Auto focus going one way only


I have a scope fitted with a Robofocus and for some reason the curve only goes up and not down. I have changed the parameters but nothing seem to make any difference. I use SGP before with a moonlite focuser and motor and it work fine apart from being very slow. Can you please point me in the right direction as I am sure it is something very small but I can’t put my finger on it. The focuser function back and forth without any issues. The focuser is a 3" focuser from TS.
Thank you for your help.


Hi Vince,
I use a RoboFocus with SGP without any problems - so the hardware should not be a problem.

If this is a new set up, I would suggest having the focuser all the way in to start with. Move it out (maybe one 10th of the total travel) and take a ‘Frame & Focus’ shot. (Could be a 5 second image at Bin2). Most likely you are way out of focus. Keep doing the same with gradually moving the focuser out, until some point that you can say…“the actual focus point is in this vicinity”. (If you have a Bathinov focus mask, then of course you can get to the spot much more quickly). In any case, only when you are near focus, does SGP autofocus routine do its job, and once you are in the ‘ball park’ of the focus point, then you can adjust the steps to get a better curve.
If for some reason, it is not working at all - then you must check you are sure that the back-focus distance to the chip is still OK with the new focuser and whatever connections you have in the image train.
Just my initial thoughts…


Do you start the focus sequence with a good focus? Your symptom indicates that focus may be way off in one direction. Start with a good focus, then when you start the focus routine, watch the focuser to confirm that it moves in one direction a certain amount, then reverses direction and goes about twice that far in the other direction. That would be the normal behavior. In your case I would expect it to just continue going further in the same direction. When you watch the focus progress dialog, what is the sequence of focus positions it goes through.
Here is what a normal sequence should look like, assuming a starting point of 18000, increment of 30, and 7 steps:
18090 (= 18000+3*30)
18003 Final best focus value

You should post your SGP program log which will document what the focuser is doing.


Thank you for your answer.
I always try to start with a good focus but to absolutely perfect.
The focuser move in one direction. (north) and doesn’t come down at all. After a few moves it restart the whole process again with the same result.
Where do I find the SGP program log?
Thanks again.


Hi Kinch

Many thanks for taking the time to answer my query.

I will do as you suggest the first clear night I get.

As it stands today I get as near as I can to a good focus and then I let SGP autofocus run. But from that point the focus get worse and worse with the focuser being moved only one way. After a while SGPM autofocus restart the process with the same result.

Can you please let me know which steps I must change to get a better focus.

Once again many thanks for taking the time to help.

Kind regards



Vince - have to ask…did you read through the help file?

Also: suggest that you save AutoFocus package (and make available for viewing)…then it may be more evident what is going on and what is needed to correct the problem.


I must say I didn’t read all the instructions and that may be the issue. I have read them now and I need to wait for a clear night to check out if it makes it better. I will keep you posted.
Thank you for your advise.


First clear evening since I read the manual and follow your instructions and now the focusing is absolutely superb.
A very big thank you for your help. And as they say in the UK RTFM.


:+1: Happy days (or nights maybe :telescope:)