Autofocus with Seletek

I´m installed the Seletek Armadillo Autofocus system and yesterday I tried to make de curve of autofocus in SGPRO.
Apparently the parameters are ok, I will like to put some screenshots or pass to somebody the screenshots that I made yesterday to see more clearly what’s is the problem.
I could make ones the curve nearly fine but a lot of times I doesn’t , it makes a line, or a half curve of autofocus.
I focused my hand and then after I run the Autofocus system.
My equipment is a Takahashi FSQ106 and Autofocus system SELETEK Armadillo 2.
The best way It will be to send by email the screenshots to understand better the problem.

You can upload screenshots directly in this editor (7th icon from the left hand side).

I use a Takahashi FSQ 106N with original rack & pinion focuser and a Robofocus stepper motor with no problem. What focuser are you using?

In order that Autofocus works flawlessly, it is important to set the focuser backlash in SGP, R & P focusers have always backlash.


Thanks Bulrichl

I used Seletek from Lunatico.
Would you mind explain with more clearly "it is important to set the focuser backlash in SGP, R&P focusers?
Thank you

Hi I use the seletek controller and an fsq 106ed with seleteks own motor…
You may find that the issue is one of backlash setting.
I can’t remember my settings but one check worth looking at is your motor polar setting, I use 13.8 volt bi polar…

It works well for me.


I also use the Seletek controller (Armadillo2). In my case, a stepper motor from Robofocus is used.

What I wanted to know is whether the focuser of your refractor is Takahashi’s original rack & pinion focuser or whether the focuser was exchanged (e.g. for a Crayford focuser). This has nothing to do with the stepper motor.

As I wrote, R + P focusers always have some backlash. This has to be compensated, otherwise the Auto Focus will not work properly. SGP has a nice feature called “backlash compensation” for this purpose. In SGP, select Control Panel/Focus and select the button ‘Other’. Select ‘Focuser backlash compensation’, Compensation direction ‘IN’ and input the number of steps in the input field that is labeled ‘Compensation step size’.

The number of steps must be determined before. Move the focuser OUT by say 200 steps, read the focuser position and write it down. Then move it IN by a much smaller number of steps (say 20) and simultaneously observe whether the focuser actually moved. Continue to move the focuser IN by this small number of steps until you detect a movement of the focuser. Now read the focuser position again and subtract this value from the initial reading. That difference is your focuser backlash in steps. You should add some steps as a precaution, a too large value will not do damage, however, the value inputted in SGP as backlash compensation must not be too low.

I strongly advise you not to alter the ‘Advanced parameter’ of Seletek’s Configuration of the Focuser Program. If you experiment with these settings without knowing what you are doing, you possibly will damage the stepper motor. These settings have nothing to do with backlash compensation.


Thank you very much for both replaies!
Great men!!!
Well I have follow the steps for calculate the backlash and it takes values of 100/120 steps??? Could be this numbers correct??
Tomorrow I will try to measured with more accurecy with a comparator.
Thanks again


100 or 120 steps is a lot of backlash. On the other hand, if everything is looking great, only this (backlash) or a mechanical problem can be the cause.

What’s been suggested is great, configure the motor as bipolar (it will have double the torque, sometimes needed for the beefy FSQ focusers) and configure the backlash - better a slowly low value than a higher one.

Also make sure the physical couplings are not slipping.

That should do.


Thank Jaime,
Well today I have made a precision measures.
First with the tube horizontal the backlash has been 100 steps.
With the tube at 43 grades the backlash has been 80 steps.
And finally with the tube at 90 grades the backlash has been 60 steps.
The motor is configured in bipolar.

Hello again,

and I’m sorry, I don’t get the email notifications so never find out when messages here are updated :frowning:

Checking my older message - now I disagree with myself. I mean, 100 or so is a lot for the motor (motor+gearing) backlash; including the one from the r&p focuser, may be perfectly normal.

My apologies for the confusion.


