HomeFind from ASCOM drivers, set new Homeposition

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This is a transfered topic from the ASA Yahoo Group.

 

 

Posted By: nomis_ch Thu Mar 28, 2013 2:17 pm

Hello !

I am a computer-science engineer working gro the University of Bern.
We recently bought an ASA DDM160.

We are using autoslew 5.1.1.5, with the addition of the small accelerometer to
facilitate Zenith parking.

The FindHome method of the ASCOM driver is not implemented, is that done on
purpose or is it gonna be implemented ?

This method would be very useful as we are developing an automated observatory
and I would like the scope to make a findhome on when Autoslew starts (either
from ascom calls or through an Autoslew option).

Furthermore, when I do a findHome from Autoslew (clicking on the button), the
mount succesfully find the home but it also modifies the Parking position 1 to
the position found by FindHome. Thus the scope is not anymore looking at the
zenith in P1 ....

Furthermore, it seems the "Set new Homeposition" function is not working. I
wanted to use it to put the new home position (reference marks ?) closer from
the Zenith position but it does not have any effect when I press on it. (It
shows a message asking if I am sure I have synched and centered the scope ... I
press yes and nothing happens)

MY colleague, M.Affolter also contacted you concerning safety concerns. If the
USB gets disconnected while a command was issued, the scope will continue moving
in that direction .... it should have a king of hardware stop ... or something
similar ...

Best regards,

Simon Ruffieux

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Posted By: lukasdemetz Thu Mar 28, 2013 4:43 pm

Hi Simon,

I am not sure, but there are a few ASCOM-specifig configuration options in Autoslew under Control -> Ascom ACL. Things like “Enable ASCOM to Homefind” and “Homefind on unpark” might do de trick for you. I use these as I need Autoslew to do a homefind as soon as it starts (used with ACP).

For the USB issue, try different cables; I had just a few days ago a similar issue with an active USB cable (funny enough, the “normal” cable is not causing these errors).

Hope it helps


Lukas

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Posted By: georgecarey47 Thu Mar 28, 2013 5:16 pm

Hello Simon,
It will be interesting to hear how you get on with a DDM160!
One bit of advice, ASA do not always read this forum regularly, so it might be
better to put in an official support request to ASA to get your questions
answered.
George

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Posted By: bernd_eppinger Fri Mar 29, 2013 12:33 pm

Hello, Simon,

concerning the homefind issues:

 

> "The FindHome method of the ASCOM driver is not implemented"

I know that it is implemented and that it works, because I have a working self-written MATLAB-script which uses this method. However, to make it work, it is necessary to change an option in Autoslew:

  Control -> Ascom ACL -> Enable Ascom to Homefind.


 

> "when I do a findHome from Autoslew (clicking on the button), the mount [...] modifies the Parking position 1 to the position found by FindHome."

When you start the mount without homefind and without synching and from an unknown position, and then do a homefind, then the homefind will change all positions in the sky, not only the park positions. For example, if you do a slew to an object before the homefind, and if you repeat the same slew after the homefind, the telescope will point somewhere else. This is an expected behaviour. For details, I'd recommend to read the Autoslew manual, but I here are some basics, as far as I understand the Autoslew documentation.

My DDM60 (and I expect also the larger mounts) has no absolute angular encoders, but instead differential encoders. This means when you power the mount up, then it does not know where it is pointing to. Autoslew tries to alleviate this by storing the last position, but of course this can only work if the mount is not moved while powered down. So for the general case, the mount must first find out the absolute angles of its encoders. This is done by homefind.

Homefind tries to find the "Reference marks" of the encoders. These are mechanically fixed marks in the encoders, of which the absolute position is known (or at least constant). This is called the Homeposition, and its value is stored somewhere in the INI-files. When the mount finds the reference marks, it knows the absolute position of its encoders, and it replaces the encoder readings with the known Homeposition. From now on, the encoder readings are "correct", as far as the stored Homeposition is correct.

If you do a slew to a park position before a homefind, then the slew (like any slew) will be to anywhere. Of course you can manually slew to any position and also set park positions, and the mount will also be able to slew to these park positions again. But these positions are relative to the encoders which have an unknown offset at this time. When you do a homefind, the offsets are corrected. If you repeat the same slew now, the telescope will point somewhere else.

