Temperature compensation by focuser or SGP


I have an Optec TCF temperature compensating focuser. And I use SGP, which also can do temperature compensation even with a non-temperature compensating focuser (as long as I have a temperature sensor properly installed).

If I tell my TCF focuser to temperature compensate on its own, AND tell SGP to use temperature compensation, am I DOUBLE compensating?

Do I just use one or the other?

Or what?



Yes, just one method


I don’t think you could connect to it two ways at the same time could you?


No, I am only connecting it once. But the driver software supplied by OPTEC includes temperature compensation. This runs all by itself out there whenever the focuser is running. SGP, also can make adjustments. That is, if the thermometer says it has dropped a degree, it will move the focuser out three steps or whatever. It does this by sending additional commands to move the focuser, not knowing that OPTEC’s control program already has!..….

So, it would seem I have double compensated.
That is what I want experiences on. What has proven successful?



I and most others have not been very successful with temperature compensation. However, I do have good results ticking the box to run auto focus every time the temperature changes by a set amount in tandem with auto focus every 30-45 minutes.



It turns out my main imaging (TPO 12 in RC) scope behaves pretty well, and does not show much change in temperature. My training run says it changes maybe one fifth of a CFZ for every degree centigrade. Also, I am in fairly dry imaging conditions, so, once the sun goes down, it gets to night time temp, and rarely changes as much as 5 degrees after that till dawn. So, I just let my Optec adjust focus as it wants, but then I re focus on every filter change, about every hour or so.



Your focuser controller and SGP handle temperature compensation differently. SGP performs a temperature compensation move only after an image completes. Your focuser controller will continuously make compensation adjustments even while an image is being acquired. The Optec controller will apply a move for every 0.1C in temperature change.

For some systems (like mine) fairly large temperature swings like we get this time of year can cause the focuser to drift out of focus in less than the time it takes to complete one narrowband image. So, I do two things - I have the focuser controller set for temperature compensation and I tell SGP to focus every 2 degrees of temperature change. So, the focuser TC keeps focus close between each focusing run.






This is generally the recommended way to setup Temperature Compensation. We typically recommend using the method inside of SGP if you either don’t want your focuser moving during an image or if your focuser doesn’t support it natively.



One more point that is slightly on a tangent, Alex. If you enable TC on the Optec, watch what it does closely. As I mentioned above, it makes a focuser move for every 0.1C change in temperature, and it does this for every 0.1C change in every direction. For my application, they do not have enough hysteresis built into the temperature reading. As a result, the temperature reading often “bounces” up and down by 0.1C. For every “bounce”, regardless of direction, the FocusLynx controller will move the focuser. This can be a real problem. As temperatures drop, the focuser should be moving “IN” to compensate. But, because of the “bouncing” in the temperature reading, sometimes the controller moves the focuser “OUT” when the temperature bounces up a tenth of a degree. Since all focusers have some backlash, this is a real problem. If the focuser only moves “IN” the backlash stays spooled up. But any bounce in the “OUT” direction moves the focuser into the backlash band. The next move back “IN” again only takes up the backlash just introduced and does not move the focuser. This will cause the compensation to not keep up with temperature changes.

I pointed this out to Jeff at Optec and he said they would look into it. When I asked him about it again several months later, I got no reply despite repeated attempts. I called their support staff a few weeks ago (didn’t talk to Jeff) and they blamed it on the larger step size of the Moonlite I am using - even though they said the Focuslynx would work fine with the Moonlite when I bought it.

I say this not to slam Optec support (though I have to say that I am very disappointed) but to make you aware of the potential issue.

I have since had to revert to the Arduino-based FocuserPro2 that I had previously built (and the TC works perfectly - thanks again Robert).