Dear Jaime,
Thanks for your answer!!
Sorry for the delay. I’m really getting crazy with the autofocus. I have a Takahasi FSQ 106 flourite tube. On saturday night I made an easy exercise, I focused with Bathinov mask and after I run the autofocus and put the mask to check if it the autofocus was working properly, the result was that it wasn’t in the proper focus point.
I couln’t made more.I’m really worried about this. I put the telescope working indoor to check if the movement in-out is fine and I saw is fine and the distance to inside and outside is the same.
Not sure if the parameters that I have in the Sgp and Seletek are ok.
Thanks Jaime

Hello, and you’re welcome!

to be thorough:

  • if your indoor tests were good (telescope vertical, etc, making things difficult) and the movements were correct, then you can rule out a mechanical problem. Unless there was very very cold outside so the focuser could get stuck. Don’t think so.

  • the above also “validates” the driver configuration, so only autofocus remains.

The V-curve had a nice shape? The vortex of it matched your bahtinov best focus position?
Is the last autofocus movement racking in the focuser? (I find this is usually best)

Good luck!

Dear Jaime,
Thanks for your answer.
I really becoming crazy with this issue.
I have calculated the step size in 18. I have made several calibration of autofocus in different parameters of backlash .
My parameters are:
Exposure for filters:6d
Bin: 2x2
Auto focus data points: 9
Step size:18
Auto focus Dialog auto close delay: 0 seg
Minimum star diameter at 1x1: 8
The backlash is in 41 IN.
As you see the graphic lines always goes to the left.
Is anybody clarify what I`m doing wrong?


well, just guessing, but for some reason the rightmost points are not being correctly measured, and the regression line is clearly not helping - other reason could be the focuser not moving correctly in that area but that you already ruled out.

Is this consistent?

Have you tried reducing the V-curve are to the inner zone?


unfortunately, you never answered my question concerning the focuser, so I will ask it again: What type of focuser (rack & pinion, Crayford) are you using: the original Takahashi rack & pinion focuser, or was the original focuser replaced? The reason for asking this is: rack & pinion focusers don’t show slippage, but Crayford focusers do.

Judging from the last “V-curve” that you showed, the step size of 18 that you calculated and set is just fine - don’t alter it. Part of your “V-curve” (from focus position 1488 to 1344) looks perfect. If I got a “V-curve” like this, I would do the following:

  1. Compare the HFR value of the validation frame with the HFR value at the focus position that Auto focus determined (in your graph, this is at a focus position of about 1388: HFR 1.6). Are these values nearly equal? If a large deviation is observed, focuser backlash was probably causing the trouble. In this case you should increase the focuser backlash value.

  2. With a result like in this “V-curve” I would cancel Auto focus, set the focus position to the minimum of the curve (in your graph: a focus position of 1416) and then repeat the Auto focus run. If the result of the repetition run then is OK (i.e. near a focus position of 1416), the cause of the trouble probably was a too large deviation from perfect focus (about 6 x 18 steps = 108 steps!) when starting the Auto focus run.


Dear Bernd,
My apologize, I didn’t realize for your message, most of the time I only have mobile to follow the forum.
Thanks for your support and for your answer.
My Taka FSQ106 has the original rack and pinion focusers.
Following your recommendatios last night I made many proves, about 40 focus and autofocus. The first impression it was clear that it was a backlash problem, I saw the HFR in manual focus and the autofocus HFR and there were close. Then I began to change the bascklash in SGP ( IN 25,30, 40, 41, 42, 35,32…many times) but there wasn´t a real improvement,yes the graphic was a bit better but still with this dots in the top (as you said It could be that the focuser didn´t work at first and the focus had this desalighment). Then I tried one thing, put the SGP backlash in “0” and change the backlash of the Seletek. This was the answer, the graphics change dramatilly!!! good responde of the focus and after a few tries I got the right “V” graphic and the autofocus worked perfectly.
Why the seleteK backlash worked better than the SGP backlash?, I think Seletek backlash worked in both sides, and it has sense,(backlash is a mechanical slack and you have in IN and OUT) and SGP backlash only works in one direction, but I´m not sure about this.
Thanks again, and thanks Jaime for your post also.
And again my apoligize for not answer before for your post Bernd

Great, congratulations, that’s a nice v-curve to have :slight_smile: