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«EQMOD VS-PEC Introduction, User Interface & Workflow Document Contents: INTRODUCTION Periodic Error: What is it? EQMOD Variable Speed Periodic Error ...»

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Introduction, User Interface & Workflow Document



Periodic Error: What is it?

EQMOD Variable Speed Periodic Error Correction (VS-PEC)

PEC & Autoguiding



PEC Playback Settings

AutoPEC Settings




PEC from Perecorder/Autoguider Logs


Third Party Autoguiders.

Using PECPrep to Analyse and filter the PE data


APPENDIX B: K3CCDTools Workflow



Q What is Periodic Error?

Q: What is VS-PEC

Q: What are the benefits of using VS-PEC?

Q: How much improvement will I see with VS-PEC?

Q: Do I have to purchase/use third party tools?

Q: Can I use third party tools?

Q: What is the advantage over auto-guiding?

Q: Can ST4 AutoGuiding and VS-PEC be run simultaneously?

Q: Can ASCOM PulseGuiding and VS-PEC be run simultaneously?

Q: Does using PEC lead to improved guiding?

Q: What do I need to record PE?

Q: How long does it take to record PE?

Q: How often would I need to record/process PE?

Q: How do I analyse my PE and create a PEC Curve for EQASCOM?

Q: What’s the difference between a PE Curve and a PEC Curve?

Q: Where is the VS-PEC data held?

Q: Must I always Park the mount?

Q: Will I loose VS-PEC synchronisation If I manually move the RA axis?

Q: I’ve heard that PECPrep is not as reliable as commercially available PEC tools, is this true?


The EQMOD Project

–  –  –


Periodic Error: What is it?

Periodic Error (PE) in a mount is caused by imperfections in the gear train causing a very slight ‘wobble’ about the axis being driven. The most significant errors are caused by the worm to worm-gear transfer and whilst this can be reduced by mechanical improvements it cannot be removed altogether, it is an inherent feature of a worm drive. The EQ6 mount has 180 teeth on its worm gear and at sidereal rate the RA worm will revolve once every 479 seconds advancing the worm gear by one tooth. The EQ5 mount has 135 teeth on its RA axis and a worm that takes 638 seconds to rotate at sidereal rate.

To the observer, periodic error appears as cyclic drift in RA occurring over the period of the worm. This can result in a total movement of up to 50-60 arcseconds on some mounts (30 arcseconds is typical). To a visual observer this does not represent any great problem, to those indulging in long exposure photography however periodic error will cause a blurring of the image as the star field move across the sensor/film during an exposure.

Although manufacturers cannot remove Periodic Error entirely they can machine the gears accurately enough to ensure that the Periodic Error response is smooth and repeatable for each revolution of the worm. If the Periodic Error can be recorded it is therefore possible to compensate for it by moving the mount in an opposite direction to the error. This is what we refer to as Periodic Error Correction or PEC.

EQMOD Variable Speed Periodic Error Correction (VS-PEC)

Before looking at how EQMOD implements VS-PEC lets first consider SYNSCAN/SKYSCAN PEC. The SYNSCAN/SKYSCAN hand controller provides a ‘traditional’ form of PEC that consists of recording the corrective actions taken by an operator who visually attempts to keep a target star centred using the controller slew controls. The hand controller records when, and for how long, the user makes corrections and when PEC is enabled the same corrections are played back for every subsequent worm cycle. Corrections are always made at the same speed it is just the duration and interval that vary. The PEC data is stored within the hand controller itself and is not an inherent feature of the mount stepper controller.

The EQMOD approach to PEC differs from the SYNSCAN approach in two key areas:

1. Recording is automatic and does not rely on the user manually observing errors and initiating corrections.

2. Correction is achieved by instantaneous adjustment of mount’s tracking speed, VS-PEC VS-PEC is an algorithm that varies the RA motor speed with the intention of cancelling any periodic errors found on the worm gear. It does this using a Periodic Error (PE) curve and synchronising it to the current worm position to determine when and by how much to change the speed of the RA motor. This is basically done to make the tracking constant at the sidereal rate minus the Periodic Errors.

