David's Astronomy - Notes 2022-11-11 (S1057)

Initial focusing of the LX200 telescope at the start of the S1057 session showed that the Foc1 Best Focus position lay at a position of around 300, which is close to the 0 step limit for the TCF-S focuser , compared to a position of 1000 at the end of the previous session (S1056) and to a mid-range (ideal) step position of 3500. Plots below show that the wide & standard focus profiles from initial Near Zenith Focusing were able to find best focus at 336 , they are both clearly curtailed below the 0 step position point.

Even though later in the night the Best Focus position increased from 300 to 900, there is risk that any further slippage of the primary mirror focus could cause focusing difficulties in future.

The increase in Best Focus Position is associated with a drop in temperature associated with a cold front whose leading front was crossed at 19:40, when there was marked by sharp humidity, dew point and wind speed changes.

Primary Mirror Slippage

There has likely to have been a gradual slippage of the LX200's primary mirror since it was last focussed & locked. Whilst the perception has been that the slippage was gradual, data from S1050 session show that it may have slipped and changed the best focus position sharply from 1900-2000 to a new position at 1400 steps somewhen between 19:50 & 20:50.

Action
The TCF-S focuser needs to be reset to c. 3500 (middle of its range) and the LX200 Primary Mirror focus adjusted for approximate focus at the start of next session, and then relocked again / refocussed with TCF-F Focuser.

Update 2022-11-24
In sessions S1058 -S1061 conditions were colder than in S1057 and focusing operations and creation of focus profiles wasn't impacted by the focuser's hard limit at all and the LX200's primary mirror position was left unchanged.

Update 2022-11-25
There was a dramatic change in Foc1 Best Focus Position from between that in session S1061 (2022-11-24) and that in S1062 (2022-11-25), which resolved the earlier concerns over focus position being close to the TCF-S focusers hard-limit at 0 steps position.

Initial Near Zenith Locate frames using the predicted best focus position of 780 (for tube temperature 6.3°C) showed significantly out-of-focus, doughnut shaped stars (see Fig 1). Observatory was taken back to Manual mode and after moving the focuser to 3500 position (in preparation for a refocus of the primary mirror, but no longer required) it was observed that focus had improved. Further nudges of the focus position showed that best focus was lay around 4200-4400.

A Foc1 Autofocus operation was manually initiated, which showed that best focus lay at 4616 positon (tube temperature 5.6°C). This compares with a best focus position at 669 position (tube temperature 7.4°C) from the final Foc1 Focus Profile on the previous night (S1061). This is a dramatic change. The large discrepency is not fully understood - it is potentially due to a unexpected movement of the LX200's primary mirror however the mirror is tightly locked. Previous large changes have been seen that are considered to be related to likely significant changes in atmospheric properties affecting focus. The primary mirror was due to be refocussed to bring best focus closer to the middle of the TCF-S focuser range (as it had begun to approach the 0 position hard-limit however this is now no longer appropriate.

Update S1151 (Oct 2023)
More cases of focus drift occured trhough Sept 2023 and led to the Main Scope's Primary Mirror position being adjusted at the start of session S1151 (2023-09-29),
It was concluded that the mirror lock on the 12" LX200 Main Scope isn't working (see Investigation - Drift in Best Focus Position (Primary Mirror Focus Slipping / Mirror Lock Not Working (2023-09-30) ), and a fix for this problem has been researched but not yet made (see LX200 - Mirror Lock Fix (2023-10-08) ).


	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 (2)

	Images from 2022-11-11 >> [ Local Files >> ]

	Focusing - Temperature and Primary Mirror Slippage Effects

This Web Page:	Notes - Session 1057 (2022-11-11)
Last Updated :	2024-03-22
Site Owner :	David Richards
Home Page :	David's Astronomy Web Site

David's Astronomy Pages
Notes - Session 1057 (2022-11-11)

Session Aims & Highlights (2022-11-11)

Operational Issues (2022-11-11, S1057)

Focusing - Temperature and Primary Mirror Slippage Effects

David's Astronomy Pages Notes - Session 1057 (2022-11-11)

Session Aims & Highlights (2022-11-11)

Operational Issues (2022-11-11, S1057)

Focusing - Temperature and Primary Mirror Slippage Effects

David's Astronomy Pages
Notes - Session 1057 (2022-11-11)