Now for something a little different.....

Taking images of the night sky definitely takes some time to learn how to do. There are lots of problems, besides the clouds, that need to be addressed. One of those is called Periodic Error. Periodic error is the result of inaccurately manufactured gears in the scope drive mechanism. Can the gears be manufactured to not have periodic error? No. Interestingly, the difference between a $500 mount and a $50,000 mount, as concerns periodic error, is that the $50,000 mount has a predictable, repeatable periodic error. Why is that important? Because the predictable periodic error can be mostly programed out. All new, computerized telescopes has PEC, or Periodic Error Correction. If we look at an image, how does this show up? If, in a perfect image, a star shows up as a dot (.), the star image of a mount with periodic error looks like a horizontal line (-). (That's not the only thing that can cause the horizontal line, but we will assume everything else is OK.) Periodic error causes the mount to speed up and slow down, above and below the tracking rate that it needs to be at to perfectly track the star. PEC in essence, records the correcting speed changes and applies them (plays them back) to correct the tracking speed as best it can.

Let's see what this looks like on my mount. (Maybe I should inject at this point that a “telescope” is actually made up of 3 parts; the base, the mount, and the optical tube.The base sits on the ground. The mount sits on top of the base and holds the optical tube. If the optical tube tracks the sky, the mount is the part that does the moving or tracking. The optical tube is the part that you look through to see the stars.)

Periodic error uncorrected

This graph shows the periodic error of my Meade LX200GPS (ca 2002), uncorrected. This is actually 3 “runs” of the gears, or 3 periods of the gear, susperimposed on each other. It's fairly consistently +10 to -22 arc-seconds. That's apparently fairly common for this type of scope and definitely would cause the star to look like a horizontal line. After analyzing and “curve fitting” a correction curve, the graph looks like the right pane of the image below.

Left, before. Right after. Both at same vertical scale

The left pane is the same before graph shown at the same scale as the after graph. Look at the vertical scale, on the far left of the graph: the after is about +2 to – 4 arc-seconds. That's a pretty good improvement; one which, hopefully, can be autoguided out.

By the way, the graph shows something called “RA worm phase.” What this refers to is the gear that moves the scope to track the stars. The gear is called a worm gear. For more information, and to see what one looks like, see : https://en.wikipedia.org/wiki/Worm_drive