TheSkyX PEC Tutorial


Important:  As of June 18 2021 daily build 13096 has a train PEC button for more automatic PEC training.  The text below is still applicable if using any build before 13096.  Couple features that are different from the old way (described in the text below) is you now have to calibrate the guider and any camera angle can be used.  Before you had to have a camera position angle of 0° or 180°.

Info for Train PEC can be found here.  With this update you use the main camera to collect data.  You also must calibrate the main camera. Be sure if you calibrate west  (recommended) you also collect data west.  Do not calibrate east and flip west, this may result in the data being uploaded inverted.  Also note you must calibrate again if an hour has past, this is new with build 13339.

Note that with builds 12966 and later win 7 is no longer supported


Note: these instructions were pulled from the Paramount user guide Revision 2.15 with tips added to aid in PE data collection. 

Before starting good practice is making sure TSXPro is up to date and that you have the latest version, Win, Mac (mac is 64 bit only now).

Note you must be logged in and have a current subscription to DL from the Bisque site.


Using the main camera is recommended.  To guide from the camera tab you will have to enable a couple of advanced features.  “Advanced Camera User Interface” and “Separate interface for autoguider’s devices”.   This is found under Preferences – Advanced – Camera, tick the boxes and restart TheSkyX.  This gives you the autoguide tab on the main imaging camera tab. 

Note if you use the main imager from the camera tab the tracking log will be and not

Follow the steps below to obtain and apply the optimal periodic error correction curve for your mount.

Step 1  Collect the Tracking Log

The first step is to obtain a tracking log for your Paramount.  A tracking log is a record of the position of a star’s centroid over four or more revolutions of the worm, without making any tracking corrections to the mount’s position. (Technically, a tracking log is merely an autoguider log without any guider corrections.)

In order to successfully acquire a tracking log, you must place a star on the detector, and that star must stay inside a “tracking box” for ten to fifteen minutes without making any corrections to the mount’s position (for several periods of the worm) so that the position of the star during that time can be recorded.

When choosing a star, the best declination is 0°, where the periodic error will cause the largest effect in sky angle, but anywhere within ±26°will be at least 90% as good. Somewhere close to the meridian gives the highest altitude and hence the best seeing.

The Paramount, TheSkyX Professional Edition and the Camera Add On must be configured to:

  • Turn off periodic error corrections. (page 144 of the Paramount user guide)
  • Turn off or disable autoguiding.
  • Turn off Tpoint Add on modeling. (see tip below)
  • Turn off tracking corrections from Protrack. (See tip below)
  • Set the mount to track at the sidereal tracking rate. (page 124 of the Paramount user guide ).
  • Remove, turn off or disable adaptive optics devices.
  • Unplug all guider cables from the mount.
  • Unplug the mount’s joystick or hand paddle.
  • Physically rotate the CCD camera to orient it north up. (see tip below)


TPoint and ProTrack can be used (enabled) but for a new user it creates another level of complexity though if you have any drift having TPoint and ProTrack enabled does help (this assumes you have a good TPoint model).


Use image link to determine the position angle and if you have a mirrored image. 0° or 180° is required for PE data collection and knowing if you have a mirrored image will help with the West check box later on.

Remember, the tracking log cannot be acquired when guider corrections are made; instead the star’s uncorrected position must be recorded over time as the mount tracks at the sidereal rate.  Collecting tracking data is typically less demanding than autoguiding since mount calibration is not required.

The resulting tracking log file (named “autoguider.log” since this log is normally used to log autoguiding data) allows TheSkyX Professional Edition to determine the best fit periodic error curve for your mount.


Its best to use your main imager to collect the data.  Just use the Autoguider tab and select your main imaging camera as the auto-guider.


Make sure you have “log autoguiding” checked, then click “Clear log” and last click on the “AutoSave” button to be sure of your path where the log will be saved.

On the Paramount

1. Unplug the autoguider cable from the mount’s Electronics box (Paramount ME II, Paramount MX, and Paramount MX+), Adaptor Panel and Instrument Panel (Paramount ME).

2. Unplug the joystick from the Adaptor Panel (Paramount ME) or the hand paddle from the Electronics Box (Paramount ME II, Paramount MX, Paramount MX+ and Paramount MyT).

3. Make sure your camera is rotated approximately north “up”.


Image link to verify 0° or 180°
Don’t worry how the camera looks on the scope, image link will tell you the correct position angle. Adjust accordingly.

Make sure the CCD camera is oriented correctly. The camera’s detector must be oriented so that telescope motion in right ascension closely corresponds to x-axis motion on the detector (see diagram below). If the detector is not oriented properly, then the maximum magnitude of the periodic error is not recorded. Note that precise orientation is not crucial; the detector should be within 5 degrees of North (or South).

From TheSkyX Professional Edition (make sure you have the latest version, Win, Mac).

  1. Choose the Connect command from the Telescope menu to connect to the Paramount.
  2. Turn off the Apply Pointing Corrections checkbox on the Setup tab of the TPoint Add On window. (Click Telescope > TPoint Add On to show the TPoint Add On window (see tip below).
  3. Turn off the Activate ProTrack checkbox on the ProTrack tab of the TPoint Add On window (see tip below).
  4. Open the Bisque TCS window (see page 125 of the Paramount user guide).
  5. Click the Periodic Error Correction tab.
  6. Turn off the Apply PEC Corrections checkbox.
  7. Click OK.


TPoint and ProTrack can be used (enabled) but for a new user it creates another level of complexity though if you have any drift having TPoint and ProTrack enabled does help (this assumes you have a good TPoint model).

Note that you can use your autoguider or imaging camera to create the tracking log. We recommend using the larger imaging camera to make the process easier.