When you have a new mount and want to maximise the precision you can achieve without a configuration / Pointing model, I would recommend to do the following once:

 

•     Telescope base should be and stay horizontal (or something equivalent with the DDM160) to make the whole thing reproducible.
•     Homefind.
•     Polar align the mount as precisely as possible (may need multiple iterations).
•     Clear the Configuration.
•     Homefind again (just to be sure; I don't know if this step is necessary).
•     Sync to a star (-> now Autoslew knows where it is pointing in the sky).
•     Set new Homeposition.

Now the stored Homeposition is "calibrated", i.e. has the correct DE and hour angle coordinates, assuming that there are no telescope errors. You should never again do a "set new Homeposition", as this would change your calibration. From now on, each time you power up the mount, the first thing you should do after magnetic angle autofind is a homefind. Only after this homefind, the positions to which the mount points are defined and reproducible. This includes the park positions.

Addendum 1: Instead of homefind, it is in principle possible to do a sync to a star, but I find homefind easier.
Addendum 2: Apart from homefind, changing the clock of your computer also influences where the mount is pointing to. Therefore, I use a GPS. As Autoslew syncs the computer time after each slew and my initial computer time may not be precise, I do a dummy slew to set the correct time, and then I do the actual slew. If apart from the time, my date is also wrong, then I have to update the date by hand; unfortunately, Autoslew doesn't do it.
 

> "it seems the "Set new Homeposition" function is not working. I wanted to use it to put the new home position (reference marks ?) closer from the Zenith position but it does not have any effect when I press on it. (It shows a message asking if I am sure I have synched and centered the scope ... I press yes and nothing happens)"

I think this is a misunderstanding of what "set new Homeposition" is supposed to do. It doesn't and cannot change the position to which the telescope is moving during a homefind, because the telescope must move to the reference marks which are mechanically fixed and cannot be moved to a different position by software. If you want to move your telescope to predefined positions, then use the park positions instead (of course after doing a homefind).

How to use "set new Homeposition": It requires, that you have made a homefind. This tells Autoslew where further positions in the sky are relative to the reference marks. After that, it requires that you synced your polar aligned horizontal mount/telescope to a known position in the sky, usually a star. This allows Autoslew to determine an offset between the reference marks and the DE / hour angle. Now when you select "set new Homeposition", then this offset is simply saved to disk. You will not notice any effect during the same Autoslew session. But if you stop Autoslew, power down the mount, then power everything up again and do a homefind, then you should realize that Autoslew will now be able to slew directly to your star, without synching.

Addendum: Homefind and "set new Homeposition" are the first step to a precise mount orientation. It does not correct for telescope errors, mount angle etc. For these things, you must make a Pointing model (sometimes also called Configuration). I have the impression that if you use a pointing model, then the "set new Homeposition" is probably redundant, but for me as a mobile user it is nevertheless useful because it helps me to do a first coarse alignment after homefind when I do not yet have a valid pointing model. When you have and use a pointing model in a stationary set-up, then the only important thing is that from now on you don't change the homeposition ("set new Homeposition"), as this would make your pointing model invalid.


Best regards,

Bernd
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Posted By: tomtom2509 Tue Apr 2, 2013 11:21 am

Hi Bernd,

you wrote:" When you have and use a pointing model in a stationary set-up, then
the only important thing is that from now on you don't change the homeposition
("set new Homeposition"), as this would make your pointing model invalid."

I don't think that it invalid a pointing file because the only thing you change
ist the delta to the reference marks. I the german AS-manual I found this:
"Nachdem Sie ein Pointingfile (welches Sie in Zukunft verwenden wollen) angelegt
haben, können Sie nun eine
neue Homeposition setzen. Dies dient dazu, dass Autoslew die
Positioniergenauigkeit der Montierung im Hinblick auf die erstellte
Pointingdatei übernimmt."
Which said that it is recommended to set a new homefind positon after making a
new pointingfile.
Or do I have a missunderstanding in this case?


BR Thomas
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Posted By: bernd_eppinger Wed Apr 3, 2013 1:05 am

Hi, Thomas,

I think your understanding of the German version of the Autoslew Manual is exactly as it is written, although the important part is in parentheses. However this is one of the parts of which I think that they could be clearer in the manual.

