Topics

Saving position at polar home #polaralignment #astrophotography #alignment


Nihar Pattnaik
 

After having used the iExos 100 for three months, I've come to notice a few peculiar things. The mount takes polar home position on whatever the RA and DEC values were when it is powered on. Even after it slews around, the "park mount" button brings the mount back to whatever position it was at the moment of startup and NOT to the actual polar home position. I was wondering if there was a way to change this. ie, to bring the OTA to perfect polar home, (with any offsets accounted for) , then save this orientation so that even if the mount is manually slewed with the power off, the control "remembers" the exact home position  , especially the DEC value. I'm sure the stepper motors report position to the processor and there's a way to save this, but I'm not sure how. Can this be done ? 

Thanks in Advance
Nihar, India


Jerry Hubbell - Explore Scientific VP Engineering
 
Edited

On Thu, Mar 12, 2020 at 10:30 AM, Nihar Pattnaik wrote:
The mount takes polar home position on whatever the RA and DEC values were when it is powered on. Even after it slews around, the "park mount" button brings the mount back to whatever position it was at the moment of startup and NOT to the actual polar home position. I was wondering if there was a way to change this. ie, to bring the OTA to perfect polar home
Hi Nihar,

The PMC-Eight controller on power up sets the motor counts to zero internally in the firmware. This is reasonable because there is no assumption that the mount's physical clutches have not been released and the mount is no longer pointed at the NCP. The Explore Stars app does not differentiate between the HOME position and the PARK position. The PARK position in Explore Stars is defined as HOME. If you use the PMC-Eight ASCOM driver, the default is also PARK = HOME, but you can set the PARK position wherever you would like to. In either case HOME is always defined as the NCP in the northern hemisphere, and as SCP in the southern hemisphere. This is equal to RADEC motor counts of zero (0) on each axis.

You could do as you say and set the PARK position where you would want to as long as you didn't ever release the clutches on your mount. This works fairly well with a mount used in a permanent observatory setup because you would only need to release the clutches when changing instruments to rebalance the load. Otherwise the clutches can stay locked.

The PMC-Eight is an open-loop control system relying on the mounts apparent position to equal the demanded position sent to the motors. There is no high-resolution encoder on the axes to send back the actual position of the mount. This works very well in practice in pointing the telescope as long as certain error terms are managed and accounted for. High-resolution absolute encoders are very expensive can cost two or even three times what the iEXOS 100 mount costs alone for one axis, let alone 2 axes. 

I hope this helps to explain what you are seeing.
 
Jerry Hubbell
Vice President of Engineering

Explore Scientific, LLC.
jrh at explorescientific.com

www.explorescientificusa.com
1010 S. 48th Street
Springdale, AR 72762
1-866-252-3811

Author: Scientific Astrophotography: How Amateurs Can Generate and Use Professional Imaging Data
             Remote Observatories for Amateur Astronomers: Using High-Powered Telescopes From Home


Mark Slade Remote Observatory (MSRO) IAU MPC W54 Equipment
Wilderness, VA
Mounts
: ES PMC-Eight G11 + Telescope Drive Master (TDM)
Scopes: ES 165 FPL-53 ED APO CF, ES 102 FCD100 ED APO CF
Cameras:  QHY174M-GPS + FW, QHY163C
Misc: 3-inch 0.7x Focal Reducer Field Flattener, Filters: Luminance,
Red, V-band Photometric, Diffuser, 200 lpmm Spectral Grating

Software: MaxIm DL 6, Cartes du Ciel, Astrometrica, AstroImageJ, AutoStakkert!