Guide

Inleiding

The Ekos Guide Module performs autoguiding using either the powerful built-in guider, or at your option, external guiding via PHD2 or lin_guider. Using the internal guiding, guider camera frames are captured and sent to Ekos for analysis. Depending on the deviations of the stars from their lock positions, guiding pulses corrections are sent to your mount’s RA and DEC axes motors. Most of the GUI options in the Guide Module are well documented so just hover your mouse over an item and a tooltip will popup with helpful information.

Opzet

Om volgen uit te voeren moet u (één keer) een volger in de profielbewerker selecteren voor het profiel dat u wilt gaan gebruiken. In profielbewerker kiest u Intern voor de Ekos interne volger of PHD2.

Om te volgen moet u ook uw optische trein voor volgen instellen. Deze tweede optische trein is bijna altijd verschillend van diegene die u gebruikt met opnemen/uitlijnen/focusseren. Zie de afbeelding hierboven voor een voorbeeld van de configuratie van een optische trein voor volgen. Merk op dat de gekozen telescoop de volg telescoop is, die misschien hetzelfde is als uw hoofdtelescoop als u een schema OAG (off-axis-guiding) of ONAG-volgen gebruikt. De geselecteerde camera is natuurlijk uw volgcamera. Het Volgen via zou uw opstelling moeten zijn, aangenomen dat u volgpulsen direct naar uw opstelling stuurt of de naam van het ST4 apparaat (bijv. uw camera) zou u ST4 volgpulsen gebruiken.

Kijk op de hoofdpagina van de volger die wordt getoond bij de start van deze sectie in de volger. Er zijn vele parameters die ook aangepast kunnen worden, sommige daarvan staan de onderstaande lijst.

• Exposure: On the main guiding page you can adjust the guiding exposure time. After the guide-camera completes the exposure, the guide algorithm computes and sends the guide pulses to the mount, then it waits a user-configurable delay, and then then begins its next exposure.

• Binning: Pixel binning for the guide image. It usually makes sense to bin the pixels 2x2. The algorithms can still find sub-pixel star positions and send proper guide pulses to the mount.

• Box: This only is applicable to guide algorithms other than MultiStar, and MultiStar is the recommended guiding scheme. Size of the box enclosing the guide star. Select a suitable size that is neither too large or too small for the selected star.

• Directions: Typically you want to keep all the directions boxes checked. Unchecking them will disable guiding in those directions. For instance it is possible to disable DEC guiding in the North direction.

• Dark: Check this to enable dark-frame corrections to your guiding image. See below.

• Clear Calibration: Check this to delete your calibration data. See the calibration section below.

• Subframe, AutoStar: These only apply to guide algorithms other than MultiStar, and MultiStar is the recommended guiding scheme.

Kalibratie

Autoguiding is a two-step process: Calibration & Guiding. Calibration is needed for the scheme to understand the camera’s orientation, relative to the RA and DEC axes, and also the effects of guide pulses (e.g. how much a 100ms RA guide pulse will typically move the RA axis). Once it estimates these values, the guider can correct the mount’s position effectively. You can see calibrated values for those parameters in the above image in the “Calibrated Values” section.

Similar to other guiders, we recommend that you carefully calibrate once, and then only re-calibrate when necessary. It is necessary to re-calibrate when the camera is moved (e.g. rotate) relative to the mount. It should not be necessary to calibrate every time you slew the mount. You should calibrate when pointing near the Meridian and along the Celestial Equator (probably just West of it). Guiding (and guide calibration) is problematic near the pole–it probably won’t work. This slide show contains good advice on how to calibrate the Internal Guider and/or PHD2.

De belangrijke opties bij de opties op de calibratiepagina (boven) zijn:

• Pulse Size: should be large enough to move your image a few pixels.

• Re-using Calibration: There are two checkboxes related to keeping your calibration. We recommend checking “Store and re-use guide calibration when possible”, and un-checking “Reset Guide Calibration After Each Mount Slew”.

• Reverse DEC...: It is also important to check or un-check (it is mount dependent) “Reverse DEC on pier-side change when re-using calibration”. To find out the right setting for your mount, you need to successfully calibrate on one pier side, make sure guiding is working well on that side, then switch to the other side. Guide for a minute or two. If DEC runs away, then you probably have the wrong setting for the “Reverse DEC…” checkbox.

