Ekos Guide Module enables autoguiding capability using either the powerful built-in ln_guider-based guider, or at your option, external guiding via PHD2. Using the internal guiding, guider CCD frames are captured and sent to Ekos for analysis. Depending on the deviations of the guide star from its lock position, guiding pulses corrections are sent to your mount Via any device that supports ST4 ports. Alternatively, you may send the corrections to your mount directly, if supported by the mount driver. Since capture and transmission of Guider CCD frames from the device to the client (in case of remote setup) can consume quite a bit of bandwidth and may result in delayed frames due to traffic congestion, Rapid Guide is a method to perform guiding analysis on the device itself without the need to transmit any frames back to Ekos, thereby considerably reducing traffic congestion.
To perform guiding, you need to set a Guider CCD in Ekos Setup. The telescope aperture and focal length must be set in the telescope driver. If the Guider CCD is attached to a separate Guide Scope, you must also set the Guider Scope's Focal Length and Aperture. You can set these values under the Options tab of the telescope driver. Autoguiding is a two-step process: Calibration & Guiding
During the two processes, you must set the following:
- Guider: Select the Guider CCD.
- Via: Selects which device receives the autoguiding correction pulses from Ekos. Usually guider CCDs have an ST4 port. If you are using the guider's ST4 to autoguide your telescope, set the guider driver in the Via combo box. The guider CCD will receive the correction pulses from Ekos and will relay them to the mount via the ST4 port. Alternatively, some telescopes support pulse commands and you can select the telescope to be receiver of the Ekos correction pulses.
- Exposure: CCD Exposure in seconds.
- Binning: CCD Binning.
- Filters: Specify filter to be applied to the image to enhance it.
In the calibration phase, you need to capture an image, select a guide star, and click Start to begin the calibration process. If Auto Star is checked, then you are only required to click capture and Ekos will automatically select the best fit guide star in the image and continues the calibration process automatically. If Auto Calibration is disabled, Ekos will try to automatically highlight the best guide star in the field. You need to confirm or change the selection before you can start the calibration process. The calibration options are:
- Square Size: Sets the square size to enclose the guide star. Use a box size slightly larger than the star width. The square is used to calculate the star center of mass so do not use too large or small squares compared to the star size.
- Pulse: The duration of pulses in milliseconds to be sent to the mount.
- Two axis: Check if you want the calibration process calibration in both RA & DEC. If unchecked, the calibration is only performed in RA.
- Auto Star: If checked, Ekos will attempt to select the best guide star in the frame and begins the calibration process automatically.
- Auto Square Size: If checked, Ekos will attempt to determine a suitable square size for the selected auto star.
- Use Dark Frame: If checked, it will automatically capture a dark frame to be utilized during the calibration and guiding processes. Ekos will ask you to cover the telescope before capturing the dark frame since most guider CCDs are not equipped with shutters. Ekos will keep the same dark frame until the exposure value for the guider CCD is updated and it will then prompt you to capture another dark frame.
- Auto Mode: Set the number of iterations the calibration process performs in each axis.
The reticle position is the guide star position selected by you (or by the auto selection) in the captured guider image. You should select a star that is not close to the edge. If the image is not clear, you may select different Filters to enhance it.
Ekos begins the calibration process by sending pulses to move the mount in RA and DEC. The Guiding tab will remain disabled until the calibration process finishes successfully. If the calibration process fails due to short drift, try increasing the pulse duration.
Calibration can fail for a variety of reasons. To improve chances of success, try the following tips:
- Better Polar Alignment: This is critical to the success of any astrophotography session. Perform a quick polar alignment with a polar scope (if available) or by using 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.
- Prefer use ST4 cable between guide-camera and mount over using mount pulse commands.
- Select different filter (e.g. High contrast) and see if that makes a difference to bring down the noise.
- Smaller Square Size.
- Play with DEC Proportional Gain or disable DEC control completely and see the difference.
