Focusser not moving to correct possition - equipment too heavy?

Hello together,

yesterday (after years of using SGPro) I wanted to start to implement the autofocus routine in my imaging.sessions.

The first problem I encountered is probably not an issue regarding SGPro. But however I would be happy, if I could get some advice or a confirmation of my assumption.

The following behaviour was reproducable over the hole night:
After getting the rough focus by a manually procedure, I started the autofocus routine. But allways the autofocus needs too much steps until coming to the lowes HFR value. And after finishing, the validation focus frame was always very off the expected result. I “fixed” that by moving the focusser 100-steps-wise inwards, take an image and evaluate manually the HFR until I came near the expected HFR value.

A typical autofocus graph looked like this:

After that I manually moved inwards:
23163: → HFR: 1,61
23063: → HFR: 1,38
22963: → HFR: 1,31
22863: → HFR: 1,23 (stopped moving inwards, as it is around the lowest value shown in the autofocus graph)

The only explanation for this behaviour is, that on moving out, the focusser actually moves more than indicated by the focuser position numbers. May it be possible, that the focusser is slipping a little bit?

Or may it be that it has to do with the backlash compensating settings? To be honest, it is very good possible that I did not understand that correctly. From my understanding the observed behaviour has nothing to do with backlash. In my opinion fhis would be the case, if the final out move to the estimated focus position would be too short. But the opposit is the case. On moving out, the focuser is moving too much. Please correct me, if I am wrong.

Unfortunately I couldn’t test the autofocus with my old camera (Moravian G2-8300). Now I have attached a G3-16200 with enhanced cooling and an external filter wheel. In addition a tilt adapter is within the image train.

I have a FSQ85 with original focusser and a HSM35 stepper motor controlled by Focus Boss II.

Thanks a lot in advance for input and best regards

Reinhard

PS:
Here is the link to the log file of that night:

@Reimer22

AF can sometimes be difficult to dial in for your rig… most of the time it is not, but sometimes gear gets in the way of software.

Reading through here answers a lot of questions about AF. Have you had a chance?

http://www.mainsequencesoftware.com/Content/SGPHelp/UnderstandingAutoFocus.html

I too have a Moravian camera (a G2-8300) and I find it heavier than the alternatives from other manufacturers. Nevertheless, I doubt this is slipping and I suspect this is a combination of backlash and step size. I’m not sure if you say what your backlash setting is. I have set mine to be quite high (100 on one scope and 200 on the other). I don’t think it matters if the backlash setting is higher than it needs to be. Clearly it does if it is not high enough.

Second thing to check is step size. Get your scope into good focus with a Bahtinov. Have your focus routine set to 9 steps. Note the HFR value at good focus (let us say, for example that it is 1.0). Run your autofocus routine. You want the HFR on the first autofocus image to be around 3-5 times the value of the good focus sub. So, in this case, somewhere between 3-5. If it is not this then adjust your step size until you get it somehwre in this range. It is pretty intuitive.

In your screenshot, the HFR at the bottom of the ‘curve’ is somewhere over 1.2 whereas the point at the extreme right is less than 3. This tells me that your step size is too low.

@Ken, @Gnomus, thank you very much for feedback!

Yes Ken, I have read this instruction.

And yes Gnomus you are correct, my step size and the number of positions are not fitting 100% the advises from the manual. But also with bigger step sizes I get similar results and I do not believe my problem has to do with step sizes and/or backlash compensation settings for following reasons:

For clarification I would like first to distinguish between “stepper motor position” and “focuser position”.

For me “stepper motor position” is the number reported in the GUI of SGPro and which would change on pushing the coarse buttons, also if the motor is not attached to the knob of a focuser. It only counts the movements of the motor.

The “focuser position” is the actual position of the camera sensor relativ to the optical relevant parts (e. g. lenses) of the telescope.

Your desire is to keep the stepper motor position and the focuser position synchronous.

To make it easy, let’s say you have a system, that if it is working in a mode without backlash (or slipping) 1 step of the stepper motor causes the focuser to change its position by 1 µm. Further assume that the system has a backlash of 500 steps.

This means (as far I have understood) that, e. g. if the last movement was IN and you move the stepper motor 1000 OUT, the stepper motor needs 500 steps before the focuser position starts to change. In result the stepper motor is moved out 1000 steps, but the focuser position has only changed by 500 µm outwards. Every further step OUT causes the focus position to change by 1 µm, but the system remains 500 steps asynchronous.
If you now move IN again, say by 1000 steps the same happens in the other direction and because of the backlash the focuser position is only changed by 500 µm inwards.
As net result you have moved the stepper motor 1000 steps OUT and then 1000 steps IN, while the focuser position has changed first 500 µm outwards and then 500 µm inwards.

