Alcor NOVA 120 User manual
NOVA 120 Pointing model user manual 1
Direct Drive Equatorial Mount
NOVA 120
Pointing model user manual
NOVA 120 Pointing model user manual 2
Changes history
Version 1 1
12
/0
5
/2014
Initial release
Here is a list of Acronym used in this document
Acronym Meaning
RA Right ascension
DE Declination
TPOINT
This is a standard formalism used for
achieving pointing model,
see here for more information
http://www.tpsoft.demon.co.uk/pointing.htm
MA or HA
Hour angle, or meridian angle
http://en.wikipedia.org/wiki/Hour_angle
NOVA 120 Pointing model user manual 3
Contents
1. Introduction .................................................................................................................................................................... 4
2. Achieving Pointing model files ........................................................................................................................................ 6
2.1. Data coming from any software (except PRISM software) ............................................................................. 6
2.2. Using PRISM software to get pointing model coefficients ............................................................................. 8
2.2.1. Prerequisites ................................................................................................................................................... 8
2.2.2. Plate solving checking ................................................................................................................................... 10
2.2.3. alibrating the first field ............................................................................................................................... 18
2.3. Sweep the sky with fields, and build pointing model field list ...................................................................... 19
3. Applying the pointing model to the NOVA mount ........................................................................................................ 42
4. Final checks ................................................................................................................................................................... 44
NOVA 120 Pointing model user manual 4
1. Introduction
A Pointing model is supported by the NOVA mounts family. It is essential to achieve it properly to create the best from
your mount. Pointing model improves pointing accuracy, but also mount tracking accuracy, because the mount software
can use the pointing model to get a speed model and also compensate for mount polar alignment errors, or any other
errors that are supported by the model.
Left is an image with no pointing model, where the user tried to center the right star (field of view is 15x15 arcmin) but
no star is present. Right is an image that demonstrates the effect of the pointing model: the star is at the image’s center.
Working without a pointing model can allow dramatic errors with a German mount. Meridian flip and optical axis non
perpendicularity error with the DE axis can lead to pointing errors more than ½ of a degree! This causes most of the
time, the target to be outside the field of view of the camera. A Pointing model can overcome this easily, allowing better
use of your telescope, and not wasting time acquiring your target.
The next picture shows what happens with a poorly polar mount alignment (18 arcmin error from the pole) and a 600s
unguided exposure.
NOVA 120 Pointing model user manual 5
The image quality is weak and shows a vertical trailing in DE direction. This is quite normal with badly polar aligned
mounts.
If the pointing model has been previously created, and injected into the mount software, the speed model will be
calculated and applied.
Despite the poor polar alignment, the 600s unguided exposure is perfect, because the mount software could
compensate for this error automatically based on the pointing model. Since the mount can never be perfectly polar
aligned, the speed model is a real asset for your mount.
Even, more the next picture shows a raw single 900s unguided exposure that would be impossible to achieve without
the speed model and refraction speeds compensations.
NOVA 120 Pointing model user manual 6
2. Achieving Pointing model files
2.1. Data coming from any software (except PRISM software)
In this document, the pointing model will be demonstrated using the PRISM software package. Nevertheless, this is not a
mandatory step, because other software in the market can use TPOINT and provide valid data that can be used with the
mount NOVA software. The purpose of this document is not to demonstrate how to achieve a pointing model with all
the software available in the market. We will do it using PRISM, because this is very easy to achieve.
An important point, for German mounts, there are two pointing models required, one for the tube located at the east
side of pier, and another for the west side of pier. These models had turned out not to be the same, by experience.
The type of file format to provide to the NOVA mount software is a simple “* cor” file, which is indeed a simple text file.
