Re: Both axis tracking with ASCOM driver #iEXOS-100 #ASCOM

Jerry Hubbell - Explore Scientific VP Engineering

Hi Martin,

As you have learned, the ExploreStars program has two modes of operation, POINT mode and TRACK mode. POINT mode uses the mount's alignment model to continuously calculate the current position of the target object and makes "micro slews" to the position based on the corrected motor counts provided by the model. The ExploreStars application does this 5 times per second. The resulting behavior is that the effective rate in RA and in DEC is dynamically adjusted to keep the object in the center of the eyepiece.

The further the physical alignment of the mount is away from the true north or south celestial pole, the larger the rate value is in DEC due to the declination drift that occurs. This is what the mount model corrects.  The ASCOM driver does not currently use a mount model and assumes that the mount is physically aligned to the  north or south celestial pole. Therefore, all that is required is for ASCOM to provide a nominally fixed Sidereal tracking rate where the DEC axis is not driven. There are always slight corrections needed due to Periodic Error and small amounts of offset from the pole causing declination drift, so an autoguiding system is used to correct those errors to get virtually perfectly round stars in your image.

I hope this helps you understand how this works. 
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

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