Permanent Focus

The PerfectStar controller has always had a “shutdown” feature that stores the current position in non-volatile memory so that there is no need to set the zero position the next time you use it.  Furthermore, the shutdown feature is invoked automatically when an ASCOM-connected application disconnects from the controller.

 Building on this feature, Permanent Focus (available in firmware version 5.2) also remembers the last established reference position and temperature (the “reference point”) for temperature compensation (TC).  To use the Permanent Focus feature, simply enable TC before executing the shutdown/disconnect.  When the controller is next powered up (or if the shutdown/lock is canceled without removing power) the controller will automatically enable TC and adjust the focus position according to the new temperature.  If you use filters that are not perfectly parfocal (most are not), you may also need to measure the focus offset for each filter and apply these in your image capture application.  The combination of filter offsets and the Permanent Focus feature means that as long as the imaging setup stays in place you might never need to focus again!

Some details:  

 • Permanent Focus relies on temperature compensation, which is supported in all PerfectStar controllers (excluding PerfectStarLite), but also requires the optional temperature sensor.

 • A TC reference point is not established until the controller gets a valid temperature reading.  While waiting for a temperature reading the controller will flash the “motor activity” LED in amber.  If this LED is solid amber it means that a reference point has been set and TC is active.

 • Implementation of the Permanent Focus feature requires a small change in behavior for the controller:  The ASCOM “disconnect” command now puts the controller in “lock” mode.  This is the same thing that happens when you select the lock/shutdown operation through the hand set.  If you want to continue using the hand set or TC functionality after disconnecting an ASCOM-connected program, you must use the hand set to exit from lock/shutdown mode.

 • When powering up the controller, Permanent Focus can be canceled by disabling TC (either with the hand set or through ASCOM) before it gets a temperature reading.  Since this always takes at least 30 seconds, you have plenty of time to disable TC.  The focus position will then be exactly where it was when you last shut down.

 • Although the TC implementation in PerfectStar avoids accumulating errors in position calculation, when focus is not checked and adjusted over long periods of use it is possible to accumulate position error through mechanical slippage.  Therefore, it is recommended that the “motor off on idle” feature be disabled and that the focuser is either locked or parked in a horizontal position before executing the shutdown.

Pricing & Availability:

 The Permanent Focus feature is a part of PerfectStar firmware version 5.2, which can be downloaded from the Support page.

About Greg Marshall

I am a retired electronics engineer and after a few months of enjoying my leisure I began to miss doing product development. My astronomy hobby always needed new solutions to unique problems, so I decided that whenever I came up with a good solution I would try to make it available to others.

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