Here is a “.cor” file sample; always starting with 789955491 as a magic number. Then pointing model coefficients
expressed in radians are listed after:
NOVA 120 Pointing model user manual 7
789955491
1.28873174669791E-0003
-1.01213383676195E-0003
-6.63990193728120E-0004
4.39074860709788E-0004
-3.73858799270869E-0005
1.58008143060133E-0003
7.43329643913604E-0004
2.46848043582684E-0004
-1.16523424111638E-0003
-5.05908232673203E-0005
0.00000000000000E+0000
0.00000000000000E+0000
-3.60434309820003E-0004
-2.61141145462940E-0004
0.00000000000000E+0000
These coefficients translate into this table with famous TPOINT standard coefficients ( H, NP, , ME, MA) for equatorial
mounts:
Raw
figure as radians
into the
file
TPOINT
Acron
Meaning
Converted in
arcmin
1.28873174669791E-0003
H
Optical axis and DE axis perpendicularity error
+
4.4
-1.01213383676195E-0003
NP
RA/DE axis perpendicularity error
-
3.5
-6.63990193728120E-0004
ME
Elevation polar axis error (polar alignment)
-
2.3
4.39074860709788E-0004
MA
Azimuth polar axis error (polar alignment)
+1.5
-3.73858799270869E-0005
ID
DE offset
-
0.1
1.58008143060133E-0003
IH
RA offset
+5.4
7.43329643913604E-0004
TF
Tube flexure
error
+2.6
2.46848043582684E-0004
FO
Fork axis flexure error
+0.84
-1.16523424111638E-0003
DAF
DE axis cantilever error
-
3.98
-5.05908232673203E-0005
DAB
DE axis flexure error
-
0.174
0.00000000000000E+0000
DNP
Dynamic non perpendicularity error
0.0
0.00000000000000E+0000
H E
RA encoder error (cosine)
0.0
-3.60434309820003E-0004
H ES
RA encoder error (sine)
-
1.2
-2.61141145462940E-0004
D E
DE encoder error (cosine)
-
0.89
0.00000000000000E+0000
D ES
DE encoder error (sine)
0
All figures coming from third party software shall be all multiplied by -1 into this file to be used with the NOVA mount
software (for historical reasons). Do tests to check all these step by step.
It is up to the user to arrange parameters in the same order as it is described above. In case of unknown parameters
leave them simply to zero and do a test and see if it improves pointing accuracy.
When performing the pointing model with other software than PRISM, do not forget to disable any pointing model at
the NOVA mount software level.
Finally keep in mind that the NOVA mount software sends to the AS OM client software “Observed coordinates” that
are corrected from refraction and not “Topocentric place” coordinates.
NOVA 120 Pointing model user manual 8
2.2. Using PRISM software to get pointing model coefficients
2.2.1.Prerequisites
Before starting the NOVA mount software, please ensure that any pointing model is already enabled, if so disable it.
Go here:
Uncheck this checkbox, if checked:
When the mount is running and connected to the software, it will mention that the pointing model is disabled, as shown
hereafter:
NOVA 120 Pointing model user manual 9
Go to “Setup” then “System setup”, press “German mount setup” and change as follows the allowable mount clearance.
NOVA 120 Pointing model user manual 10
This step is achieved to avoid unwanted meridian flip, because two pointing models are required for German mounts,
one for the tube located at the east side of pier, and another for the west side of pier Since pointing models are
created by slewing to areas only in one side of the pier, these settings forbids the mount from meridian flipping when
located close to the meridian.
Using PRISM software from AL OR-SYSTEM, please, connect the mount to PRISM, and keep eye on side of pier
information that is sent by the NOVA mount software to PRISM.
This panel helps doing so:
Say, that the pointing model will be built for the telescope located on the east side of the pier, aiming at targets that are
on the west side of the Sky.
A D camera attached to the telescope is mandatory. There is no way doing visually with an eyepiece; it will never
reach to the required accuracy. The larger the field of view is the better to allow a fast pointing model to be built. But
also a sampling of 3 arcsec per pixel is the maximum sampling rate required.
2.2.2.Plate solving checking
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