OPTEC Temp Comp focuser


#1

Hello, does anyone have advice on how to set up a TCF-S Focuser to play nice with SGP autofocus?
I’m confused about the whole temperature compensation training procedure and the atuofocus A / B switch.

Thanks
Walter


#2

Hi Walter,

the Temperature compensation in SGP is currently not working as expected, see Temperature Compensation Implementation
A Feature request Temperature Compensation Fix was put to fix this issue. Before a fix is applied, enabling of Temperature compensation in SGP is not advisable.

Independently from this issue you can determine the temperature dependence of the focus position of your scope by now. Generally you want a night with large temperature shift and do an Auto Focus run whenever a large enough temperature change happened. The Temperature Compensation Trainer does this for you. I think the description in the manual is well written, and the operation is very simple: you start the Trainer, take a note of the current temperature and stop it, when the temperature changed by 5 degrees at least.

You can do it also manually, without the trainer. Then you have to start the Auto Focus manually whenever a large enough temperature change happened. You will have to extract the data from the history (which must have been enbled before) and transfer it into a spreadsheet software. HFR values (as y) at best focus are then plotted versus Focus result values (as x) in a xy plot. The negative slope of the linear regression curve is the temperature coefficient that is needed for the Temperature compensation. The coefficient of determination R² is a measure for the quality of the data.

It should be mentioned that you want to measure the temperature of the scope. Therefore it is crucial to protect the temperature sensor from wind. (I don’t know whether your Optec focuser has an internal or external temperature sensor.)

Bernd


#3

Thank you, Bernd!

Would you recommend using the auto temp compensation feature built into the OPTEC focuser (after you train) or just query SGP to autofocus periodically? I know that each time I autofocus using SGP it takes a while and sometimes (for unknown reasons to me), the focus gets corrupted in some fashion and the HFR values get larger.


#4

Would you recommend using the auto temp compensation feature built into the OPTEC focuser (after you train) or just query SGP to autofocus periodically? I know that each time I autofocus using SGP it takes a while and sometimes (for unknown reasons to me), the focus gets corrupted in some fashion and the HFR values get larger.

Since it depends on your specific focuser (which I do not know), I cannot give general recommendations here.

If you have a rack & pinion focuser there will be some Focuser backlash and it is very important to have a good Focuser backlash compensation. This applies both for Auto Focus and Temperature Compensation. You can judge the quality of the Focuser backlash compensation if you compare calculated HFR values of Auto Focus runs in SGP with the HFR value of the validation frame. If these values differ substantially, Focuser backlash compensation seems to be inadequate. Bad Focuser backlash compensation will make for an error when the focuser - at the end of the Auto Focus routine - returns to the best focus position. Therefore Focuser backlash compensation has to be adjusted correctly before you determine the temperature dependence of the focus position of your scope. Whether Focuser backlash compensation is better set in the focuser software or in SGP has to be tested. I think SGP does a good job in Focuser backlash compensation.

With a Crayford focuser, there may be some slippage, but this cannot be balanced with backlash compensation.

Regarding Temperature compensation: SGP will correct for temperature change only between frames. This may be right depending on the mechanical adjustment of the focuser: If focus corrections also result in a lateral shift of the focusing unit, it is not advisable to correct focus during an exposure; the guider would not be able to correct for the shift rapidly enough. However, if the focuser does not move in lateral direction, a correction even during an exposure (by the focuser software) may be better.

Naturally both Focser backlash compensation and Temperature compensation may be enabled either in the focuser software or in SGP, but not both.

In summary it’s not easy to give recommendations here, it depends on the system that you are using, and it has to be checked.

Bernd


#5

Thank you for the thoughtful response.
Walter


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