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

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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

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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.

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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.

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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:

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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.

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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:

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Go to “Setup” then “System setup”, press “German mount setup” and change as follows the allowable mount clearance.

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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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Plate solving, that is having each pixels X and Y image’s coordinates to be translated into RA and DE coordinates, is a
fundamental step and must work.
PRISM embeds this step automatically by matching the star of the image with a catalog. Nevertheless, this feature shall
be checked and shall work; otherwise no pointing model can be build
Slew to a star that can be identified with the sky map, which is not too bright, and be sure this star is located into the
camera’s field of view.
The next image shows the star and the sky map that matches to it.
Then re-center the star so that it is located at the image’s center.

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Once achieved, click on the star in the sky map that matches to this star (ensuring that the mouse is set to identification
mode), and this window appears, click to the “calibrate with” button. Then the Sky map and the telescope are now
synchronized.
Then now acquire another image, this image will have the proper rough RA/DE coordinates that will be used for plate
solving. Be sure to be focused as best as possible.

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Then now select this menu:
This form sets the parameters to be used for automatic plate solving.

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The RA/DE comes from the telescope system and is embedded into the image’s header. As well binned (or not) pixel
size is included. The focal length comes from either from a previous plate solving or from the user when camera was set
up.
Do put a valid focal length (10% error is allowed). The catalog used for plate solving is important, use either GS -A T star
catalog or U A 2/3/4 catalog.
These star catalogs are available for free on download here:
http://www.prism-astro.com/us/catalogs.html
U A 4 is recommended, but it works also well with GS -A T catalog.

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When the “OK” button is pressed the software matches the star catalog and stars from the image and after some
seconds the next form is displayed, saying that plate solving went fine.
104 stars have been found in this case. This is just a 10s exposure from a 400 mm F/3.5 telescope binned 2x2 image and
turned out to be sufficient. In this case field of view is 1°25’
Select “Polynomial degree (plate solving)” from 2 to 4. Then click “compute”. The image is calibrated and this has
worked out fine in this case.
Sometime plate solving might not work, and here it a list and proposed solutions
Cause of failure
Solution
Focal
length no set correctly
hange focal length, do trial and tests
The center coordinates are not good
Refine them; be sure to get the right one. They do not
need to be accurate.
All sky plate solving can help retrieving these coordinates
Star size (FWHM) t
oo big. When star FWHM exceeds 1
5
to
20 pixels, it might not work.
Use binning to reduce star
FWHM
size. Be sure to be
properly focused
Too few stars
Increase exposure time, or/and binning
A combination of failures above will result in failure, they do not compensate, so go step by step.
Failure to achieve PRISM’s plate solving will result in pointing model failure, so this is a mandatory step and need to
work out!

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Focal length is being computed and is accurate to the mm.
If you go back into the camera setup form, the focal length can be updated. The camera orientation can be also been
entered in this case, this has no function for achieving the pointing model, but can be convenient for other purposes.

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This “other purpose” is to use telescope shift with the mouse. Left click over the image and then use “Shift telescope”

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Then put the mouse where you want the star (or other object) to be inside the image, the telescope will slew so that the
image is centered as you wish.
This not a mandatory step to make functional the pointing model, but is very useful.
Now, you are ready to create your first point model.
2.2.3.Calibrating the first field

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2.3. Sweep the sky with fields, and build pointing model field list
To build pointing model, the telescope needs to go throughout the sky. Instead of using a single star, the telescope
moves to any field and computes plate solving with many stars present into this field. The amount of stars used can
reach one hundred stars and leads to a far better positioning that he could be achieved with a single star. So there is no
need to center to a single star used as reference and to select some.
In this document a German mount will be assumed. The tube’s side of pier is located Westside, meaning that the
telescope is looking at the East side of the sky, thus meridian angle http://en.wikipedia.org/wiki/Hour_angle is from
minus 12H to 0H.
PRISM embeds a feature that will allow sweeping across the sky, and allowing you to carry out the pointing model.
lick to this menu:

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This form should appear, and is the control panel that will allow you to build the pointing model field list.