David's Astronomy - Notes 2022-01-01 (S947)

Session Aims & Highlights

- Observing Result
- Night Summary Plot
- Session Event Log

Operational Issues

- Critical Issues (0), Major Issues (0), Minor Issues (5), Small Defects (2), Continuous Improvement (1)

Images from 2022-01-01 >> [ Local Files >> ]

2022-01-02

Review - LX200 Temperatures (":fT#")

SBIG Camera Bias Level (Sky Brightness Issue & Fix)

2022-01-02

After discovering that a CommandString(":fT#", True) command could be used to access a temperature reading from the observatory's LX200GPS Telescope monitoring routines were modified to read the temperature every 20s and plot it as a line on the Focus Temperature charts alongside other temperature readings from the observatory. It was qucikly discovered that the LX200GPS Temperature showed a very poor correlation with other that from other sensors. Applying a trim (correction) setting didn't help nor did sending a command to ensure that scope's GPS is turned off (as this uses circuitary that is understood to warm the readings returned by the LX200GPS Temperature.

The following plot from the S947 sessions displays the LX200GPS Temperatures returned by CommandString(":fT#", True) as a magenta line. This is plotted in degC after conversion from raw (degF) values from the scope. The plot provides further proof that the LX200GPS temperatures are unreliable and of no pratical use for observatory monitoring.

In the following S948 session the temperature returned by :fT# started out returning temperatures of 23.8 to 18.8°C, far higher than the observatory ambient temperature of 1°C, and then at 18:24 the temperature returned from the LX200 telescope suddenly dropped from +18.8°C to -10.4°C in 20s, after which it gradually fell to an eventual low of -16.4°C. This is an impossibly low number unless the scope has a built in freezer !

Conclusion
Although only recently introduced, enough data has now been collected to show that the temperatures returned by the observatory's LX200GPS telescope is anomalous & unreliable for most of the time (being either too hot or too cold) and with trends that simply don't reflect the trends seen by other tenperature sesnors used inside & outside the observatory. Reading the temperature value too frequently can also cause the LX200 Telescope/Driver processing to slow down to the point that major problems are seen during slewing, centering and guiding.

Action
Withdraw Scope.Temperature from AstroMain.
- remove Scope.Temparture property (stop using :fT# command)
- remove ScopeTemperature from the Focus Temp Chart & from the ObsPic Chart
- remove Scope.Temperature from the Observatory Tab & Virtual Model.

Following replacement of the Observatory Computer with a new one on 2021-12-10 it has been noticed that session plots showing the Sky Brightness as determined by background ADU rates per second appeared to show much brighter sky that expected. These are determinated from Locate Frames taken with C Filter. Initial sessions after changing to the new observatory computer corresponded with a near full / full moon, and the issue was initially dismissed.
A vignetting artefact seen in recent (reduced) images was noticed but there had been no change to the imaging setup to explain this.

Subsequent sessions in which the moon lay below the horizon showed that the initial was real.
See following Sky Brightness plot from the recent S947 session (2022-01-01).

With the possibility that the problem related to stray IR light from the Observatory WebCam coding was changed to turn off the camera/IR lights during Locate Frames (in addition to main image frames) but this didn't resolve the issue.

Further Analysis
Tests taking dark and bias frames showed that they were giving much higher average/peak values than dark/bias frames taken before changing to the new computer. Normal bias reading (from 3x3 binned frames) is around 160-170 ADU, but the new frames returned values around 1000 ADU.

It is speculated that a vignetting effect seen in recent images that have been reduced using older Flat frames is an artefact of this bias level problem.

Fix
It was hypothesized that this related in some way to the drivers installed for the SBIG camera when setting up the new Observatory Computer.
Investigating it was noticed that on the driver's 'Configure Driver' tab there was a 'Auto Bias Level Correction' option which was unchecked.

This option was checked, applied (using Ok) and the drivers reinstalled. Post-modification checks showed that the camera was again producing Bias frames and short dark exposure frames with the expected normal low values of around 165 ADU

Confirmation
Sky Brightness plot from the following S948 session (2022-01-03) shows that Sky Brightness values derived from CCD Images have returned to their expected values.

Update 2022-08-10
The same issue was noticed 2022-08-10 in images taken in session S1024 (2022-08-04) which followed reinstallation of Windows 10 / reinstallation of SBIG Camera on 2022-06-21. Problem wasn't noticed initially as the bright background was put down to bright twilight skies in June/July, but by August it was evident that something was wrong.

Again the same solution to SBIG Driver Configuration appears to have fixed this later problem.

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Notes - Session 947 (2022-01-01)

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2024-03-22

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David Richards

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Notes - Session 947 (2022-01-01)

Session Aims & Highlights (2022-01-01)

Operational Issues (2022-01-01, S947)

2022-01-02

Review - LX200GPS Temperature (":fT#")

SBIG Camera Bias Level (Sky Brightness Issue & Fix)

David's Astronomy Pages Notes - Session 947 (2022-01-01)

Session Aims & Highlights (2022-01-01)

Operational Issues (2022-01-01, S947)

2022-01-02

Review - LX200GPS Temperature (":fT#")

SBIG Camera Bias Level (Sky Brightness Issue & Fix)

David's Astronomy Pages
Notes - Session 947 (2022-01-01)