David's Astronomy - Notes 2021-12-29 (S946)

Session Aims & Highlights

- Observing Result
- Night Summary Plot
- Session Event Log

Operational Issues

- Critical Issues (0), Major Issues (0), Minor Issues (2), Small Defects (2), Continuous Improvement (8)

Images from 2021-12-29 >> [ Local Files >> ]

2021-12-31

Guiding Review - Lock Position Adjustments

2021-12-31

Lock Position Adjustments
Lock Position adjustments are being tried during guide settling in order to overcome issues with Dec backlash & backlash overcompensation. In situation where guide star starts out on the wrong side of zero line for planned uni-directional guiding or where first guide pulse pushes the guide star onto the wrong side of the zero line, a simple adjustment of lock position can be used to quickly bring the guide star 'back' to the zero line without any of the side effects of swamping the Dec Guide Mode only to swap it back later on, with the extra uncertainty that brings to Dec position.

The technique has been usefully applied during sessions S945 & S946 to adjust guide star position during settling/focusing stage, prior to acquiring main frames.

There is opportunity to extend the technique to make adjustments between main frames to help reduce guide error in two particular situations.

1) where Dec Guiding is proving to be ineffective in bringing the guide star back to the zero line
(this can happen particularly where the target is lying close to zenith)

2) where guide position has moved to the wrong side of the zero line where principal Dec guide mode can not appply guide pulses to bring the guide star back to the zero line.

Examples of these two situations from session S945 & S946 are shown below.

1a) Non-Effective Dec Guiding - Guide Run 1 (Near Zenith)

At the end of Frame 2 and certainly at the end of Frame 3 & 4, an adjustment in Lock Position would have been helpful in bringing the guide star back to zero position. Doing this in this case particular case would have reduced the average guide error over the entire guide run, but wiuld probably not have changed the quality of an image made up of the 5 aligned and stacked frames.

1b) Non-Effective Dec Guiding - Guide Run 5

In this example a lock shift adjustment at end of Frame 1 would have arguably reduced the guide errors during Frame 2. Resetting the Dec Guide error back to zero, would have reduced the likelihood of seeing the 3-4 arc sec movement in position at 21:51 when the Dec Pulse guides finally returned the guide star back to zero line part way through Frame 2 exposure. .

2) Guide Star on Wrong Side of Zero Line - Guide Run 22 (close to Meridian)

In this example lying close to Meridian there is no strong direction of star drift and there is every chance that the Principal Dec Guide Mode may be inappropriate or not optimised for the precise target position. Possibly related to cloud/atmosphere change at 02:51:30 the Guide Star has become positioned on the wrong side of the zero line where unidirectional Dec Guiding is inactive. Adjustment in Lock Position at end of Frame 1, 2 and 3 could have limited the drift in position which eventually reached 4" during Frame 4. Eventually a change in DecGuideMode was triggered at end of Frame 4, but a single guide pulse saw telescope position move from +3 to -4. which again left the guide star on the wrong side of the zero line for the fifth and final frame.

A change in lock position should have been employed to bring the reset the zero position, which might have prevented the overshoot in position caused by the change in Dec Guide Mode. A purple line around the time of the Dec Guide Mode change shows that a lock position chnage was made, but being coincident with the Dec Guide Pulse might have been a factor in the overshoot (from two changes being made at same time)

Logs associated with change in DecGuide Mode and Lock Position just before Starting Frame 5 :

Actions

Lock Position Adjustments. Use Lock Position adjustments to correct Dec Guide Errors before starting a new image instead of changing Dec GuideMode. This is to minimize overall Dec Guide Errors and prevent overshoots due to Dec Backlash/Overcompensation. Lock Position adjustment to be made where Dec error is <-1.0 or >+1.0 (Dec Guide mode is only changed when crossing Meridian).

This Web Page:

Notes - Session 946 (2021-12-29)

Last Updated :

2024-03-23

Site Owner :

David Richards

Home Page :

David's Astronomy Web Site

David's Astronomy Pages
Notes - Session 946 (2021-12-29)

Session Aims & Highlights (2021-12-29)

Operational Issues (2021-12-29, S946)

2021-12-31

Guiding Review - Lock Position Adjustments

David's Astronomy Pages Notes - Session 946 (2021-12-29)

Session Aims & Highlights (2021-12-29)

Operational Issues (2021-12-29, S946)

2021-12-31

Guiding Review - Lock Position Adjustments

David's Astronomy Pages
Notes - Session 946 (2021-12-29)