EQMOD Goto Accuracy – Explained (Author: Mon)
EQMOD Goto Accuracy – Explained (Author: Mon)

The key to GOTO accuracy is basically a two step method;

  1. Alignment of the mount with respect to a series of reference point on the sky (alignment stars)
  2. Properly Positioning the RA and DEC stepper motor after the GOTO slew to the correct location.

Step 1 is achieved by the N-Point alignment.

You can still achieve accurate GOTO's even with a 1-Point if the mount is properly polar aligned and have minimal cone errors (we can still overcome cone errors on a 1 star align by choosing stars on the same meridian side on both the alignment and the GOTO). If the final position crosses the meridian opposite where the 1-Point alignment is, then you need to do a SYNC first to overcome cone errors on the opposite side.

Step 2 has already been achieved within the EQMOD

It is achieved using the GOTO function codes and through the “Iterative Goto” implementation. Achieving the correct stepper position both for RA and DEC at the same time compensating for the earth's movement and the time the GOTO was implemented were the keys for an accurate iterative GOTO function.

The “iterative GOTO” uses the following two control lines found at

the 'EQMATH.bas' source code;

NUM_SLEW_RETRIES As Long = 5retries
gRA_Allowed_diff As Double = 10

It says that Goto should not stop until you get within 10 microsteps of the desired position (10 * 0.144arcsecond ) or roughly 1 arcsecond. To achieve this you need to at least perform an iterative GOTO retry. Iteration will stop if more than 5 attempts. I have observed that most GOTO's on this mount stops at the 2nd or 3rd iteration. You will notice this on your mount as stepper “clicks” / “bursts” after a GOTO operation and as “Slew Retry” messages on the EQMOD window.

CoordSlew: RA[ 06:00:03 ] DEC[ +44:57:00 ]
SlewRetry[ 4]: Diff at : 1444 Target is: < 10
Goto Slew Complete. Diff at 2

Using the sample EQMOD message above, without iteration, GOTO will stop at 1444 microsteps of discrepancy (.144arcsec/microstep = 207 arcseconds). A slew retry has brought the mount only at 2 microsteps discrepancy. With good alignment, a 2 microstep discrepancy should put the object dead center on the FOV.

The driver has to impose a 5 retry limit to avoid any indefinite oscillations in the iteration process. If you get more than 1 arcsecond of stepper positioning error visually on the FOV, then the problem is NOT on the stepper position but on the mount alignment (polar/cone/etc).

/home/welshdra/public_html/eqmod/data/pages/eqmod_goto_accuracy_explained.txt · Last modified: 2009/08/29 20:43 (external edit)
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