Saturday, October 12, 2013

Accurate GOTO with plate solving alignment module in KStars

Any amateur astronomer must have experienced the woes of aligning their mount. First, your mount has to be aligned with the polar axis. Second, you need to perform an alignment procedure to enable the built-in GOTO firmware to slew and track your objects of choice. Often we are offered to align the mount with 2 or 3 bright stars spread all over the sky which then enables the firmware to build a simple model for the mount errors that it has to correct for when it slews to a target. Also, at this point, the firmware knows where the telescope is looking at in the sky, or so it thinks as we find out below.

While the simple 2 or 3 star alignment is often sufficient for visual observation, it becomes a source of frustration in astrophotography. After slewing to your target, you often have to perform framing to center the actual object within the CCD desired field of view.  Once that's done, your mount is now tracking the object, and you can begin to take your photos.

For deep sky astrophotography, you typically have to take multiple long exposures frames and later stack them. The mount tracks the object sidereally (i.e. in RA), but most commercial mounts suffer from manufacturing defects in the worm gears and other parts that makes accurate sidereal tracking difficult. Furthermore, your mount has to be perfectly aligned with the earth polar axis as any deviation will cause tracking errors.

This is where Ekos Alignment module comes into play. Alignment module performs the following:

  1. Highly accurate GOTO.
  2. Determine polar alignment errors.
Ekos Alignment Module

The way it works is by capturing an image of a star field, feeding that image to solver, and getting the central coordinates (RA, DEC) of the image. The solver essentially performs a pattern recognition against a catalog of millions of stars. Once the coordinates are determined, the true pointing of the telescope is known. Often, there is a discrepancy between where the telescope thinks it is looking at and where it is truly pointing. The magnitude of this discrepancy can range from a few arcminutes to a couple of degrees. Ekos can then correct the discrepancy by either syncing to the new coordinates, or by slewing the mount to the desired target originally requested.

Furthermore, Ekos can measure the misalignment in the polar axix by taking a couple of images near the meridian and east/west of the meridian. This will enable the user to adjust the mount until the misalignment is minimized.

With the addition of the alignment module, Ekos is now the ultimate astrophotographer tool under Linux!

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