Re: Advice on exos2 dec movement. #EXOS2

Walter Vinci

Hi Jerry,

Your point makes totally sense. Just as a reference point for a new Exos2 owner, what would you consider a good range for polar alignment offset? Anything as large as to avoid field rotation? Any insight from experienced users would be great. As you said, alignment tools like Sharpcap could also used very easily to achieve a desired offset in the alignment. 


On Feb 6, 2021, at 8:32 PM, Jerry Hubbell - Explore Scientific VP Engineering <jrh@...> wrote:

On Sat, Feb 6, 2021 at 11:32 AM, Robert Hoskin wrote:
Basically, less appears to be more where DEC guiding is concerned. 
Hi Bob,

I agree with you in terms of DEC guiding. In fact I go much further in my assessment. The following may go against the conventional wisdom a little bit and impacts the need for a precise polar alignment. You may already know where I am going with this.

I would say that for less expensive mounts where the quality of the gear train is not in the upper tier, obtaining a near perfect polar alignment with the great tools available including the QHY Polemaster, and others is not the optimum solution. This works great in higher end mounts where the quality is higher because the system can handle the switching from North to South direction of the control pulses to maintain tight control over the DEC position. To tell the truth, with higher end mounts with a near perfect polar alignment, I contend that you should not be guiding in DEC at all unless you are pointing lower on the horizon (< 30 degrees altitude) to correct for refractive tracking errors.

In the case of the lower end mounts, allowing PHD2 to correct excursions in DEC by switching between North and South pulses (which you will need to do with a near perfect polar alignment) in the presence of any amount of gear lash makes things worse. A much better solution is to leave a bit of polar alignment offset so that you can control and mitigate any switching of the pulse-guide direction. You also need to bias the equipment load to be "camera heavy" on the DEC axis so you can ensure a positive/constant worm/wheel contact which minimizes the effect of any gear lash in the system. This will generally result in much better performance for lower end mount systems such as the EXOS 2 and iEXOS 100 PMC-Eight systems. 

This has generally been my experience and also the experience of a lot of customers I have spoken to over the years. IMHO, the advent of these new polar alignment tools over the past few years has ironically made the problem worse for astrophotographer's using lower end equipment as I said above. 

I would love to hear everyone's thoughts on this

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)
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