Jerry Hubbell - Explore Scientific VP Engineering
On Fri, Oct 2, 2020 at 09:11 PM, Mario wrote:
Quick note, some others seem to be observing this same behaviour:Hi Mario,
There is a documented 1.3 second oscillation in the motor driver chip which I have talked about previously ( https://espmc-eight.groups.io/g/MAIN/message/3894 ) as some of our customers identified on the G11. We identified this issue about 2 years ago but the magnitude of the problem is typically < 0.5 arc-second. In fact it has been demonstrated to be smaller than that as some users have reported an total RA guiding error ~ 0.5 arc-second including scintillation effects during long exposure imaging. This behavior does not affect long exposure imaging, nor lunar/planetary imaging at long focal lengths as the stacking software can manage the small amount of "jitter" caused by the motor driver. The only real observing that was impacted was visual observing at very high focal lengths typical when observing Mars, Jupiter, Saturn, etc. Focal lengths of > 2500 mm when visually observing these planets were impacted.
At the time of discovery, after some investigation, we found that an increase in the motor current on the G11 helped to reduce this 1.3 second "jitter" significantly, although we also found that it depended on the equipment load and could be exacerbated by having weight at long moment arms which increased the inertial load.
Based on your video, I made some measurements and assumed the following:
Video is a 1080p resolution image
The full width of the moon would effectively stretch the full width of the frame if centered
The width of the moon is 2160 km and 30 arc-minutes (1800 arc-second)
I measured the motion of the video using the crater Hell which is 33 km wide
I found that the oscillation (peak to peak) was about 1/2 the width of crater Hell, or about 15 km
The pixel scale for the image was about 2 km per pixel (2160/1080) or about 1.7 arc-seconds/pixel (1800/1080)
Based on this, the oscillation is equal to about (15 km / 2 km) * 1.7 arc-seconds which equals 12 arc-seconds peak to peak.
This is equal to about 2 arc-seconds RMS which is about 4 times what we would expect based on our previous experience.
It appears that there is something in your equipment configuration that could be aggravating the appearance of this tiny oscillation, perhaps the total load or how it is mounted.
If you could give me some details in that regard I would appreciate it. Also, it might be possible to increase the motor current setting to help mitigate this issue and reduce the
magnitude of the oscillation.
Again, it is important to realize that this motion is just one component of the motion you will see when observing the moon or planets at a large focal length either through the
eyepiece or via live video. There is the natural drift in Declination, there is the local seeing conditions which can cause the motion to be at least the same magnitude or even more
than what you are seeing here.
Thanks for your report Mario.
Vice President of Engineering
Explore Scientific, LLC.
jrh at explorescientific.com
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
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!