APPM tracking error

Craig Young


F/W: VCP4-P01-10
APCC Pro version: V1.6.0.3
V2 ASCOM driver: V5.10.02
Maxim DL version: 6.17
plate scale: 0.57"/pixel
Location: southern hemisphere (New Zealand)

In the past I have not been able to get APPM to accurately point or track, even with models up to 200 points,  so I gave up and used a guide camera.  But recently the guide camera failed so I learned how to use the RA and DEC correction rates in the V2 driver to carefully offset tracking errors.  This has worked very well the last few weeks but does require updating the correction rates every 15 minutes or so, probably due to changes in refraction and the alignment of the system.  For example, if the star drifted to the left in the image I knew I had to increase numerically the DEC correction rate in the V2 driver.

Last night I performed a 110 point model over both hemispheres (55 points in each hemisphere).  I then enabled APPM pointing and tracking correction.  Selecting a random star, APPM displayed the tracking correction:

APPM correction: RA 1.23 sec/hr, DEC 38.3 arc-sec/hr

After a few minutes the star drifted noticeably in both DEC and RA, with the DEC drift much greater than the RA drift.  This hinted that the tracking error in both axis scaled with the APPM correction amounts above.

To see if the APPM corrections were being applied in the opposite direction I calculated V2 corrections:

V2 RA corr = -(APPM RA corr * 2) / 3600, V2 DEC corr = -(APPM DEC corr * 2) / 3600.

V2 RA corr = -(1.23 * 2) / 3600, V2 DEC corr = -(38.3 * 2) / 3600

V2 RA corr = -0.00068, V2 DEC corr = -0.02128

The reason for multiplying the APPM corr rate by 2 is because the V2 corr rate is numerically added to the APPM corr rate, and since I needed to reverse the correction I needed X1 to cancel the correction and X2 to reverse the correction.

I entered these two values into the V2 driver and after several minutes the star drift in RA was 0 but DEC was much worse, which meant the sign was incorrect in DEC.  So I changed the V2 settings:

V2 RA corr = -0.00068, V2 DEC corr = 0.02128 figuring the DEC directions were reversed

After several minutes tracking both RA and DEC drift were now 0.  So it does appear the APPM corrections are being applied in the wrong direction.

Looking back at my notes when using V2 correction I noticed that when pointing in this area of the sky I used:

V2 RA corr: -0.00050 and V2 DEC corr: 0.01000

converting this to APPM values:

APPM RA corr: -1.8 sec/hr, DEC corr: 36 arc-sec/hr

this matches fairly well with the above results.  The APPM RA corr rate needs the opposite sign (- instead of + in the tests above), and the APPM DEC corr rate needs to move the DEC axis in the opposite direction.  In other words, the APPM corr rate is displayed correctly but is applied in the wrong direction to the DEC motor.

There seems to be two possible explanations.  First, since tracking and pointing are reasonably accurate without APPM, implies the configuration of the mount is correct.  In other words, the RA and DEC directions for the southern hemisphere agree fairly well with the sky and the motors do indeed move in the correct direction.  But with APPM correction not working well for either pointing or tracking it would indicate that APPM corrections are being applied in the opposite direction to the motors.

The other possibility is there is a configuration problem in APPM that does not correct for southern hemisphere operation.  If other telescopes operating with similar hardware and software are operating successfully in the southern hemisphere with APPM then there must be a configuration problem on my end.

Any suggestions?


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