposure and calculate its astrometry. Set
exposure time properly so the image contains enough stars
allowing astrometric reduction. SIPS calculates the location
of the apparent pole (this means the pole as
projected by refraction) and highlights it in the image.
If the refraction is to be included in calculations, also
the position of the real pole (where the pole would
be seen is there is no atmosphere).
Hint: The Polar adjustment dialog box runs modally, so no
other controls are accessible. This is why the Image
Zoom control is available withing the dialog box. Also,
the mouse wheel image zooming is propagated to the
image view and can be used to find a desired visible sub-frame
from the whole image. Step 3: Mount rotation
In the next step, SIPS needs to determine where the R.A.
axis of the mount actually points. This is done by rotation
the mount around the R.A. axis. The rotation angle is not that
important; however, it should be large enough to allow
calculation of the rotation center with enough precision.
Angle close to 90° is optimal, but any angle between 60° and
120° should work as well.
Step 4: Pole Shot Two
Another exposure of the imaging camera allows SIPS to
determine rotation center.
Step 5: R.A. Axis Adjustment
The desired state is the mount rotation center and the
celestial pole overlaps. Use the final step of the Pole
alignment dialog box to start exposures, process them and
display actual angular deviation between the pole and the
mount rotation axis. The distance is shown in the
Deviation field. Also, the deviation is displayed
within the image.
SIPS also calculates and displays the projection of the
angular deviation into Azimuth and Altitude part. As the mount
polar axis adjustment is typically done separately in azimuth
and altitude, seeing deviations in both coordinates
independently helps with the adjustment.
Hint: SIPS does not distinguish negative and positive
directions in both axes, as different mounts use different
placement of adjustment elements—sometimes the user observes
the mount from southern direction, sometimes from northern
direction etc. Using the trial-and-error (move the mount in
one direction and check if the deviation increases or
decreases) proves to be more effective than keeping track of
what direction is interpreted as positive and which is
negative. There is only one celestial pole displayed in this step. If
the refraction is included in calculation, only the real pole
position is shown. Otherwise, only the apparent pole is
used.
The dialog allows to check the Repeat exposures
option, which initiates next exposure, including the
astrometry processing and calculation of celestial pole
position and deviation. The next exposure is started after the
defined Delay between images, which is intended for
actual R.A. axis adjustment.
The best achievable pole adjustment precision
depends largely on the robustness, stability and precision
of the used mount and its adjustment mechanism.
Deviation < 15” is excellent. Such
deviation should not meaningfully affect the telescope go-to
precision or tracking accuracy. Deviation < 1’ is good. Any go-to or
tracking uncertainties should be easily compensated by
precision go-to (multi-step go-to including plate solving
and mount syncing) and guiding. At least, this precision
should be achieved on permanent setups
(observatories). Deviation < 5’ is acceptable, especially
for portable and ad-hoc setups. Higher pole adjustment
deviation may negatively affect mount functions.
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