• Max Move, Iterations: We recommend you keep iterations large (e.g. 10) and Max Move large (e.g. 20+ pixels). This way you should get a good estimate of the guiding calibration parameters. Calibration should be something you do rarely, so it is best to take a little extra time and get right.

To (re)calibrate, clear your calibration on the main guiding page, and then simply click on the Guide button. Note that if calibration was already completed successfully before, and you didn’t clear the calibration, and you are re-using calibrations, then the autoguiding process will begin immediately, otherwise, it will start the calibration process.

Ekos begint de calibratie met het sturen van pulsen naar de opstelling om die te laten bewegen in RK en dec. Het stuurt pulsen uitwaarts van de RK-as, daarna pulsen terug inwaarts. Daarna verplaatst het een beetje in dec om speling die zou kunnen bestaan te wissen en daarna pulsen uitwaarts en terug in voor dec. Om dit grafisch te bekijken, klik op het sub-tabblad “Calibratieplot” op de hoofdpagina voor volgen.

Mislukkingen van calibratie

Calibratie kan door diverse oorzaken mislukken. Om de kans op succes te vergroten, kunt u de volgende tips proberen:

• Slechte hemelcondities. Als uw hemelconditie niet geweldig is, kan het het niet de moeite zijn om met volgen/calibratie het gevecht aan te gaan.

• Camerafocus volgen.

• Leave algorithm to the default value (SEP MultiStar) in the Guide Option tab.

• Probeer de “Guide-standaard” SEP sterdetectieparameters (in het tabblad Guide-optie) en pas ze, indien nodig, aan.

• Better Polar Alignment: This is critical to the success of any astrophotography session. Use the Ekos Polar Alignment procedure in the Align module.

• Set binning to 2x2: Binning improves SNR and is often very important to the success of the calibration and guiding procedures.

• Maak dark frames om de ruis te verminderen.

Volgen

Once the calibration process is completed successfully, guiding begins automatically. The guiding performance is displayed in the Drift Graphics region where Green reflects deviations in RA and Blue deviations in DEC. The colors of the RA/DE lines can be changed in KStars color scheme in KStars settings dialog. The vertical axis denotes the deviation in arcsecs from the lock position and the horizontal axis denotes time. You can hover over the line to get the exact deviation at this particular point in time. You can also zoom and drag/pan the graph to inspect a specific region of the graph. Another convenient place to examine guiding performance is in the Analyze tab.

There are two types of algorithms used in the internal guider, and you have choices of which variations to use for both. The first is the Star Detection algorithm. The guider captures images of the sky and automatically detects stars in these images. Once this is done, it can determine your mount’s drift in RA and DEC from its original position. The second type of algorithm is the Guiding Algorithm used to compute the RA and DEC guide pulses that should be sent to your mount to correct that drift.

You can find star-detection algorithm choices in the Guide Settings page (above image) at the top of the Other Settings section. By far the most accurate is the (default) SEP MultiStar algorithm. It uses the detected position of many stars (in the above settings, up to 50) to determine its best estimate for the current drift. It is dependent on accurate star detection. Thus, it may be important to adjust star-detection parameters. Start with the default Guide-Default SEP profile, and optionally edit its parameters if you feel stars are not being detected accurately. Pretty much the only reason not to use SEP MultiStar would be if you can’t get your SEP star-detection to perform adequately.

Guiding algorithm choices are made at the top of the settings page. You can choose separate algorithms for RA and DEC. Here are the possibilities:

• Standard: The traditional proportional guide algorithm. It computes a pulse to correct the computed guide drift. The aggressiveness parameter decides what proportion of the error is corrected. Integral gain can be used but is not recommended. Errors smaller than MinError won’t be corrected. Max response limits the largest correction. The hysteresis parameter is not used.

• Hysteresis: Hysteresis is like the standard algorithm but weights in the previous correction according to the hysteresis parameter.

• Linear: This is similar to the PHD2 Lowpass2 algorithm. It computes the error pulses based on a short history of recent errors. This may be applicable to very stable mounts and is similar to the PHD2 LowPass2 algorithm.

