Guide Module

 Introduction

Ekos Guide Module is based on the excellent ln_guider application. 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. 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.
  • Filters: Specify filter to be applied to the image to enhance it.

 Calibration

In the calibration phase, you need to capture an image, select a guide star, and click Start to begin the calibration process. Once an image is captured, 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.
  • Auto Mode: Set the number of iterations the calibration process performs in each axis.
  • Two axis: Check if you want the calibration process calibration in both RA & DEC. If unchecked, the calibration is only performed in RA.
  • 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.

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.

 Guiding

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.

 Dithering

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.

 Adaptive Optics

Ekos supports 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

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.

 Control

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.

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