When I look into the Advanced Pointing Control window, I can see that each configuration has its own "Synch Offset". Different configurations have different Synch Offsets, which is obvious for my mobile set-up. Under "Pointing -> Show Corrections", I can find them as "Offset Constant". This shows clearly that each configuration stores its own Synch Offset.

It seems also obvious that this Synch Offset must be adapted, if the angles which Autoslew assumes for the reference marks change. The only alternative I can think of would be that Homefind does not correct the encoder reading, but instead modifies the Synch Offset. However I have checked that this is not the case.

Following these findings, there's some room for interpretation of the manual text, especially the sentences that follow. I personally don't think that Autoslew changes the Synch Offsets of all configurations I have stored somewhere on disk when I do a "Set new Homeposition". The manual says that I should load the configuration that I want to use in future, then save it as "Use now, save and use on next start", and then do a "Set new Homeposition".

Interpretation 1: Autoslew does not correct any configuration on disk. In this case, I will in future need to synch after the homefind (unless I make a new pointing file), which would not be the purpose of homefind.

Interpretation 2: Autoslew modifies the Synch Offset of the currently loaded configuration, or of the configuration that is automatically loaded on start, when a "Set new Homeposition" is done. This would then make all other saved configurations invalid, and the modification of the configuration would be somewhat unexpected.

It would in principle be possible to try and find out what Autoslew actually does during "Set new Homeposition", but I don't want to lose my saved homeposition.

Anyway, I think it is safest to do "Set new Homeposition" only once, then make a new configuration, and then never again change the homeposition. This should hopefully avoid the effects of these unclear parts of the manual.


Best regards,

Bernd

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Posted By: nomis_ch Fri Apr 5, 2013 10:27 am

Hello everyone,

First of all, thanks to all for your replies.

Concerning the FindHome function from Autoslew, I indeed found the "hidden" option; thanks for pointing that out.

The "HomeFind" and "set new homeposition" procedures are much more clear now. Still not as clear as I would like to, but your explanations helped me a lot (Specially bernd's detailed explanations). I am still working on that part to be sure it will perfectly work with complete remote automation.

Concerning the USB-cable disconnection problem, it is not a problem of the cable. I disconnected it myself to see the behavior. The problem is ASA not having taken into account the fact that a USB cable may be disconnected or that the program/computer where Autoslew is installed may fail (blue-screen, power off, ...). So in the case of a failure, it should (hardware wise) detect it and stop the motors instead of continuing forever in the last directions received. Because the actual behaviour could break the scope (hitting the pier). Indeed all limits and controls are software, there should be mechanical/hardware stops on the mount.... or something similar. We already informed ASA and they told us they were working on it. We thought about adding sensors ourselves and shut down power in case of emergency, but with the inertia, the scope may still hit the pier.

Otherwise, we are quite satisfied with the mount itself. (Just a bit disappointed with manuals and software not being too clear, notably translations being sometimes a bit approximative). I also would like some automatic way to detect east/west orientation using the small accelerometer ... that would not be too complicated to develop, I guess.

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Posted By: dwein6889 Mon Apr 8, 2013 4:29 pm
Dear Simon,

About the accelerometer, did you get this part from ASA? So what will you
develop in that case?

Best regards,
Dietmar Weinzinger
ASA

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Posted By: w.boot49 Mon Apr 8, 2013 6:35 pm

Hallo Dietmar,

I would think that is a secundary function of the encoders...
Or am I totally wrong here?

Best Regards,
Waldemar

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Posted By: dwein6889 Tue Apr 9, 2013 8:56 am

Hello Waldemar,

This is a new unit(USB plug), we are in the Beta test level now.

Best regards,
Dietmar Weinzinger
ASA

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Posted By: nomis_ch Wed Apr 17, 2013 9:36 am

Hallo Dietmar,

I will not develop anything using the accelerometer (ASA sensor) that was
provided from ASA.

What I meant, is that, using this accelerometer, you should be able to
automatically detect the current pier side (east or west) of the mount.
Currently we have these two options "Ask" or "Decide". Therefore you should
provide an option "Decide using ASA Sensor" which would make much sense when
remote operating the mount.

Simon