Re: Odd behavior of temperature-compensated focuser

Dave Schwartz

You should have the pull request now.

There is also a debate about whether TC-driven changes to the focuser position should actually be reflected in the position that is given out.

Here's a section from the PerfectStar manual on that:

"TC also involves a heated philosophical debate, if you'll pardon the pun. The issue is whether corrections to compensate for temperature changes should be reflected in the reported position. The argument against reporting is that the “actual” position (the distance from the telescope objective to the image sensor or eyepiece) does NOT change because TC is holding it constant by increasing the draw tube extension as the OTA dimension shrinks, for example. In fact, under this philosophy the position value DOES change with temperature if TC is NOT enabled. This might make a lot of sense if the reported position were actually the focal length of the telescope (perhaps in millimeters). Since the usual case is either reporting the extension of the draw tube in millimeters or in the somewhat arbitrary unit of “motor steps”, I think that the opposing philosophy is preferable: PerfectStar reports the number of steps out from a user-defined zero point, including any adjustments made for temperature compensation."

So PerfectStar includes TC-driven position changes in the reported position. However, Moonlite takes the opposite approach - the reported position does not change but the internal position (visible on the display of the newer controllers) does.

PerfectStar also has a hysteresis parameter which makes a lot of sense. Here's his wording on that:

"The hysteresis value defines how large a change in temperature is allowed before a correction is applied to the focus position. Setting it to “0” means that a new position is calculated every time that the temperature is measured (approximately every 5 seconds). However, if the resulting position change is less than 1 step no action is taken.

Normally, the hysteresis is set to something larger than “0” because it is not necessary to make such small changes and moving the focuser unnecessarily is a risk. Good quality focusers will not shift the image when focus is changed even a large amount, but there is no point in risking it when the adjustment will not improve anything.

To determine the optimal value for hysteresis, use this formula:

TC hyst = ((CFZ / SS) / 4) * (1/TC coef)

where CFZ is the “critical focus zone” for your telescope, SS is the linear step size for your focuser and motor (the distance the focuser actually moves for a single step of the motor), and “TC coef” is the temperature coefficient determined in the previous section. CFZ and SS must be in the same units (typically microns) and TC coef and hysteresis are both in degrees C. The idea here is that no correction should be made until the resulting movement is at least a quarter of the critical focus zone. For example, if CFZ = 100 microns, SS = 5 microns per step, and TC coef = 10 steps per degree, the suggested TC hyst value would be:

TC hyst = ((100 / 5) / 4) * (1/10) = 0.5 degrees "

Sitting here with the TC focuser running in the background, I can hear it making occasional adjustments, probably as the sensor waffles between tenths of a degree.

Any chance of implementing a similar TC hysteresis parameter in OnStep?

Join to automatically receive all group messages.