Problems Focusing with 2483mm scope


I have in the past used the autofocus routines successfully with a 925mm scope and feathertouch focuser with the focuslynx motorised focuser.

I have a new RC 12 scope (2483mm fl) and a moonlite focuser and high res stepper motor. I have set the focus step size to 37 per the instructions in the SGP manual. I am struggling to get the focus routine to work. I am not getting v curves but rather fairly random patterns. Often the thing gets completely lost and ends up way away from focus. I have tried adjusting step size and Nebula rejection parameters without success. I can get reasonable focus using the hand controller on the focuser so I don’t think the focus hardware is to blame as such. Sometimes SGP gets a reasonable focus point but the curves are not convincing like I get regularly with my other scope (V curves). I am only trying to focus through the Lum filter for now.

I have read a few other comments about problems focusing longer focal lengths with SGP. What else should I be trying?


Until we refactor focus for long FL and secondary obstruction, your only
options is to find a combination of data points and steps per data point
that take you to the edges of producing “donut” stars, but not to the point
where they actually do. SGPro does not recognize centroids properly when
stars appear like this.


G’day Brett - I’m in a similar boat, new RC 10" scope (2000mm FL) with Moonlite focuser and Lakeside stepper motor. I’m getting ‘OK’ results with a much smaller step size (15) for 9 steps. (+/- 220 microns). You also need to start the AF routine when it is close to focus. I’m guessing both our scopes are f/10, so our DOF’s will be similar. Two other things, if the collimation is not that great, it will be difficult to focus and unless you have a field flattener, the stars in the periphery will be elongated, so restrict the AF area by about 20%.
Hope that helps


Thanks Ken for the explanation, hope you guys can find a way to improve the focusing routine for long fl RC scopes in the next release? I will try reducing my step size again and try staying within the donut range and see how I go. I might disable that smart focus idea as well as this will take it into donut territory.

Buzz, thanks for the post. Nice to know I am not the only one having issues with these scopes. I will put that AF area restriction in place, didn’t even notice the feature until you brought it up so thanks.

If there are any other long fl users out there feel free to chime in with your ideas.




I have had some success focusing an Edge11 f/7 at around 2000mm with an atik 383 at 0.57"/pixel using robofocus on the primary. I have discussed it recently in a thread related to reversing the focus direction - but the main issue for me was getting backlash handled properly. I am focusing with robofocus on the primary focus knob, i.e. focusing by moving the primary mirror, and as long as everything behaves consistently it works well - but I haven’t had much time to test it since I recently got backlash working the right way. I did post a nearly perfect parabolic curve - indicating it can work well.

I don’t understand the concern about focal length or focal ratio - since the focus curve amounts to a roughly parabolic curve when sampled at some step size. The main issue with long focal length is that the bottom of the curve is somewhat flattened out by seeing - but the width of the curve is determined by the focuser step size - and that depends on the motor and how it is geared to the focuser. As long as you figure out the width of the parabolic part of the curve where the focus is well behaved - and then divide that by 9 steps or so - you should trace out a parabola and it should work.

To make it work as well as possible I would:

  1. use exposure of 1-2 seconds
  2. find a fairly uniform star field or cluster with many stars that are bright but don’t saturate in 1-2s exposures
  3. use the central 50% of the frame or so - to avoid aberrated field stars
  4. tune the nebulosity rejection and so forth so that the stars it identifies are true stars and are measured properly. You can do that manually by stepping through the range of focus and making sure there are not many false or badly measured stars
  5. make sure a good amount of backlash compensation is in use - and that it goes the right way so it isn’t invoked on every step
  6. make sure the stars don’t turn into donuts that aren’t measured properly
  7. for me, click “disable smart focus” in the autofocus options - so that smart focus is disabled
  8. I used bin x2
  9. good seeing helps

In my case I use 9 steps of size 20 - but the ideal step size for another system could be completely different - even if the focal length is the same. It shouldn’t take much guesswork to find a good step size since you can manually go through focus and see the range over which the HFR value looks correct for the star size. Then divide the range by 9 and that is your step size for 9 steps. Then start near focus and let it run - again after confirming the backlash compensation works right.



Frank, how are you handling SCT image shift? I know you have an Edge scope, but if I understand your setup, you’re focusing with the primary focuser, which - again if I understand correctly - means you don’t have the mirror locked. If you’re only using 1-2 second focus exposures, isn’t the mirror shifting during the exposure?

With my non-Edge C11, I use 15 second exposures so that the mirror is stable over most of the exposure time. That does give some saturated stars, but it seems to work pretty well.



I am a big proponent of just using a stepper motor on the primary focus knob of an sct, and using automatic focus with backlash compensation to find focus. There is no need for mirror locks and you can re-focus as you choose during an imaging session. This has the advantage that the imaging train is attached rigidly, and you save in backfocus without an additional focuser. There is no downside to not locking the mirror - as long as you use OAG and refocus occasionally.

I don’t know what you mean about the mirror drifting during short exposures. I use a full turn of backlash compensation - and then I proceed through the focus curve pushing up on the mirror. Every small increment of motion immediately stabilizes and - most importantly - the focus curves are repeatable.

