I have introduced an update to the Ekos polar-alignment (PA) tool found in the Ekos Align Tab. There are some User Interface (UI) changes and also some new functionality.
This is in the 3.6.0 beta code and (soon) in the nightly releases. If you see the screenshots below, including the LED line below the image display, you have the new software. Please feedback and issues or suggestions you might have.
(Note that this is post 1 of 2, as the forum software doesn't allow all the attachments I'd like to include in this post).
Summary of Changes
The original polar-alignment error measurement scheme has not changed. The user interface has changed slightly--different messages and a new LED display to indicate progress.
The original polar-alignment refresh/correction method still exists (if you choose the "MoveStar" or "MoveStar & Cal Err" refresh methods).
A new polar-alignment refresh/correction method is provided called "PlateSolve". It allows for corrections of larger polar-misalignment in a single pass, does not depend on the image display, and may be more reliable if your plate solving is working well.
***Detail Below***
Background
There are two distinct phases to the polar-alignment process.
- The first phase is the Measurement phase. In this phase, PA captures 3 images, and your telescope is rotated along the RA axis between the captures. These three captures are used to estimate the mount's RA axis of rotation, and, given the geographical location of the telescope, the polar-alignment error.
- In the second phase, the Refresh phase, the user (you) adjusts the mount's altitude and azimuth knobs to correct (reduce) the polar alignment error, following instructions from the PA process. (Ekos typically cannot control the altitude and azimuth adjustment directly, or else this could be fully automated). The PA process regularly captures images, and using these images, estimates the new/adjusted RA axis, and thus the remaining polar-alignment error after the corrections the user has made to the altitude and azimuth knobs.
Usage Instructions
The Measurement Phase.
Point the telescope. At the start, point your telescope at an open patch of sky where your mount can rotate in RA 30-60 degrees and still be pointing clear sky, and not crash into the tripod. Typically, one points at or near the North or South Celestial Poles. However if the pole is obscured, one may point near the meridian (e.g. just west of the meridian and slew to the west, or just east of the meridian and slew to the east). In fact, any part of the sky could work.
Select the Measurement Phase parameters. Select the slew direction, the slew speed, and click Start. The process will start, and if all goes well capture and solve 3 images, with mount slews between them, and, when done, report a polar-alignment error. During this time LEDs, text message, and the log window help communicate the system's progress. See the screenshots below.
The user has not started the polar-alignment process yet. The telescope should be pointed
at an appropriate place in the sky (preferably but not necessarily near the Pole). The direction of rotation and slew speed should be selected, and the Start should be clicked.
The system is in its measurement phase. The second image is being captured.
The Refresh Phase.
Once the measurement phase is complete, the Refresh phase begins.
Select the Refresh Phase parameters. There are two main parameters to choose. The image exposure time, and the refresh algorithm.
- Choose the exposure time such that images with clear stars are captured, and that your computer is fast enough to process the images in a similar amount of time. 2s is often a good choice.
- There are two main algorithms, the traditional MoveStar algorithm, and the new PlateSolve algorithm.
The MoveStar Method. The existing MoveStar algorithm can work with no further computer smarts. You click on a star in the image, and the system overlays a colored triangle on the image so that the vertex between the yellow and purple sides touches the star. You then adjust the altitude knob to "move the star" along the yellow side of the triangle. Doing this should correct the altitude polar-alignment error in your mount's RA axis. Then adjust the azimuth knob to "move the star" along the green side, until the star finally reaches its target. At this point, your mount should be polar aligned.
A related algorithm called
MoveStar & Calc Error is similar. In addition to the above, the system assists you by circling the star it believes you are tracking, and by letting you know its estimate of the remaining polar-alignment error. These extra features are nice, but are not necessary to achieve polar alignment.
The advantage of the MoveStar approaches is that they are simple, potentially fast, and don't necessarily need computer smarts (you can track/move the star without them). The disadvantage is that it cannot correct errors larger than the image field-of-view. It can point you in the right direction, but you need to iterate, and finally have an error less than your field of view before you can eliminate your polar-alignment error.
MoveStar & Calc Err algorithm is selected with 2s refresh.
Error directions and magnitudes are displayed.
User has zoomed in, and clicked on a star. Triangle moves to the star.
User has clicked Refresh. User's star is circled.
PA error is estimated, but as user hasn't adjusted knobs yet, it's the full error.
Star has been moved along the yellow line by adjusting the altitude knob, and now along the green line to near the target. The process is almost done.