Identifying Peaks in the PECPrep Periodicity Plot In the PHD2 Guide Log - Somewhat #ExploreScientific #iEXOS-100 #EXOS2 #TECHNICAL

Jerry Hubbell - Explore Scientific VP Engineering

I would have posted this in the MOUNTS subgroup, but only a little over half of the members (158 out of 271) have joined the MOUNTS subgroup. If you haven't joined the MOUNTS subgroup, please do.

I wanted to talk about the fundamental periodic errors that will show up when using PECPrep to look at your PHD2 Guide Logs. These Periodic Errors (PE) show up based on the mechanical operation of your mount and are influenced by how your gear train and belt drive system is setup mechanically in terms of mesh, belt tension, and eccentricity. The rate that the system moves is a fundamental factor in determining what peaks will show up in your periodicity plot. First some basic information about the mechanical portion of your mount, this applies to both the EXOS 2 and the iEXOS 100 as they have identical motor/belt drive systems.

The main gear has 144 teeth which turns once every 24 hours or 1440 minutes. The worm needs to turn at a rate that will go through all the teeth during that time so the worm rate is equal to 1440 minutes / 144 teeth, or 10 minutes per tooth, or 10 minutes per worm revolution. So this is the worm fundamental period - 10 minutes or 600 seconds. We will be using seconds instead of minutes in this discussion. Typically all mounts have a certain amount of PE caused by runout or other issues with the worm, and it will show up as a period of about 600 seconds. we are concerned with the worm period PE because this is the longest PE that will impact our imaging and how well we can guide. The wheel with a fundamental period of 24 hours may have PE because of runout or other issues, but it is such a slow change in rate that it does not impact our  imaging.

The motor belt drive system is the next source of periodic error that you will find in your periodicity plot. The belt drive system on the EXOS 2 and iEXOS 100 mounts reduce the rotation rate of the motor down to the worm and multiplies the torque provided to the worm also. The pulley ration is 4.5 to 1. This means that for every turn of the worm (in 600 seconds) the motor turns 4.5 times. So if the rotation period of the worm is 600 seconds the rotation period of the motor is 600 / 4.5 or 133.3 seconds. This is the next period that you may find a peak for - 133.3 seconds. This PE is caused by the small pulley having some amount of runout where the pulley seems to "wobble" a tiny amount on the shaft. This can be caused by a couple of things that you should check. First the pulley may indeed by tightened on the shaft not quite straight. The hole in the pulley should be concentric with the shaft and should be very tight when slipped onto the shaft. Any variation in this will cause some PE at the 133.3 second period. The second thing that may cause a small amount of PE at the 133.3 second period is the belt may be tensioned too tight. This would have a tendency to pull the motor shaft and pulley to the side inducing a wobble in to the pulley.

Another periodic error that you may find in your guide log periodicity plot is related to the belt/pulley toothed belt. Each of the pulleys on the drive system has teeth and uses a toothed belt. The large pulley has 45 teeth and the small pulley has 10 teeth. The PE comes about because for every rotation of the small and large pulley, there are a given number of times that the teeth on the belt are engaged into the pulley "gaps" and disengaged. This happens continuously. Every time a tooth is transitioned into the gap on the pulley, it may encounter some resistance depending on the amount of belt tension. If the belt is tensioned correctly, then the amount of resistance is minimized but probably not eliminated. The periodicity plot will show a peak at the tooth engagement period of the revolution time of the pulley divided by the number of teeth. So for the 45 tooth and the 10 tooth pulley this period is the same. It is equal to 133.3 / 10 seconds or 13.33 seconds. If your belt tension is too tight, you will probably have a significant amount of PE at this period.

So those are the fundamental PE errors you will find in your periodicity diagram.  I hope this 

Here is is example plot for the iEXOS 100 mount
This first plot shows the 3 periods discussed above This shows that the belt tension is too tight but the pulleys are very well centered on the shaft as the 133.3 period shows a smaller  PE. I think the PE at the 133.3 second period in this example is caused by the belt tension being too much.

Here is another example plot showing the iEXOS 100 mount with a reasonable belt tension and some residual PE at the 133.3 second small pulley fundamental showing a tiny bit of "wobble" at the small pulley. I am not sure what is causing the peak at around 800 seconds which interestingly enough is equal to 1.333 x 600 seconds. Any thoughts?

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
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