David's Astronomy - Notes 2020-11-05 (S0827)

Issue: TheSky6 encountered an exception during slew to Target 22/49 (SS Cyg) and produced a modal error dialog which blocked further session progress until the dialog was manually acknowledged.

Description: Target 24/49 was noted to be showing a slew failure when session progress was checked. A modal dialog window (with a TheSky6 icon in taskbar) was entitled "ASCOM.DeviceHub" and read "SlewToCoordinatesAsync: set '-0.0155881..' is an invalid value. The valid range is: 0 to 24." suggesting an issue with the RA coordinate. DeviceHub and AstroMain User Interfaces remained active (no lock up or hang)

A brief look at the log file records "Exception during Telescope Slew" at 22:38:33.49 with the exception message "Command in progress Error code = 219 (0xdb)". This was within 10ms of the command to Slew. The target coordinates for the slew were RA= 21 43 29.782 Dec= +43 40 21.06 | Epoch Current, so it's hard to see how the invalid RA value was given to TheSky since the routine operates perfectly fine for targets before and after SS Cyg. The coordinates correspond to Az 274.2 / Alt= 52.4 which is a perfectly normal/valid sky position for a slew. Pre-Slew coordinates were Ra= 02 53 45.000 Dec=+16 45 42.00 Epoch Current. Code needs to be examined to see how it might have been possible for RA 21 43 29.782 (21.0.7249394) to be converted to a value of -0.155881 for the slew and why on just this occasion. Although the Slew failure was recorded as a 'Red' Session Event it should also have been recorded as a Red Session Alert & Alarm.

In-Session Fix
After acknowledging the error (by clicking on dialog window's Ok button), the session automatically resumed without further incident.

Investigation: Investigating further showed that SS Cyg was the in fact the 4th of 4 consecutive targets that failed with a slew error. Three of the targets including SS Cyg had failed with the "Command in progress, Error Code=219" but this was just the sideproduct of an open dialog window associated with a slew failure exception from the first of these targets ( Target 19/49 (C/2019 L3 Atlas) ). Looking through the logs it feasible to understand how the issue occurred.

The target had object coordinates of RA= 23 59 41.513 Dec= +64 10 37.56 | Epoch 2000, which has current epoch coordinates of RA= 00 00 46.443 Dec= +64 17 43.67. On top of this there was a slew adjustment of 1.48 arc min (W= 1.28', N= -0.76') giving slewing coordinates of RA= 00 00 34.712 Dec= +64 16 58.27. (Coords need to be double checked in case any of them were in fact negative, but the logs don't indicate this as present).

After calling TheSky6's TeleASCOM.SlewToRaDec command the telescope targets would have received an additional adjustment form the TPoint Model. It is presumed that the routine for doing this in TheSky contains a bug that led to a negative and invalid RA coordinates which were blocked and raised as an exception. '-0.0155881 would be equivalent to - 00 00 56.12. Given that target was of 00 00 34.712, equivalent to +0.0096422 the angular difference is only 22 arc min which would be well within the bounds of a T Point correction.

Conclusion: In rare cases of a target very close to RA 0 or RA 24, a bug in TheSky6 can allow a RA value to be generated and sent to telescope which is outside the range 0 to 24.

Fix: With the impossibility of obtaining a fix to TheSky6 as it is no longer supported by Software Bisque and it being unreasonable to ask Device Hub to be modified to accept -0.0155881 and convert it to 24-0.0155881, a workaround is necessary.

One 'fix' would be a modification of AstroMain (and even AstroPlan) code to either not schedule targets that are very close to RA 0 / RA 24 and prevent the target from attempting a slew to a target that is very close to RA 0 / Ra 24. However such targets would simply not be able to acquired.

The proposed alternate 'fix' is to slew to targets that close to RA 0 / RA 24 using ASCOM Telescope.SlewToCoordinatesASync command instead of using the TheSky. Telescope.SlewToRaDec command. This would prevent potential invalid RA coordinates from causing an Out of Range exception that blocks automated Telescope operation viaTheSky6, but would miss out on the more accurate slewing that can be achieved with a TPoint model. The latter loss can be largely mitigated by target centering operations (taking a locate frames, plate solving it and making the appropriate jog correction(s))

AstroMain Scope.SlewTheSkyScope routine has been modified to using DeviceHub.Telescope.SlewToCoordinatesASync when target RA < 0.04 or RA > 24 - 0.04
Fixed 2020-11-06 (AstroMain 3.32.4)


	Session Aims & Highlights
	- Observing Result - Night Summary Plot - Session Event Log

	Operational Issues
	- Critical Issues (0), Major Issues (1), Minor Issues (12), Continuous Improvement (4)

	Images from 2020-11-05 >>

	Investigation - Slew Failure (Invalid RA)

This Web Page:	Notes - Session 827 (2020-11-05)
Last Updated :	2024-02-21
Site Owner :	David Richards
Home Page :	David's Astronomy Web Site

David's Astronomy Pages
Notes - Session 827 (2020-11-05)

Session Aims & Highlights (2020-11-05)

Operational Issues (2020-11-05 S827)

Investigation - Slew Failure (Invalid RA)

David's Astronomy Pages Notes - Session 827 (2020-11-05)

Session Aims & Highlights (2020-11-05)

Operational Issues (2020-11-05 S827)

Investigation - Slew Failure (Invalid RA)

David's Astronomy Pages
Notes - Session 827 (2020-11-05)