–  –  –

The top graph shows a typical PE trace – all high frequency signals have been filtered out to reveal the signal contributed by the mounts RA worm. The bottom graph shows in blue the rate at which the PE changes –so the highest rates correspond to the steepest parts of the PE curve and the lowest rates the flattest part. The magenta trace shows rate corrections that EQMOD sends and you can see that where the rate changes quickly, i.e. at the top and bottom of the PE curve, EQMOD sends many corrections. Where the PE curve is relatively linear, i.e. on the rising and falling slopes of the PE curve, EQMOD sends fewer corrections.

This approach is essentially the same as used in Satellite Tracking where the speed of the motors is varied based on another curve, the ALT/AZ orbital speed and position of the Satellite. The only difference between PEC and Satellite Tracking is that PEC applies the motor speed changes to cancel out the PE curve whilst satellite tracking applies the motor speed changes to follow the target speed curve.

–  –  –

PEC & Autoguiding Traditionally there have been two schools of thought on the subject of using PEC whilst autoguiding. Some believe using PEC reduces workload of the autoguiding system whilst others believe it is better for all corrections to come from a single source and that having two competing control applications, one open loop and the other closed, leads to worse overall performance. The situation is further complicated by the fact that there are two autoguiding approaches – ST4 and pulse-guiding With ST4 guiding the EQMOD driver has no knowledge of how the guiding program is controlling the mount. If VS-PEC is running in this case it will send out its own corrections regardless of whether the autoguider is already making corrections of its own. In such a situation there is probably no advantage in running VS-PEC at all and autoguider performance may suffer.

With ASCOM pulse-guiding the EQMOD ASCOM driver is able to arbitrate between the autoguiding application and VS-PEC. EQMOD ASCOM and has been written to ensure that VSPEC and pulse-guiding complement each other and don't fight. It works as follows:

• When a pulse guide command is received the correction pulse is applied using the current VS-PEC rate not sidereal.

• During the pulse period VS-PEC rate calculations continue but are not applied to the mount.

–  –  –

Using VS-PEC in combination with Pulse-Guiding has been shown to reduce the size and frequency of corrections issued by the guiding application. This we would expect. and doesn’t necessarily deliver any improvement in overall performance. If using a bright enough guide star that allows the autoguider to use short exposures (0.5 seconds or faster) then the autoguider should be able to issue corrections quickly enough without using VS-PEC. Using VS-PEC as well will not do any harm and will simply mean the autoguider doesn’t have to work quite so hard.

Where using VS-PEC and pulse-guiding together becomes advantageous is when using a mount that has a large periodic error (+/- 25 arc seconds) AND when guiding on a fainter star that requires 3 to 5 second exposures. VS-PEC can correct for much of the error before it happens and in between autoguider exposures. This greatly improves the overall guiding performance compared to autoguiding alone. The major advantage of this is that guiding on fainter stars permits many orders of magnitude more guide stars, greatly increasing the ease of locating one.

To use VS-PEC in combination with guiding it is important to start VS-PEC prior to guiding.

The reason for this is as follows:

VS-PEC issues corrections to return the mount to the 'PEC centre' position. The guiding software also attempts to maintain a guide star (and therefore mount) centred. There is no guarantee that guiding target position will correspond to that of PEC, this will depend on the phase the PE at the time PEC/Guiding is started. If VS-PEC is started first it will hold the mount at 'PEC centre' ensuring that the PEC and guiding targets are in sync.

–  –  –


To ensure correct synchronisation of the VS-PEC playback it is important is that the mounts stepper positions always remain in synch with their positions when the PE curve was originally recorded. The only way to do this over mount power cycles is to PARK and UNPARK the mount using EQASCOM.