My main focusing has been with my own code and I have only recently been able to work with the sgp focus routine due to the backlash direction issue. At the same time I have very cloudy skies and haven’t been able to test sgp since I got it working. But my preliminary tests look promising and I don’t see a problem with donut stars or zig-zag focus curves.



It’s interesting that you don’t see image shift. I have a similar focus routine to yours, using backlash correction first and then moving continuously inward during the AF run so that more backlash comp isn’t necessary. However, I find that each quick nudge of the focuser inward pushes the star a bit off center, and it takes a couple seconds before it comes back to center. I’ve always attributed this to SCT image shift. Maybe my non-Edge SCT just has more play on the baffle tube than an Edge. OTOH, I haven’t looked at this in well over a year, so I probably ought to check into it a bit more thoroughly. I’ll let you know what I find.



I would make sure you are giving it plenty of backlash compensation, and that you are focusing in the ccw direction so you are pushing up. What you are describing would indeed be a problem - but I haven’t seen it in normal C11, Edge8, or Edge11.

I would also watch the focuser knob carefully while stepping through focus. If there is any relaxation of the belt or something to allow it to unwind clockwise a bit after a focus move - that could explain what you see.

But the whole idea of winding way back and pushing forward through focus is that the mirror stays mechanically loaded - and for me and many others that does work well.

You may also need to re-lube it or something - but the only scope I opened up to re-lube was my old c11.



Wow. I just really looked at my focuser for the first time in at least a year, and in fact I am focusing - wait for it - backwards. Ending on CW, not CCW. And I’ve been doing this, with pretty good success, for at least a couple years now. That explains what I’ve been seeing. Just goes to show that no matter how much you think you have this stuff all figured out, you just might, um, not.

Now I have to figure out how to switch it all around to run the other way. Both my focus controller software and SGP have options to reverse focus directions. I’m guessing it’s best to switch it in the controller SW, so I’ll try that first. Not sure what that will do to my preset focus positions, since my planetary imaging setup is 30K steps inside - er, outside - my deep sky setup. Mostly cloudy tomorrow night, so a good night for troubleshooting.

Thanks as always, Frank.



Hah - well that explains it. Yes I recommend everyone periodically study their focuser behavior in operation to make sure the mechanics are sound. I once had horrific behavior I didn’t understand - and it turned out to be a loose setscrew that was allowing slippage.

I’m not sure what focuser you have, but with robofocus and sgp there are ways to “reverse” the meaning of in and out - but I don’t have a way to make the numerical direction of the focus values get reversed. That has been my problem with sgp autofocus - that it insists on going from high numbers to low. If that direction is CW for your system, and you can’t change it, then autofocus won’t work well. So that is something you should be aware of. I solved the problem by building a bracket to support the motor upside down.

If you have been getting adequate focus going CW - then I expect when you get things working CCW and with backlash compensation - it will be much better.



I have a Rigel Systems GCUSB focus motor and controller. I just tried to reverse directions using the RigelSys software, and while it works as expected when I’m focusing through that software - all it does is change what “in” and “out” mean - it does not play well with SGP. When I tried to do an AF run, the focuser did its backlash comp move as expected, but then ran continuously after that. I had to unplug it to get it to stop. I’ll try switching directions in SGP next. Hopefully there is some way to get this to work.



Changing it in sgp will do the same. We don’t have a way to reverse the AF as of yet. Your options are to do it in the driver (seems this does not work), reverse the motor (as Frank did) or rewire the plug.




Ask RigelSys if swapping some wires in the cable that connects to the motor would do the trick to reverse the motor direction instead of customizing the bracket. I read in another forum (I think it was Cloudy Nights) that it was suggested to swap some wires to reverse the motor direction. I believe it was for RoboFocus but I can’t be sure.

I found the link about reversing wires:



Jared, I plan to get in touch with the Rigel Sys folks to see if they have any thoughts on how to deal with this. To help with that, can you speculate at all on why the direction reverse in the RigelSys software doesn’t work well with SGP? I’d like to give them some ideas to run with.

Peter, I’ll also ask about rewiring the pins. Electrical work is not something I’ve ever done, however. I’ll feel free to come back to you for guidance. :wink:




Yes I probably should have just reversed a pair of wires to the RoboFocus stepper motor. I have been working on various projects with different steppers and different drivers - plus different mechanics - and somehow I was fixated on inverting the motor mechanically rather than switching two wires.

It should be possible with any stepper just by inserting an intermediate cable that does the switch.

Thanks for the pointer, Peter.



It’s likely just reverses the behavior of the buttons on their UI rather than reversing the overall direction of the focuser (2 very different things). I suspect that to really do this you’d need to do it in the hardware. Maybe there is some incantation that can be performed on the hardware to reverse it? You’d probably have better luck asking Gene and team at RigelSys how to accomplish this if it’s possible.



Thanks Jared. I’ll let you know what I hear. One more question, though: can SGP be modified to allow the user to choose “out” rather than “in” as the direction for AF moves?


Sorry, let me be more specific. I gather that the issue is that SGP needs AF moves to be done in a direction of decreasing step numbers. So can this be changed to allow the user to select increasing step numbers instead? (Or perhaps I don’t really understand the problem, always a possibility.)


I knew what you meant :slight_smile:

Yes, this is currently on our “ToDo” list but we don’t have it slated for any specific version at the moment.