Re: G11 slewing past meridian pointing to the ground...

Steve Lang

Hey Jerry.

Thanks for the detailed explanation.

Cheers -

On 6/09/20 2:47 pm, Jerry Hubbell - Explore Scientific VP Engineering wrote:
On Sat, Sep 5, 2020 at 07:51 PM, Steve Lang wrote:
I've had no problems like this in the past at all that I can remember.

Any idea what might have gone wrong? Could it be a bug/condition that I
triggered (because I'm in southern hemisphere)?
Wes emailed about this, here is how I responded:
"The problem is not the meridian flip done at zenith. All the mount cares about is what side of the meridian it is on, it doesn’t give a hoot about where DEC is. The problem here is related to the plate solve when near the meridian.

I have talked about this at least 2 times in the forum when people run into it. What happens is that if you are near the meridian and the mount is slewed to an object which the mount thinks is currently on the east of the meridian (which it is). You take an image of the sky, and the pointing is off enough where the current position is west of the meridian. The pointing was off enough from where the mount thought it was. In this case, as soon as you do a plate solve and a SYNC, you now tell the mount, “hey, you are actually west of the meridian!” and calculates the motor counts according to that position. The problem is created at that point.

The calculation of the motor counts depends on which side of the meridian you are on of course for both RA and DEC. So now the mount is 180-deg out of phase (the motor counts) so when you go to slew to the object again, it will point the mount to the ground.  My advise on the forum is to never do an initial plate solve on an object within 10 arc-minutes of the Meridian.

You must make sure that any object you are pointed at in the sky and you have photographed is actually on the side of the Meridian that the telescope is pointed at before doing a SYNC."

Jerry Hubbell
Vice President of Engineering

Explore Scientific, LLC.
jrh at
1010 S. 48th Street
Springdale, AR 72762

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
: 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!

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Mounts: ES PMC-8 G11, Orion Atlas
Scopes: ES FCD100-80, GSO RC8, Orion XT10i
Cameras: QHY 8L, QHY163m, QHY5L-II, Polemaster
Software: NINA, PixInsight

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