Re: Possible defective unit - SOLVED! #iEXOS-100

Wes Mcdonald


The movement of the steppers is not perceivable in images.  The steps are way to small to be discerned by your camera.  Just so you know when you are actively guiding, where the motor rate is altered at a two or one second rate (one of your many guiding parameters) the mount maintains pointing to 1 arc second or so.   This is smaller than your imaging pixel so that is how you can integrate for 2-5 minutes and not have any smeared images.

If you are imaging with explorestars, not recommended, if you are seeing movement it is because the thing is in P mode.   In that mode the mount only updates it’s position 5 times per second.  It is literally still for most of the time between pulses.  Objects move in camera-discernible amounts in 200ms.  You must image in T mode which is tracking.  In T mode the mount runs at sidereal rate in RA and not at all in DEC.  there is essentially no interval between stepper shaft rotations (ok there is , it is 23micro seconds).  This is 43,000 times per second.  Your mount has inertia and believe me, those tiny nudges 42000 times per second are silky smooth by the time the mount filters them.   

If you are not guiding then your explorestars (it any other control app) basically needs you to have very good polar alignment.  This is where the most error in your images is going to come from with out guiding if you are in T mode.  If you insist on not guiding then you are going to have to learn how to get superb polar alignment.  And then operate explorestars in T mode .  

For bright solar system objects any small difference between sidereal rate and the object won’t mess up your short exposures .   But with poor polar alignment the objects will slowly march across the camera field of view with slight minor rotation between images.  You just have to live with it and post process it out.

Hope this helps your understanding of this mount.  Believe me, if stepper motors were somehow inferior to servos virtually every manufacturer of mounts would not be using them in new designs.  As far as I can see they are superior in almost every way.


Wes, Southport NC
EXos2-GT PMC-8, iExos 100
ES ED 127, 10" LX200GPS+wedge, Astro-Tech 8" Newt, ETX-90, 60mm no-name guide scope ~ 260mm FL
Polemaster, Orion ST-80 and SAG, ZWO 290MM, D5300 astro modified
Nina, Bootcamped Mac Mini control computer, RDP to iMAC
110 amp hour lead acid deep discharge battery for field power
Electrical Engineer, Retired

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