Q What is Periodic Error?
A: Periodic Error (PE) is movement in a mount caused by imperfections in the gear train resulting in a very slight 'wobble' about the axis being driven. The most significant contribution to PE is the worm to worm gear transfer. PE can be reduced by mechanical improvements but it cannot be removed altogether, it is an inherent feature of a worm drive.
Q: What is VS-PEC
A: VS-PEC is Variable Speed Periodic Error Correction. This is a technique developed for the EQMOD project that varies the mount tracking speed to compensate for Periodic Error. This differs from PEC as applied by the SYNSCAN hand controller which uses constant speed corrections of varying
Q: What are the benefits of using VS-PEC?
A: VS-PEC basically provides an improvement in tracking performance. Typically this is an advantage when using the mount for astrophotography.
Q: How much improvement will I see with VS-PEC?
A: Its difficult to give absolute figures because every mount differs in its periodic error. PEC is most effective if the mount periodic error varies smoothly and is consistent in amplitude over many cycles. If the periodic error appears erratic then you should first consider mechanical adjustments.
Q: What is the advantage over auto-guiding?
A: A lot depends on the image scale you chose to image at and the magnitude of Periodic Error that your mount exhibits. For instance with VS-PEC it may be possible to reduce the periodic error movement of the mount to a level where for a given exposure time the movement experienced is smaller than the resolution of your imaging system. In this case VS-PEC has an advantage of not requiring a separate autoguider thereby saving weight, less processing
load, less cables etc.
Q: Can ST4 AutoGuiding and VS-PEC be run simultaneously?
A: It is not recommended to use VS-PEC in combination with ST4 guiding it is likely that this will lead to overcorrection. No tests have been performed to verify this, so if you do try it let us know your results.
Q: Can ASCOM PulseGuiding and VS-PEC be run simultaneously?
A: Yes. EQMOD VS-PEC is designed to work alongside pulse guiding and can actually improve guiding performance when tracking on fainter guide stars.
EQMOD VS-PEC: Q&A (continued)
Q: What do I need to record PE?
A: A star, an imaging device and a capture program. The simplest approach is to use the EQMOD perecorder application and a Webcam.
Q: What is pereccorder and why should I use it?
A: perecorder is an application written specifically for the capture of PE data for use by EQASCOM's VS-PEC. It provides automatic calibration to automatically determine the resolution of your imaging system and integrates closely with EQASCOM to record not only the Periodic Error, but also the associated stepper motor position. Perecorder is the most accurate means of capturing PE
data for EQASCOM.
Q: What imaging devices does perecorder support?
A: Currently perecorder only supports webcams.
Q: I don't own a webcam, how can I record my PE?
A: If K3CCDTools or PHD supports your imaging device then there is a way to import log file data into PECPrep. This process is slightly more involved than using perecorder.
Q: How long does it take to record PE?
A: For an EQ6 type mount you really need to record at least 40 minutes of data. For an EQ5 mount 55 minutes of data. With brighter stars you can record PE during dusk or periods if during poor seeing when other observation is impractical (the smoothing process is able to remove seeing and wind disturbances in the recorded signal).
Q: How often would I need to record/process PE?
A: You might need to re-record your PE if you change the loading of the mount, or if you fail to park the mount (there are means to resynchronise manually if this happens but re-recording may be the simpler option).
Q: How do I analyse my PE and create a PEC Curve for EQASCOM?
A: Use PECPrep. PECPrep has been written specifically for this purpose and contains a number of analysis and manipulation tools.
Q: What's the difference between a PE Curve and a PEC Curve?
A: There isn't a big difference but the PE Curve is the raw response of your mount, whilst the PEC curve is the data formatted for EQASCOM to apply corrections to the mount.
Q: Where is the VS-PEC data held?
A: VS-PEC data is stored in a text file on your PC and loaded by the EQASCOM driver. EQASCOM uses the file to send rate change commands to the mount. Whilst the mount may continue to track at sidereal rate when EQASCOM has been disconnected, it cannot track using VS-PEC.