#Mach2GTO #APCC #Absolute_Encoders #APCC #Absolute_Encoders #Mach2GTO


John Upton
 

Hi Everyone,

   Being new to Astro-Physics mounts and hampered by cloudy skies since getting my Mach2 a couple of weeks ago, I have a bunch of small questions. I'll be asking these over the next few days.

Here is Question #1:
What is the difference between the Mach2 Home and Park-3 positions?

   I have noticed that if I tell APCC to Find home (from the AE Tab) and then query its position (from the GoTo/Recal Tab) it gives me position almost exactly as I expected -- Alt/Az is equal to my Latitude and Azimuth of 0. The mount is not parked after this Find Home operation but tracking is turned off.

   Now, If I Park the mount to Park-3 using the Park Tab of APCC, the mount will slew ever so slightly -- practically invisible to the eye. However after this miniscule slew, if I again query the mount's position as before, I see that the reported position is ever so slightly different from that reported after the Homing. The reported Alt/Az now shows the altitude a little off a tiny amount from my Latitude and the Azimuth is now up to 10 arc-seconds from 0 and seems to vary from one Park to the next.

   What I have read in the manuals, says the Find Home is highly accurate and is set via the encoders at the factory. That is easy to understand. However, why is this not consider to be the same physical position as Park-3?

   It's not like this is terribly important because the difference in positions is tiny but why is there a difference at all?

John


Ray Gralak
 

Hi John,

Home is a physically fixed position. However, the park positions can vary slightly because they are relative to the last synched or recalibrated coordinates, which may be "off" slightly because of polar alignment error, equipment flexure, refraction, etc.

-Ray

-----Original Message-----
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of John Upton
Sent: Friday, April 16, 2021 8:31 PM
To: main@ap-gto.groups.io
Subject: [ap-gto] #Mach2GTO #APCC #Absolute_Encoders

Hi Everyone,

Being new to Astro-Physics mounts and hampered by cloudy skies since getting my Mach2 a couple of weeks ago, I
have a bunch of small questions. I'll be asking these over the next few days.

Here is Question #1:
What is the difference between the Mach2 Home and Park-3 positions?

I have noticed that if I tell APCC to Find home (from the AE Tab) and then query its position (from the GoTo/Recal
Tab) it gives me position almost exactly as I expected -- Alt/Az is equal to my Latitude and Azimuth of 0. The mount is
not parked after this Find Home operation but tracking is turned off.

Now, If I Park the mount to Park-3 using the Park Tab of APCC, the mount will slew ever so slightly -- practically
invisible to the eye. However after this miniscule slew, if I again query the mount's position as before, I see that the
reported position is ever so slightly different from that reported after the Homing. The reported Alt/Az now shows the
altitude a little off a tiny amount from my Latitude and the Azimuth is now up to 10 arc-seconds from 0 and seems to
vary from one Park to the next.

What I have read in the manuals, says the Find Home is highly accurate and is set via the encoders at the factory.
That is easy to understand. However, why is this not consider to be the same physical position as Park-3?

It's not like this is terribly important because the difference in positions is tiny but why is there a difference at all?

John


Roland Christen
 

Find home sends the mount to the Park 3 position but does not park the mount. It also re-calibrates the mount to this position so that you can recover from mount being lost due to an errant sync or recal. The mount will also resume tracking at the sidereal rate. Therefore the RA position will not change (mount continues to track the RA) but the Altitude/Azimuth will slowly change as the mount tracks the earth's rotation.

Park3 sends the mount to the same position but does not recalibrate the mount. It also shuts off power to the motors which stops tracking in RA. The RA position changes as the earth turns on its axis, but the Altitude/Azimuth position will remain the same. The exact Alt/Az position will change very slightly as the motors are de-energized because being steppers, they will drop into the nearest null position of the rotor/stator alignment. This amounts to +- 7.2 arc seconds of precise position change when the holding current is removed. However, this means nothing since the absolute encoders keep track of the precise shaft position at all times when the mount is de-energized in Park or even when power is removed. Therefore, to be absolutely accurate when turning power back on, simply start the mount from present position instead of forcing it to start from Park3 or other park positions.

Rolando



-----Original Message-----
From: John Upton <upton@...>
To: main@ap-gto.groups.io
Sent: Fri, Apr 16, 2021 10:31 pm
Subject: [ap-gto] #Mach2GTO #APCC #Absolute_Encoders

Hi Everyone,

   Being new to Astro-Physics mounts and hampered by cloudy skies since getting my Mach2 a couple of weeks ago, I have a bunch of small questions. I'll be asking these over the next few days.

Here is Question #1:
What is the difference between the Mach2 Home and Park-3 positions?

   I have noticed that if I tell APCC to Find home (from the AE Tab) and then query its position (from the GoTo/Recal Tab) it gives me position almost exactly as I expected -- Alt/Az is equal to my Latitude and Azimuth of 0. The mount is not parked after this Find Home operation but tracking is turned off.

   Now, If I Park the mount to Park-3 using the Park Tab of APCC, the mount will slew ever so slightly -- practically invisible to the eye. However after this miniscule slew, if I again query the mount's position as before, I see that the reported position is ever so slightly different from that reported after the Homing. The reported Alt/Az now shows the altitude a little off a tiny amount from my Latitude and the Azimuth is now up to 10 arc-seconds from 0 and seems to vary from one Park to the next.

   What I have read in the manuals, says the Find Home is highly accurate and is set via the encoders at the factory. That is easy to understand. However, why is this not consider to be the same physical position as Park-3?

   It's not like this is terribly important because the difference in positions is tiny but why is there a difference at all?

John

--
Roland Christen
Astro-Physics


John Upton
 

Rolando & Ray,

   Thank you both for the details and deep dive into how this works. I appreciated the depth of the information.

   Now that Rolando mentions that the tracking is not turned off when Home Position is found, I recall that I also observed that and had left out a step in my procedure in the initial post here. Immediately after the Home Position had been found, I had to Toggle Tracking off before querying the mount's position. I had not considered the slight offset that happens when the motors are de-energized following a Park command compared to just stopping the tracking after finding Home.

   Your explanations tied all of that together nicely.

John