# Temperature compensation questions

HI folks. I am new to this forum. I am trying to set up temperature compensation and have a I few questions.

1. Do I input the steps/degree that I measure, or do I change the sign to input the steps/degree that I want to be applied to the focuser position?

2. Does reversing the direction only change the display, or does it change the compensation calculations? In other words, if I reverse the display, do I also need to change the sign on my compensation?

David Crall

Hi David,

As to point 1:
From the PDF manual, page 139: â€śNegative temperature compensation ratios indicate that your focuser will move INWARD as temperature drops.â€ť

A positive temperature coefficient is observed when the shrinkage of the tube with dropping temperature is the main factor. Some (perhaps all?) refractors show the opposite behavior, because the refractive index of the glass in the objective exhibits a strong temperature dependence which often overcompensates the tube shrinking. If this is the case, an overall negative temperature coefficient results.

The temperature coefficient (with correct sign) is best determined by using Mikaelâ€™s SGP Auto Focus LogViewer, see SGP AutoFocus LogViewer . The download URL is https://sourceforge.net/projects/sgp-autofocus-logviewer/ .

As to point 2:
From the PDF manual, page 49: â€śReverse focuser direction: Reverses what is considered In and Out movementsâ€ť.

This has to be adjusted correctly, but you have to do it only once for a given focuser. The adjustment of the correct focuser direction has to be made at first.

Please note that the adjustment of focuser backlash (PDF manual, pages 49 and 50) is critical for the correct operation of Auto focus and temperature compensation. In SGP, the backlash value needs not be accurate, it must be large enough.

Bernd

1 Like

Bernd,
Thanks for your help. The log viewer shows the steps/deg slope to be negative, but instructs me to put in a positive value. I guess that means SGP is looking for the desired temperature compensation and not the measured relationship between position and temperature.

Dave

Yes, thatâ€™s right. You have to input the value of â€śSGP Temp Compensation Coeffâ€ť.

Bernd

@Dwcrall1

If your focuser supports temperature compensation, I would recommend that you enable TC in your focuser and not in SGP. SGP only moves the focuser prior to starting the next frame but your focuser will implement continuous TC. If you are shooting a 30 minute sub, you will definitely benefit from having TC adjustments being made during the exposure when significant temperature changes are occurring.

Whether the temperature compensation coefficient (steps/degree C) is positive or negative depends on how your OTA responds to temperature changes. For example, the OTAâ€™s of a Meade ACF or Celestron EdgeHD are aluminum and shrink with dropping temperature. This moves the primary mirror closer to the secondary causing the focal plane to move rearward. So dropping temperatures means the focuser needs to move outward to keep the camera in focus.

However, an APO or a carbon truss tube scope may need either positive or negative coefficients depending on their net response to temperature changes.

Most focusers with built-in temperature compensation will move outward (increasing step count) with dropping temperatures and positive coefficients but verify that with your focuserâ€™s vendor.

Charlie