Quantifying the Paramount ME's Corrected Periodic Error

PrecisionPEC is a very powerful tool that can be used to quantify the Paramount ME's uncorrected periodic error, and generate the optimal periodic error correction curve for your mount.

 

However, after programming the Paramount ME's PEC table using PrecisionPEC's computed periodic error curve, a very common question arises:

 

"How do I measure (quantify) the Paramount ME's corrected periodic error?"

 

In other words, how can I demonstrate that PrecisionPEC actually computes the optimal PEC curve, and that the Paramount ME's periodic error correction algorithm is performing optimally?

 

There are several techniques that can be used to show the Paramount ME's corrected periodic error. Some are outlined below.

 

Valid Technique

Detailed Explanation

Generate a tracking log with PEC enabled and use PrecisionPEC to analyze the results.

By now, you should be familiar with the process of collecting a tracking log with CCDSoft, loading the resulting file into PrecisionPEC, and then using the Fit command to determine the magnitude of the mount's uncorrected periodic error.

 

The identical process can be used to show the corrected periodic error with PrecisionPEC.

 

Here's how.

 

  1. Program the Paramount ME's PEC table using the PEC curve computed by PrecisionPEC. Click here for details.

  2. From TheSky6 Professional Edition, click Telescope | Link | Establish and home the mount if necessary.

  3. From TheSky6 Professional Edition, enable periodic error corrections by clicking Telescope | Options | PEC | Enable.

  4. Again, following the procedure described here, collect 10 minutes of tracking log data, with PEC enabled. The resulting tracking log will record the mount's "tracking errors" when PEC is enabled.

  5. From PrecisionPEC, click File | Open to open the new tracking log, and then click Data | Fit.

 

PrecisionPEC will attempt to fit a periodic error curve using this data. Remember, this tracking data should be mostly noise, since the periodic error will be removed.

 

See Interpreting PrecisionPEC's Output for details about how to adjust the scale of the graph to quantify the corrected periodic error.

 

In general, the corrected periodic error should be less than one arcsecond.

 

  • Do not use the "PEC enabled" PEC curve program the Paramount ME!

 

Intentionally move the Paramount ME's polar axis away from the pole, then acquire a single CCD image that shows the mount's periodic error with and without PEC enabled.

Polar mis-alignment introduces drift in declination (only), and this drift can be used to acquire an image that shows the PEC enabled vs. PEC disabled results. Using the scale of your image, you can measure the magnitude of the periodic error.

 

  1. Adjust the mount's polar alignment so that the polar axis points about 1 degree (or so) off the celestial pole.

 

For this demonstration,

 

  • Paramount ME was intentionally moved off the pole by 1.5 degrees in azimuth (shifted too far to the east).

  • The period of the worm of the Paramount ME is 2.5 minutes (150 seconds).

  • The image was acquired with an SBIG ST-8 camera (9 micron pixels).

  • The optical system was a Celestron C-14 with 110 inches of focal length.

  • The following is an 8 minute exposure with a scale of approximately 1 arcsecond/pixel. 

  • The star trail shows several periods of the worm (3.2 periods). Note that the star trail is somewhat wide because of poor seeing in the foothills of Colorado.

 

Uncorrected Periodic Error


 

The above image shows the resulting star trail during an 8 minute exposure. A  magnified view showing the mount's uncorrected periodic error is below. 

 


 

Corrected Periodic Error


 

The above image represents 16 minutes of exposure.  8 minutes without periodic error correction, followed by a 15 arcsecond jog and then another 8 minutes with PEC enabled.

 

Magnified View

 


 

Note the difference in the star trail before (top trail) and after (bottom trail) PEC is enabled. The magnitude of the corrected periodic error is "in the noise". That is, the effects of seeing (random errors caused by atmospheric turbulence) are larger than the mount's periodic error.

 

The technique demonstrates, without a doubt, that the mount's periodic error correction is performing optimally.

 

 

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