Has anyone else had this problem and how did you solve it.
I have a new electric motor focus running with FCUSB. Works just fine (ish)
Connects fine - moves fine so while I monitor the focus between filters it’s no problem.
BUT
I want to use it with SGP without monitoring it - the way SGP works is that it counts the MS between each filter and uses that to adjust isnt it ?? Is there another way using fwhm for example and having SGP adjust it automatically?
My problem is that the distance it travels to do 20000 ms in forward is different to the distance it travels in reverse.
When the camera (qsi683)is attached to the ED80 it doesn’t return to the original point with the same MS used. It comes out 20000 and goes back in 20000 but needs another 3000 ms to reach the same starting point.
I suspect it doesn’t like the weight of the camera, because it looks like it runs correctly when the camera is not attached.
One thing I did already try was to grind the focus sleeve on the telescope flat to be sure it was running straight. Unfortunately, that made no difference.
You can see the gap in the photo attached.
Any ideas. Is this likely to happen on all telescopes? What do others do to get SGP to work with an electric focusser? Should I be using something else more compatible with SGP?
I don’t want to be a downer, but I am not sure how I see that you can do auto focusing with SGP if you aren’t assured that the focuser will return to where it is told to go? For focus offsets and for running the v-curves and for backlash compensation, you are counting on the focuser going to an absolute position. If it doesn’t quite make it back, I don’t see how it is going to work.
I’m afraid you will need a stepper based focuser instead of your actual motor. A DC-motor cannot be used for precise positioning - unless there is some encoder involved - and as such cannot be used for autofocusing.
I had the very same problem when I first tried to do autofocusing with SGP. I had a FeatherTouch focuser and their focus motor that had a friction drive. It worked pretty well, but not well enough to always get back to the same place when lifting the camera. Eventually, I changed to their PosiDrive system where the drive gear is screwed onto the flat of the focus shaft and it is driven with a toothed belt. Between those interfaces and the R&P drive on the focuser there is no longer any place for any slippage. Once that happens, then the focusing worked just like it is supposed to… Offsets all work reliably and the v-curves look like v-curves.
I am sure there are other systems that work well, but this is the kind that works for me. I think in general, a r&p focuser is best (because the tube cannot slip like it can in a Crayford), and eliminating friction drives in the motor/focuser interface is the most reliable. Again, I am not saying that others won’t work. I am just sharing how I thought about this and how I tried to solve it.
I know that my solution was to just buy something, and I know that isn’t always the right answer nor was it the point that I was trying to make… I more wanted to let you know that yes, others have faced this same problem!
If you understand the basic requirements for things to work, then you can decide what gear you need, or alternatively, you may decide that a different focusing strategy is needed for your current configuration.
I also want to make the point that requirements are certainly different for manual motorized focusing than they are for fully automated focusing with filter offsets. If you choose to use a Bahtinov mask or watch star metrics at your monitor while remotely moving the focuser, you can be more tolerant of some slippage. If you want to become fully automated, then you really do count on being able to reliably return to the same position.
I evolved from focusing manually with a Bahtinov mask to remotely focusing with a Bahtinov mask to finally fully automated focusing. I had to make equipment changes at each step to make it work.
This is pretty much the exact reason why we don’t support DC motor focusers directly. There is no guarantee of accuracy or precision with them. Asentioned Stepper Focusers provide both, which are needed for auto focus.
