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eqmod_goto_accuracy_explained [2009/08/29 20:43] (current)
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 +==EQMOD Goto Accuracy โ€“ Explained ​ (Author: Mon)==
 +The key to GOTO accuracy is basically a two step method;
 +
 +  -Alignment of the mount with respect to a series of reference point on the sky (alignment stars)
 +  -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;
 +
 +
 +
 +<code vb>
 +NUM_SLEW_RETRIES As Long = 5retries
 +gRA_Allowed_diff As Double = 10
 +
 +</​code>​
 +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.
 +
 +
 +
 +<code vb>
 +CoordSlew: RA[ 06:00:03 ] DEC[ +44:57:00 ]
 +SlewRetry[ 4]: Diff at : 1444 Target is: < 10
 +Goto Slew Complete. Diff at 2
 +
 +</​code>​
 +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 ยท Last modified: 2009/08/29 20:43 (external edit)
 
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