The backlash compensation of SGPro “only” helps that, if the last movement was IN and you move 1000 steps out, the focus position is actualy changed by 1000 µm, (the system remains synchronous, after a single direction change)

For me that means, that even if you have the backlash compensation disabled you should achieve the focus position always at the same stepper motor position ± backlash of the system in steps.

But unfortunately my system behaves not like this.

The autofocus first moves IN (e.g. 100 step wise) to figure out the stepper motor position of the lowest HFR. It is moving inwards until the HFR starts getting higher for a while. Then the autofocus has to move out again to get this “lowest HFR position”. But every time this position is too far out, and I have to move IN again manually to get the actual lowest HFR position, which always results into a lower stepper motor position than originally estimated.

As I have started a few days ago, I achieved the best HFR position at around 32000, in the meantime my best HFR position is around 16000.
That means in average the IN stepper motor movements are resulting less focuser movements than the corresponding OUT movements. Or in other words: to achieve the same focuser movement inwards and outwards, more stepper motor steps are required for the IN movement.

But what might the reason for this?

So far I have following hypotheses:

  1. Slipping of the focuser or the stepper motor as the equipment is to heavy.
    Counterargument: I think I should have observed more erratic changes of the HFR on moving the stepper motor

  2. I have backslash only on moving inwards (on every single move)
    Counterargument: I can observe continiously HFR changes on moving the stepper motor 20 step wise inwards. And if this sort of backlash is very small, it would not affect the auto focus routine at a single run.

Any ideas?

Sorry for that long post and thanks a lot in advance for help with my problem.

Have a happy new year!

Reinhard

Will autofcus only ever move the “final” focus position to one of the values it sampled during the autofocus routine - i.e. will it only choose one of the “steps” it took an exposure? Or is the routine intelligent enough to move the final focus to a position in between two adjacent steps during the routine? I have not had enough clear night to bottom this out as yet…
Thanks.

Are you sure that the screws are tight on the connector between stepper motor and focus shaft? I had a similar isssue recently where it was not going back to the correct point of focus and nailed it down to a loose screw…

Good luck.

Thank you for reminding me to update this thread, i inteded to do since some weeks :slight_smile:

As I was not sure, if it is the focuser or the stepper motor, causing the problems, I decided to replaced both.

I replaced the HSM35 (http://starlightinstruments.com/store/index.php?route=product/product&path=96&product_id=356) by the Posi Drive motor system (http://starlightinstruments.com/store/index.php?route=product/product&path=96&product_id=407) and the original TAK focuser by a Feather Touch and the problems described above are gone. Now I get reproducable focus positions, even if the scope is pointing to the zenit. As I have no seperate tests, I cannot be 100% sure, but I would bet it was the HSM 35 causing a slipping.

The HSM35 has no simple screws, which might be not tight enough. You have to play around with the positioning of the gears, which has to be in a very specific position to clamp the focusser. As you have to unmount the motor und unscrew the motor housing for each try, it was a very time consuming and frustrating business for me.

My next steps regarding focussing will be to get an overall flat field, with help of a tilt adapter which still needs to be adjusted and then I will finally find out, if it is possible to focus a FSQ85 with a 16200 sensor.

@step_change: as far I have seen, SGPro uses the intersection point of the green lines as focus point.

Best regards

Reinhard

@Reimer22 My understanding is that the imaging circle of the FSQ85 cannot fully illuminate the 16200 large sensor and a FSQ106 is needed to do that with its 88mm imaging circle at focus. That sensor size is effectively “wasted” with the Baby Q.
Oliver Penrice is an expert at this so contact him or @swag72

@step_change

According the specification the image field of the FSQ85 field is 44mm (without reducer). As the 16200 sensor is 27.2 x 21.8 (35 mm diagonal) this should no problem. For sure the illumination drops toward the borders. But this is no issue, as this can easily be corrected with flats.

The problem is to get an overal flat field. By design the flat field of the FSQ85 is only achieved in focus. And the tolerance is very samall. If you focus on a center star, It may be that the stars in the inner 50-75% of the image circle are very well regarding roundness, but in the corners all stars are distorted.

There were already some discussions regarding this topic.

See here:

The next step for me is to ensure that tilt is 100% eliminated. Then I can start again with autofocus supported by SGPro.

Best regards

Reinhard

No, SGPro will select the most appropriate focus position based on the intersection of two regressions or the weighted average of the lowest points. The actual focus position values captured during the run are just data and do not define themselves as the master list of available focus positions.