• GPG: (RA Only) The GPG algorithm tries to predict periodic error and linear drifts. It uses the aggressiveness, min and max parameters here, and more parameters on the separate GPG tab. It is very similar to the PHD2 PPEC algorithm. For technical details see this paper. There is more detail on GPG below.

A good starting choice is Standard or Hysteresis (with a 0.1 hysteresis parameter). You may want to use GPG for RA, as it is probably the best performing algorithm for many mounts, however it is more complex to set up (see below). Linear is recommended for some high-performance mounts.

Good advice in choosing parameters is available on the internet, e.g. from the above slideshow.

The main parameter choices you have are below. They are applicable to all the guiding algorithms.

• Aggressiveness. This controls how quickly you want the guider to correct the error. Values of 0.5 to 0.7 are usually best (i.e. correcting roughly half the observed error). Unintuitively, it seems that correcting 100% of the error can cause poor performance as the guider may oscillate with overcorrections.

• Min error. This controls the minimum deviation (in arc-seconds) for which a correction will be made. Adjusting this can avoid chasing the seeing.

Ditheren

To enable automatic dithering between frames, make sure to check the Dither checkbox. By default, Ekos should dither (i.e. move) the guiding box by up to 3 pixels after every N frames captured in Ekos Capture Module. The motion duration and direction are randomized. Since the guiding performance can oscillate immediately after dithering, you can set the appropriate Settle duration to wait after dither is complete before resuming the capture process. In rare cases where the dithering process can get stuck in an endless loop, set the appropriate Timeout to abort the process. But even if dithering fails, you can select whether this failure should terminate the autoguiding process or not. Toggle Abort Autoguide on failure to select the desired behavior.

Dithering resulteert niet in een lange wandeling van de originele doelpositie. Ekos onthoudt de originele en huidige doelposities en verplaatst het doel terug naar het oorspronkelijke doel als de positie te ver is weggedreven.

Eén-puls dithering is een interessante snellere optie die een puls naar dither stuurt, maar niet verifieert dat de dither zijn gewenste locatie bereikt. Het is mogelijk dat de dithering voor elke gegeven dither niet is wat is gewenst, maar het gehele effect zou goed zijn.

Non-guide dithering is also supported. This is useful when no guide camera is available or when performing short exposures. In this case, the mount can be commanded to dither in a random direction for up to the pulse specified in the Non-Guide Dither Pulse option.

Drift Graphics

The drift graphics is a very useful tool to monitor the guiding performance. It is a 2D plot of guiding deviations and corrections. By default, only the guiding deviations in RA and DE are displayed. The horizontal axis is the time in seconds since the autoguiding process was started while the vertical axis plots the guiding drift/deviation in arcsecs for each axis. Guiding corrections (pulses) can also be plotted in the same graph and you can enable them by checking the Corr checkbox below each Axis. The corrections are plotted as shaded areas in the background with the same color as that of the axis.

U kunt in de plot zoomen en zwenken, en wanneer u de muisaanwijzer boven een grafiek houdt ziet u een tekstballon met informatie over dit specifieke tijdstip.Die bevat de volgdrift, en de gemaakte correcties, naast de lokale tijd waarop deze gebeurtenis werd genoteerd. Met een verticale schuifbalk rechts in de afbeelding kunt u de hoogte van de secundaire Y-as voor pulscorrecties aanpassen.

The Trace horizontal slider at the bottom can be used to scroll through the guide history. Alternatively, you can click the Max checkbox to lock the graph onto the latest point so that the drift graphics autoscrolls. The buttons to the right of the slider are used for autoscaling the graphs, exporting the guide data to a CSV file, clearing all the guide data, and for scaling the target in the Drift Plot. Furthermore, the guide graph includes a label to indicate when a dither occurred so the user knows guiding was not bad at those points.

The colors of each axis can be customized in KStars Settings color scheme.

Driftplot

A bulls-eye scatter plot can be used to gauge the accuracy of the overall guiding performance. It is composed of three concentric rings of varying radii with the central green ring having a default radius of 2 arcsecs. The last RMS value is plotted as with its color reflecting which concentric ring it falls within. You can change the radius of the innermost green circle by adjusting the drift plot accuracy.

