What is PEM?

PEM is the term used by Astro-Physics for periodic error correction. When it was first introduced, we chose PEM (Permanent Error Memory) rather than PEC (Periodic Error Correction), which was in common use at the time by other manufacturers. At the time, mounts with PEC had to be retrained each observing session - a real drawback. We chose PEM to indicate that our mounts retained the error correction in memory so that the process of correcting it did not have to be repeated each session. Later mounts on the market unitized the acronym PPEC (Permanent Periodic Error Correction). We have chosen to continue using PEM for consistency, although it can be confusing at first. Generally speaking, PEM and the market use of "PEC" are interchangeable terms.

The servo drive electronics contain a very sophisticated permanent periodic error compensation circuit. Because a majority of the periodic error is due to the RA drive worm and is quite predictable, it is possible to significantly reduce it by simply having the controller “memorize” the corrections you make with your button inputs and automatically play them back for each cycle of the worm. This circuit has some unique features that makes it easy to operate and almost totally foolproof.

A most valuable feature of this circuit is that it automatically compensates for any drift inadvertently introduced by the operator in the process of recording the gear error. For instance, if the mount was not properly polar aligned in altitude, the right ascension drift rate would be in error. The circuitry automatically subtracts this slow drift from the memory, so that the overall long-term drive rate remains exactly at the sidereal rate. This way, the compensated driving rate will always be correct for any other position in the sky and for any other time that the mount is set up in the field.

There are several ways to correct the residual periodic error in Astro-Physics mounts, as described below.

PEM Already Programmed at Astro-Physics

The heart of your Astro-Physics mount is a precision gearbox using custom high-quality fine-pitch gears, coupled to a highly accurate worm and worm wheel. These parts are manufactured and assembled at our facility and individually tested to meet or exceed our periodic error standards before adding any periodic error correction. The PE is smoothly variant with extremely low ripple or moment to moment error. As a final step to the mount testing process, we optimize performance even further  using a special version of Ray Gralak's PEMPro developed specifically for our test equipment. We generate a unique PE curve for each mount that we program into the Control Box. Simply use the Keypad or your Astro-Physics software (ASCOM V2 driver or APCC) to turn the PE Correction "on" and you will see results significantly better than our published 5 or 7 arc second spec (depending on the mount).

Use PEMPro by Sirius Imaging to Record PEM

PEMPro (Periodic Error Management Professional), Version 3, by Sirius Imaging is a very powerful Windows software application that allows you to correct your mount’s periodic error, polar alignment and backlash using your astro camera  to dramatically improve tracking and guiding. As mentioned previously, our extensive testing procedures include several PEMPro runs so that your new mount will perform with optimum PE correction out of the box. Although you will not need to use PEMPro when you receive your mount, you may wish to do a PEMPro run at a later date after the worm gear has run in. Depending on usage, re-measuring error and re-loading periodic error correction from one to four times a year is a good practice, especially if you do some unguided images from time to time. The corrections have been stored in the GTOCP3 Control box and can be enabled via the Keypad, the ASCOM driver or APCC in subsequent sessions.

Use APCC to Record PEM

NOTE:  Like the Keypad method below, using APCC to record PEM will use the GTOCP3's servo recording capabilities, and will only record a single worm cycle.  DO NOT replace a good PEMPro correction model with this single cycle model.  Think of this as an emergency tool for getting a quick PE curve into the mount, NOT as an ideal way to program your PEM, or as a way to improve on a PEMPro model!.

Polar align your mount so that you have minimum R.A. and Dec drift. Be sure that the night is steady enough with minimum atmospheric motion, and pick a star near the zenith (refer to the LST value in the Telescope Position group box) to minimize atmospheric refraction errors. You must use a CCD camera or webcam, which will faithfully remember each error correction that it makes. These cameras can respond quickly to changes so they are extremely accurate and will place into memory a very smooth correction of the gear error. The corrections will be stored in the GTOCP3 Control box and can be enabled via the Keypad, the ASCOM driver or APCC in subsequent sessions

1. Select 1x response rate for N-S-E-W buttons on the Rate Settings tab.

2. Go to Setup tab of APCC. Refer to Setup Tab for a discussion regarding the other functions on this tab.

3. Choose PEM Record button and click OK in the confirmation window that will appear. The PEM Record Countdown display box will show the amount of time remaining for the recording. The length of time may vary depending on the mount model. You cannot cancel the recording session once it begins.

4. If you are not satisfied with your results, record again using the same procedure.

Use the Keypad to Record PEM

If you plan to use a crosshair eyepiece, this is the best choice since you can use the keypad as you stand at the eyepiece. Follow instructions in the Tools Menu section of the Keypad Manual. The corrections will be stored in the GTOCP3 Control box and can be enabled via the Keypad, the ASCOM driver or APCC in subsequent sessions.  Like the APCC Recording method above, this should not be used to replace a perfectly good PEMPro model.