SGPro Auto Focus - flat graph between data points

[Andyb90]

Hi Everyone,

I have a Lakeside motor focuser unit for my Esprit 100 scope and I'm using SGPro auto focus with it. The generated V curve, with 9 points, looks okay except that its flat between points 1 and 2, which I think is skewing the focus point. If I operate the focuser manually the action looks smooth. There doesn't seem to be a mechanical problem.

I followed the 'Understanding Auto Focus' guide in the SGPro help to do the initial setup.

Any advice on what might cause this issue and how to resolve it would be appreciated. I'm wondering if there is a setting I need to tweak in SGPro.

[iapa]

I am wondering if it the the step size that you have selected.

May be that it is not enough to actually change focus, then the 3rd get you moving?

Don't have a Lakeside tho', use the SkyWatcher and I needed to increase the 'big' step size to 20 with 'small' at 5.

[Davey-T]

How did you work out your initial step size ?

There is an Astro Imaging Channel video on SGP autofocus on YT, will have a look for it

Dave

Here you go

https://www.youtube.com/watch?v=bB84JS5Uefo

 

[Barry-Wilson]

Andy

The 'flat' area between the first two points is characteristic of backlash in the R&P focuser, quite normal as you no doubt know.  This effect has been documented on the SGP forum a number of times.

I would experiment with a backlash setting of say 50 steps IN, and see if this eliminates the kink in the V-curve.

The R&P focusers on my 3 scopes all have backlash and using the backlash compensation settings in SGP solved this issue and gave me a symmetrical curve.

HTH.

[Andyb90]

3 hours ago, Davey-T said:

How did you work out your initial step size ?

There is an Astro Imaging Channel video on SGP autofocus on YT, will have a look for it

Dave

Here you go

https://www.youtube.com/watch?v=bB84JS5Uefo

 

I followed the instructions on the 'Understanding auto focus' page in the SGPro help to calculate the step size. I used the calculation, which gave me a step size of 20.

Andy.

[Andyb90]

2 hours ago, Barry-Wilson said:

Andy

The 'flat' area between the first two points is characteristic of backlash in the R&P focuser, quite normal as you no doubt know.  This effect has been documented on the SGP forum a number of times.

I would experiment with a backlash setting of say 50 steps IN, and see if this eliminates the kink in the V-curve.

The R&P focusers on my 3 scopes all have backlash and using the backlash compensation settings in SGP solved this issue and gave me a symmetrical curve.

HTH.

Ah right I hadn't realised there was an option to compensate for backlash. Will try it set to 50 steps IN and see how I get on. Thanks for the advice.

Andy.

[Davey-T]

10 minutes ago, Andyb90 said:

I followed the instructions on the 'Understanding auto focus' page in the SGPro help to calculate the step size. I used the calculation, which gave me a step size of 20.

Andy.

According to Lakeside if you drive the focuser in using the Lakeside Utilities or the Lakeside controller, measure the gap between focuser and scope using calipers, drive the focuser out 1000 steps and remeasure then divide the result by 1000 it gives you the step size in microns.

Also says you should set some backlash but it's not critical, I've got mine set to 20

Dave

[Barry-Wilson]

Hi Andy

As a guide, I would say that a good step size to use as a starter would be 20 as Davey suggests, perhaps increasing by 5 each time you try a V-curve until you are satisifed with the curve.  My FSQ106 and FSQ85 both have a step size of 21, 9 points; my WO 132FLT uses a step size of 45, 9 points.  You are aiming for the range of HFR between the minimum value at the bottom of the curve to the highest value at the end of the curve to be 2x or 3x.

HTH

[Andyb90]

17 hours ago, Barry-Wilson said:

Hi Andy

As a guide, I would say that a good step size to use as a starter would be 20 as Davey suggests, perhaps increasing by 5 each time you try a V-curve until you are satisifed with the curve.  My FSQ106 and FSQ85 both have a step size of 21, 9 points; my WO 132FLT uses a step size of 45, 9 points.  You are aiming for the range of HFR between the minimum value at the bottom of the curve to the highest value at the end of the curve to be 2x or 3x.

HTH

I've updated my profiles to backlash comp 50 IN and will start from step size 20 and see how I get on.

Also just wanted to check does the 'Focus pt' need to be set with approximate values for the filters in the filter setup window (shown below) ? Or does it not matter if you are approximately focused when the auto focus routine starts?

Andy.

[Barry-Wilson]

Hi Andy

The short answer is it doesn't matter, it only matters if you choose the option of having the focuser move to the absolute position of the filter.

As I image on a permanent pier I leave my focuser at the position it remains after the end of a night, if needed I will move it out a few multiples of my step size at the beginning of a following night's imaging.

For most instances, the relative position of the focus point for each filter is the important information that SGP uses if you have checked the option to adjust focus per filter.  Then if you have filter off-sets enabled the focuser will adjust to the offset you have specified when you move between filters; you can still enable AF at filter change of course if you wish (I do).

8 seconds per sub is also a good place to start.  I also use a dark frame of the same duration as my AF sub and this will help too.

[Andyb90]

48 minutes ago, Barry-Wilson said:

Hi Andy

The short answer is it doesn't matter, it only matters if you choose the option of having the focuser move to the absolute position of the filter.

As I image on a permanent pier I leave my focuser at the position it remains after the end of a night, if needed I will move it out a few multiples of my step size at the beginning of a following night's imaging.

For most instances, the relative position of the focus point for each filter is the important information that SGP uses if you have checked the option to adjust focus per filter.  Then if you have filter off-sets enabled the focuser will adjust to the offset you have specified when you move between filters; you can still enable AF at filter change of course if you wish (I do).

8 seconds per sub is also a good place to start.  I also use a dark frame of the same duration as my AF sub and this will help too.

Okay, will leave the focus pts alone at the moment. I'll also setup some dark frames too. I did a couple of tests with the camera uncooled, but I guess it makes more sense to have it cooled for testing.

Andy.