Volgen met meerdere sterren

In standard guiding the system selects a guiding star. In non-MultiStar systems, the measured movements of that star relative to its original positional measurements are converted to RA and DEC offsets which are the guiding drift errors. In MultiStar guiding the system selects many reference stars and measures all their offsets relative to their initial positions. The guiding error is computed as the median displacement of the individual reference stars from their original positions. The magic the system needs to perform is to find this noisy 2-dimensional pattern of reference stars in the guide image, but finding this pattern is more robust than finding a single guide star that may have moved significantly or may not have been detected at all. We recommended you choose this way to guide by selecting the guide Algorithm SEP Multi Star.

There are a few options you may wish to consider. Max MultiStar Ref Stars is the maximum number of reference stars the system can use. The main reason to limit this is computation cost, thought it is not a very expensive computation. 50 is a good choice. The setting Min MultiStar Star Detections tells the system to fallback to a single guide star if there are fewer than that many star detections. Invent Multi-Star Guide Star should be left checked, and Max MultiStar HFR is an old parameter that likely has little effect anymore.

Guiding with GPG

Met GPG-volgen gebruikt de interne volger voorspelbaar en adaptief volgen voor de RF-as. Deze adaptief modelleert de periodieke fout van de opstelling en voegt zijn voorspelde bijdrage toe aan elke volgpuls.

The main settings to consider are Major Period and Estimate Period. If you know the worm period for your mount, perhaps by examining this table, then uncheck Estimate Period and enter your known Major Period. If not, then check Estimate Period. Intra-frame dark guiding can be used to “spread out the GPG prediction. For instance, if you guide at 5s, you can set the dark guiding interval to 1s and its prediction is pulsed every second, but the guiding drift correction would be sent every 5s. In this way, it outputs the predicted corrections much faster than the guide camera exposure rate, effectively performing periodic error correction and allowing longer guide camera exposures. All the other parameters are best left to defaults.

Dark Frames

Dark frames can be helpful to reduce noise in your guide frames. If you choose to use this option, then it is recommended that you take dark frames before you begin your calibration or guiding procedure. To take a dark frame, check the Dark checkbox and then click Capture. For the first time this is performed, Ekos will ask you about your camera shutter. If your camera does not have a shutter, then Ekos will warn you anytime you take a dark frame to cover your camera/telescope before proceeding with the capture. On the other hand, if the camera already includes a shutter, then Ekos will directly proceed with taking the dark frame. All dark frames are automatically saved to Ekos Dark Frame Library. By default, the Dark Library keeps reusing dark frames for 30 days after which it will capture new dark frames. This value is configurable and can be adjusted in Ekos settings in the KStars settings dialog.

Het wordt aanbevolen dark frames op te nemen voor diverse binnings en belichtingstijden, zodat die door Ekos transparant kunnen worden gebruikt wanneer dat nodig is.

PHD2 ondersteuning

Indien gewenst, kunt u de externe PHD2-toepassing selecteren voor het volgen, in plaats van de ingebouwde volger.

If PHD2 is selected, the Connect and Disconnect buttons are enabled to allow you to establish a connection with the PHD2 server. You can control PHD2 exposure and DEC guide settings. When clicking Guide, PHD2 should perform all the required actions to start the guiding process. PHD2 must be started and configured before Ekos.

After launching PHD2, select your INDI equipment and set their options. From Ekos, connect to PHD2 by clicking the Connect button. On startup, Ekos will attempt to automatically connect to PHD2. Once the connection is established, you may begin the guiding immediately by click on the Guide button. PHD2 performs calibration if necessary. If dithering is selected, PHD2 is commanded to dither given the offset pixels indicated, and once guiding is settled and stable, the capture process in Ekos resumes.

Log van volgen

Ekos’ internal guider saves a CSV guide log in PHD2 format data that can be useful for analysis of the mount’s performance. In Linux this is stored under ~/.local/share/kstars/guidelogs/. This log is only available when using Ekos’ internal guider. It should be compatible with PHD2’s guide log viewer.