Provided you always PARK at the end of your sessions, UNPARK at the start of the next, and don't move the worm in between, VS-PEC synchronisation will be retained.

Loosening the clutch and rotating RA axis shouldn't affect the worm or RA gear. The clutch disengages the axis from the RA gear not the RA gear from the worm. Provided you have Parked the mount you can therefore rotate the RA axis manually without loosing VS-PEC synchronisation. You will however loose astronomical synchronisation and unless you can accurately return the mount to its original position any saved alignment data will become void.

–  –  –


The EQASCOM PEC is one of several displays accessed by the “Display +” button. Right click this button to get a menu of all the available displays or just repeatedly click it to cycle to the PEC display.

In order for PEC to be applied a PEC file must first prepared and loaded. Once loaded EQASCOM will remember where to find that file and will automatically load it on future startup.

You can tell if a PEC file has successfully loaded because the correction curve will be plotted and PEC file name displayed.

Once loaded PEC playback can be started or paused via the associated button.

When PEC is active the Sidereal Rate tracking rate button will show a red sine wave superimposed on the white star. The PEC status area will display “Correcting” followed by the currently applied correction rate (in arcsecs/sec).

When Tracking at Sidereal+PEC the Tracking Status will show “Sidereal+PEC” The TimeStamp button is used to associate a particular motor position with a time within the EQMOD.ini file and is provided so that motor position data can be generated for PE files produced by third party PE recording programs (K3CCDTools, PHD etc). This will be discussed in more detail later.

–  –  –

The PEC Setup button provides access to various parameters that govern how PEC Playback and automatic PE capture operate.

PEC Playback Settings There are two “Advanced” controls that can be used to fine-tune the playback, the gain slider and the phase shift slider.

The gain control applies a factor to the rate correction as determined from the PEC algorithm.

If when tracking at Sidereal+PEC, the rate corrections don’t appear great enough to cancel all of the periodic error then try adjusting this control. The gain control can compensate for error in the original PE capture process due to variable such as focal length, pixel size, star declination, camera orientation etc.

The phase shift control allows you to advance (or retard) the curve either to either perform manual synchronisation or to compensate for time lags in the control system. Once applied the PEC curve can be re-saved with the phase shift applied (via the [Save] button) such that on subsequent load a phase shift is no longer needed.

AutoPEC Settings Capture Cycles sets the number of worm cycles of data that is used to build the PEC curve.

Usually the more cycles the better.

The two filter setting govern how EQASCOM will apply an FFT filter to the captured data in order to filter out fast transients (seeing fluctuations).

AutoPEC outputs two files – a PEC file and a PE file that can be loaded into PECPRep for further analysis. The “Timestamp files” checkbox dictates whether these files will be given unique names for each separate capture session. The location where the output files are placed can be changed.

–  –  –

The “Auto Apply PEC” checkbox determines whether the PEC file generated by AutoPEC will be automatically loaded on AutoPEC completion. If, for instance you do not wish to run PEC but do want to record your mounts PE then you would uncheck this box.

WORKFLOW Before EQASCOM can provide Periodic Error Corrections it is first necessary to record the mounts Periodic Error so that a correction curve can be built and stored in a PEC file. This File is then loaded into EQASCOM. There are two basic procedures that can be followed to

achieve this:

AutoPEC – a feature built into EQASCOM itself that automates the entire process.

–  –  –

Capture and process guide logs using PECPrep. This method is more technically • demanding but gives you finer control over the PEC produced.

–  –  –

Preparation As our initial aim is record a PE curve that represents only the error induced by the mount RA gear train it is best to try to minimise error signals such as ‘drift’ cause by alignment, tracking rate errors etc.

Get as accurate a polar alignment as you can. If having done this you target star still drifts in RA, and this appears constant in different areas of the sky, it may be there is a tracking rate error present. In this case adjust the EQMOD drift compensation slider. Each increment/decrement of the slider equates to approx 0.024 arcsecs/sec change in rate.

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