Equipment Profile - Focusing Question with nSTEP and my SCT


I’m a little confused with trying to figure out how to setup the Rigel nSTEP STEPPER system in SGP. with my currentl SCT scope. I’ve been using the Optec TCF-S focuser for well over a year using the SGP autofocus on the Celestron 8’ SCT without any issues. However the TCF-S focuser is consuming a lot of back focus and forcing me to push the primary mirror forward which changes my F10 scope to almost a F12. Even with a 0.63FR I can only get to approximately an F8. Therefore I decided to go to a primary mirror focusing system and decided on the Rigel nSTEP STEPPER kit. I went with the Rigel system mainly because it has worked so well with SGP and my ED102 steup…

Here/s what I am discovering with this setup using the Regel focuser, SGP and the SCT - For the SGP autofocus to work with this setup I need the primary mirror focusing knob to be turning in the clockwise direction and have the step count numbers to increase so SGP autofocus can move out of focus and then lower the step numbers to achieve the “V” curve. In the Equipment Profile under the Focus tab there is a check box to “Reverse direction” however during my testing it does not appear to work.with or with “Reverse direction” checked or unchecked, when the focuser knob is turning clockwise the step numbers are still decreasing. I also tried in the ASCOM driver to use the reverse direction and that seem to really screw up things, it was like adding a negative number, say I was at step 20,000 and I wanted to go to say step 21,000 and I put 21,000 in the GoTo it would end up at 19,000 not 21,000. Like it was adding a negative number.

Maybe I’m thinking this all wrong or I’m doing something wrong but I just can’t seem to work out the correct settings. Any advice/help would be greatly appreciated. Sorry for being so long winded.



I have reported on this a few times. With RoboFocus and Rigen nSTEP - there is no way to reverse the direction of focus in terms of the numbering - and SGP won’t do it either. So if it happens that going from high numbers to low numbers turns the knob clockwise - you are stuck with that and that’s the way the autofocus curve will be taken.

What you may be able to do is switch two of the wires on the stepper motor to make it go the other way. That is what I did and everything now works. But there is good reason for the focus drivers not to change direction that way - because it would mean completely changing the number range of the positions. The real fix needed is for SGP to be able to take the curve either going from low to high or high to low.

You may be ok taking the curve with the knob going clockwise - but you will just have to see. The things to watch for are focus shifts after the move completes - and a lack of repeatability of the curves.



Hi Frank,

Thanks. So my question is will the autofocus still work just that the “V” curve starting high point will be on the left and the finishing point will be on the right? Or in this situation autofocus is not going to work at all unless I do the modification?



The problem is there and you can’t get around it without rewiring - but it’s different from what you describe.

In SGP the curve is always taken from right to left - and the high numbers are always on the right and go down to the left.

There is no way with robofocus or Rigel - or perhaps any focuser - to make high to low be either clockwise or counter clockwise. All the software and drivers let you do is change what “In” and “Out” mean. But that doesn’t help with autofocus because SGP doesn’t care what In and Out are - it just goes from high to low.

The hope for you is that it won’t matter much to be taking the curve while turning the knob clockwise. It is not recommended to focus by turning clockwise - but it may be ok. But unless you change the wires or do something else mechanical to reverse the direction - you are stuck taking the curve clockwise.

Just make sure there is a good amount of backlash compensation - and it is only invoked at the start of the curve - when moving ccw way up into the high numbers - and then coming back down to start the curve.

If you find that the V curves aren’t very repeatable or don’t look good - you probably need to change things so the curve is taken ccw.



I finally got it figured out in my head. I don’t believe it really matters which direction the focusing knob is turning during autofocus. It will always move from out of focus to in focus and back to out of focus of the other side of the curve. So which way the focuser knob moves I don’t believe matters. I think what is important is ensuring the backlash is setup correctly so the knob finishes each time pushing up on the mirror which for my setup the knob should always finish turning in the counterclockwise direction when viewed from the back after each move. If the move finishes in the clockwise direction then the backlash should be invoked which is the way I have it setup. Now I just need to wait for some clear skies which here in Ohio at this time of year might be awhile :joy:



I agree the most important thing is for backlash to be setup correctly - so it is only invoked at the start of the curve.

But at the same time the whole focusing process is different when pushing up against gravity - as opposed to letting the mirror slide down on its own weight. If it works for you well and you get good curves and focus doesn’t drift then you are fine. But if problems remain then focusing ccw is the next thing to try.