- Leave algorithm to default value (Smart)
You can immediately start autoguiding by clicking on the Start Autoguide button. The guiding performance is displayed in the Drift Graphics region where Green reflects deviations in RA and blue deviations in DEC. The vertical axis denotes the deviation in arcsecs from the guide star central position and the horizontal axis denotes time.
Ekos can utilize multiple algorithms to determine the center of mass of the guide star. By default, the smart algorithm is suited best for most situation. The fast algorithm is based on HFR calculations. You can try switching guiding algorithms if Ekos cannot keep of the guide star within the guiding square properly.
The info region displays information on the telescope & FOV, in addition to the deviations from the guide star along with the correction pulses sent to the mount.
Ekos supports automatic dithering of the guide star between subsequent exposures. To enable dithering, check the Dither checkbox and specify the dithering distance in pixels. After an exposure is complete, Ekos will move the guide star in a random direction for the amount of pixels specified in the dither input box. The next exposure will be automatically started once the dithering process is complete.
Ekos supports preliminary Beta guiding via Adaptive Optics (AO) units in addition to traditional guiders. If you selected an AO unit in the Ekos setup, you can set the maximum guiding error to correct for with the AO. Set the maximum guiding error in arcseconds (default 2) in the AO Limit input box. If the guiding errors exceed this value, Ekos will automatically guide the mount mechanically to correct for the guiding errors until the guiding errors are within the threshold of the AO unit, in which case it will be used again for correcting guiding deviations. The switch between the AO unit and the traditional guider is completely automatic.
Rapid Guide is a mode where the CCD driver, and not Ekos, determines the center of mass of the guide star. Using Rapid Guide, if supported by your CCD driver, enables very fast measurements of the center of mass of the guide star. This is because all the calculations are done within the CCD driver without sending any images back to Ekos for analysis. The CCD driver reports the guide star data back to Ekos where it analyzes it and makes corrections accordingly. If you are using an Adaptive Optics (AO) unit as your guider, Rapid Guide is especially important since the AO requires very fast changes compared to the regular mechanical mount guiding. Furthermore, if the guide CCD is operated remotely, the upload and processing of FITS images, especially over slow and/or unreliable networks, can adversely affects the performance of the autoguiding process.
Therefore, it is recommended to use Rapid Guide in the following situations:
- Using AO unit as the guider.
- Operating the CCD guide camera remotely over slow or unreliable network.
If the CCD guide camera is connected directly to your PC, and you are using ST4 port to guide your mount, there are no major benefits to enabling Rapid Guide.
You can fine tune the guiding performance in the Control Section. The autoguide process works like a PID controller when sending correction commands to the mount. You can alter the Proportional, Integral, and Derivative gains to improve the guiding performance if necessary. The default guiding rate is 0.5X sidereal, which is equivalent to a proportional gain of 133.33. Correction pulses are sent to both axis (RA & DEC) when necessary, but you can control which access receive correction commands by enabling or disabling the Enable Directions checkboxes for each respective axis.
You can opt to select external PHD2 application to perform guiding instead of the built-in guider. To select PHD2, go to Ekos Options and select PHD2 under Modules tab. The PHD2 hostname (or IP address) and port must be set as well. By default, the hostname is set to localhost and port to 4400.
If PHD2 is selected, the Calibration tab and most of the Guide tab is disabled since most of the functionality is off-loaded to PHD2. From within Ekos, you can start/stop the guide process and optionally set Dithering.
After launching PHD2, select your INDI equipment and set their options. From Ekos, connect to PHD2 by clicking Connect PHD2 button. On startup, Ekos will attempt to automatically connect to PHD2. Once connection is established, you may begin the guiding immediately by click on the Start button. PHD2 shall perform calibration if necessary. If dithering is selected, PHD2 shall be commanded to dither given the offset pixels indicated and once guiding is settled and stable, the capture process in Ekos shall resume.
Ekos saves a CSV guide log data that can be useful for analysis of the mount's performance under ~/.local/share/kstars/guide_log.txt. This log is only available when using the built-in guider.