  • Click the Telescope Setup command from the Telescope window and expand Autoguider.
  • Double-click Camera and select your main imaging camera imager for the detector that will collect the tracking log data.
  • DISABLE AUTOGUIDING CORRECTIONS IN BOTH AXES. Click the Autoguider tab. (Click the Autoguider command from the Display menu if this window is not visible.)
  • On the Autoguide tab, click the Setup… button.
  • On the Relay Settings tab, turn off both Yes check boxes for Relay Enabled in the X-Axis and Y-Axis columns (See image below). Turning these options off prevents the Camera Add On from making corrections to the position of the guide star.
  • Click OK.
  • On the Autoguide tab, click the Clear Log button to delete the autoguider log file. Click Yes to confirm deletion of the file. (You will not be prompted when no log file exists).
  • Turn on the Log Autoguiding checkbox. Autoguiding logs are saved to a folder that is named using the current date (for example, January 01 2018) in the Autoguider (or Imager folder, depending on which camera was used to collect the tracking log) folder of the CameraAutoSave the operating system-specific Application Support Files folder. See TheSkyX Professional Edition User Guide for details about the location of this folder on your operating system.
  • Locate an appropriate guide star and then click the Autoguide button to begin collecting tracking log data.

Periodic Error Tracking Data Collection Recommendations

  • The guide star should be a relative bright star on the west side of the meridian, near 0 degrees declination and near the meridian (within a few degrees).
  • Track the star for at least four periods of the worm; 10 minutes using a Paramount ME or Paramount ME II, about 15 minutes 30 seconds using the Paramount MX and Paramount MX+ and about 20 minutes for the Paramount MYT.
  • Make sure to compute and record the image scale for your imaging system. It is required for the next step.

Step 2 Generate the PEC Curve

  • From TheSkyX Professional Edition, connect to the Paramount by clicking the Connect command on the Telescope menu.
  • Open the Bisque TCS window (see page 125 of the Paramount user guide).
  • Click the Periodic Error Correction tab.
  • Click the Open Tracking Log button.
  • Select the tracking log file and click Open. The “raw” periodic error data is shown in the image below (make sure ‘Raw tracking data’ is ticked).  Your results will vary, but the noisy-looking data is normal.  For this example, each pixel of motion recorded in the tracking log equates to one arcsecond of error.  The maximum and minimum error values are displayed on the right side of the graph (these values are not easily visible in the image below).
  • Enter the image scale of your optical system in arcseconds per pixel. If you do not know the scale, use TheSkyX Professional Edition’s Image Link to determine this value.
  • Turn on the PEC Data Was Collected When Telescope Was Pointing West checkbox (see tip below) if the tracking log data was collected using a star on the west of the meridian. This information is needed to determine the phase of the error. Turning this checkbox on when the mount was pointing to the east while recording the tracking log results in a “doubling” of the periodic error. So, if the periodic error is higher after applying PEC, chances are this is the cause. Simply turn off the check box, and proceed with the next step.
  • Click the Fit button. The PEC Table graph is updated with the optimal periodic error curve for your mount


The West check box, this part can be tricky.  This assumes the scope was set up pointed East with the camera position angle (PA) at 0°.  If you go to the West side now the camera PA is 180° (mount flipped), check the box.

If you start out in the East with the camera at 180° and go West, now the camera PA is 0° (after the flip) no check in the West box.

Easiest way to remember this is if the camera PA is 0° do not check the West box and if the PA is 180° check the West box,  regardless of pier side. 

Also be aware if your image detector produces mirror images this can affect the results of the check box as well.  If image link reports mirror image = Yes then at 0° you would check the box.

Bottom line is you have a 50 50 chance of getting this tricky part right.  If your error doubles just do the opposite (check or un check) and re apply (save to mount) the same log.

Click the Periodic Error Curve for Bisque TCS checkbox (Blue arrow below) to display the graph of the curve that will be transferred to the Paramount control system’s PEC table.


The above PEC curve is not “smooth” by design.  The graph shows the minimum resolution for mount “moves” (approximately 0.01 arcseconds) when applying periodic error corrections.  This data is saved to the control system’s Flash RAM, and will always be used to correct the mount’s periodic error (provided the Apply PEC Corrections checkbox is turned on).

Step 3       Save the PEC Curve to the Paramount Firmware Bisque TCS PEC Table)

Click the Save to mount button to save this curve into the Paramount’s PEC table (that is, the curve is saved in the control system’s flash RAM for permanent use).


Step 4      Apply PEC Corrections

  • From TheSkyX Professional Edition, connect to the Paramount by clicking the Connect command on the Telescope menu.
  • Open the Bisque TCS window (see page 125).
  • Click the Periodic Error Correction tab.
  • Turn on the Apply PEC Corrections checkbox.

Your Paramount will now use the optimal PEC curve for the errors in your worm/gear.

Not Done Yet

Now that you’ve generated the PE log, seen the raw data, fit the data and saved it to the mount it’s time to check your work.

To do this go ahead and tick the apply PEC box and track a star (as above, with guide corrections off) again to generate a log file.  Repeating the above tutorial, check the raw data and fit the data, this is your corrected PE. 

Don’t save this to the mount as its already there!

Now that you’re done training the PE to be as low as possible a common question is this;  What if I need to re balance or move the mount for some reason, will I need to retrain PEC?

No you wont and the reason is as simple as the home sensor on the Paramounts.

This is one of the purposes of the homing sensor.  1 arc second or less repeat-ability from night to night on the gear positions.  If you have to move the mount by putting it in balance mode to re balance or any other reason, be sure and re-home the mount before you start imaging to re-establish the encoders (not to be confused with “absolute encoders”).  That’s all there is to it, home and go.