# Starting out with a Pegasus motor focuser

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Just now, StuartT said:

so I assume the white points on the graph are actual measurements of focus (star width?). So if those are actual measurements, surely I can just read off the x-axis number of the lowest one and enter it into the focuser? Then it should be in focus, right? I assume 50 or so either side won't make a lot of odds

No, those white dots are the point in which the image is taken to check focus, each white dot is an image to check, as the graph goes down it means the star is getting more into focus, as it starts to go up it’s going out of focus, so your focus point is at the very bottom, of the V curve, so in the example  above that would be approx 24600 steps…

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If you look at the bottom under the graph where it says position, 24.616, that is the focus point in steps…

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18 minutes ago, Stuart1971 said:

No, those white dots are the point in which the image is taken to check focus, each white dot is an image to check, as the graph goes down it means the star is getting more into focus, as it starts to go up it’s going out of focus, so your focus point is at the very bottom, of the V curve, so in the example  above that would be approx 24600 steps…

Yes. That's what I meant. Sorry, I said lowest one, but I meant the x-value of the y minimum point, so 24600.

Problem is, now I have attached a motor, I can't use a Bhatinov mask like I used to (because the motor seems to prevent you turning the focus knob). Unless I just take the motor off the scope.

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1 minute ago, StuartT said:

Yes. That's what I meant. Sorry, I said lowest one, but I meant the x-value of the y minimum point, so 24600.

Problem is, now I have attached a motor, I can't use a Bhatinov mask like I used to (because the motor seems to prevent you turning the focus knob). Unless I just take the motor off the scope.

You can use the motor manually with the in and out buttons in the software, and take an image and then tweak it…moving say about 20 to 50 steps at a time…

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Why do you need to use a mask, if focus is good it should set it for you at the end of the autofocus routine…?

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5 minutes ago, Stuart1971 said:

You can use the motor manually with the in and out buttons in the software, and take an image and then tweak it…moving say about 20 to 50 steps at a time…

excellent! Thanks! I don't want to waste a clear night because I can't work my new focuser!

Only problem is, it's very hard to judge what the best focus is on a laptop screen! It's going to be around there though, I guess. 20 or so either side prob isn't noticeable

Edited by StuartT
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Just now, StuartT said:

excellent! Thanks! I don't want to waste a clear night because I can't work my new focuser!

The autofocus routine seems to have done it’s job, so just go with that, take a 3 min image and check the stars…

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26 minutes ago, Stuart1971 said:

Why do you need to use a mask, if focus is good it should set it for you at the end of the autofocus routine…?

So this is the bit I don't understand. I was thinking the backlash meant the autofocus doesn't work? But maybe that is wrong?

Do you mean that the flat part on the right doesn't mean the minimum is in the wrong place?

Edited by StuartT
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My understanding is the flat part of the graph is indicating the slack (backlash) in the focuser/motor is being dealt with by the first few steps, once this is done the  autofocus routine continues, as Stuart says, run the routine, take an image and check how good the stars are

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11 hours ago, dark knight said:

My understanding is the flat part of the graph is indicating the slack (backlash) in the focuser/motor is being dealt with by the first few steps, once this is done the  autofocus routine continues, as Stuart says, run the routine, take an image and check how good the stars are

Exactly…
there is always some play in a mechanical gear system, like a rack and pinion focuser, so when you move from inward to outward it changes direction in the gearing and so has to remove any slack first, before it will move the actual gears..as you see in the image below, this then means that a certain amount of steps is needed to remove this backlash first before any movement will be noticed…which are the flat parts on your V curve graph…

Edited by Stuart1971
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Thanks. That's a nice illustration.

Except that I don't think this is backlash, as even dialling the OUT backlash to 1000 doesn't fix it. So there must be something else going on. I think I probably need to dismantle it and try again. Maybe the grub screw isn't quite on the flat part of the shaft or something.

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9 minutes ago, StuartT said:

Thanks. That's a nice illustration.

Except that I don't think this is backlash, as even dialling the OUT backlash to 1000 doesn't fix it. So there must be something else going on. I think I probably need to dismantle it and try again. Maybe the grub screw isn't quite on the flat part of the shaft or something.

If something was slipping, then you would have flat parts all over the graph, not just at the start, this shows it’s backlash….

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3 hours ago, Stuart1971 said:

If something was slipping, then you would have flat parts all over the graph, not just at the start, this shows it’s backlash….

ok, take the point. But isn't a setting of 1000 going to take care of that?

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2 minutes ago, StuartT said:

ok, take the point. But isn't a setting of 1000 going to take care of that?

Yes it should do, have you tried putting the backlash setting in the other box, and then put zero in the one you are using ATM…? Just don’t put it in both places…

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4 minutes ago, Stuart1971 said:

Yes it should do, have you tried putting the backlash setting in the other box, and then put zero in the one you are using ATM…? Just don’t put it in both places…

yes. I tried that last night. The graph looked even worse.

Maybe the focuser motor is defective?

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9 minutes ago, StuartT said:

yes. I tried that last night. The graph looked even worse.

Maybe the focuser motor is defective?

Well you can feel backlash, when the focus motor is fitted and all tightened up, try turning the other focus knob, if there is a lot of backlash you feel it, it will rock back and fourth, like there is play in it….

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3 hours ago, Stuart1971 said:

Well you can feel backlash, when the focus motor is fitted and all tightened up, try turning the other focus knob, if there is a lot of backlash you feel it, it will rock back and fourth, like there is play in it….

you know what? I now feel like a dummy.

Just been out to check the rig and sure enough, there was quite a bit of play between the coupler/shaft when I turned the coarse knob! I was SURE I'd tightened that up! Anyway, I have now tightened both grub screws really tight and there is now no play (at least, no visible play). So if I get a clear night again tonight, I'll run another autofocus curve.

Edited by StuartT
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Yes!! I can't tell you how pleased this makes me

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Good to hear.

I read up a bit moe on autofocusing and how the settings in NINA worked as well as looking for details on the Pegasus Focuer.

On average it seems the adjustment is around 100, for the NINA setting you add a safety margin to that so about 130 would be a figure to use if it works as expected.

A youtube video showed someone measuring the backlash in downtime (cloudy or during day)

Basically you start at zero position and tell focuser to goto 1000 - that sets the gears ready to start.

Next you measure the focus extended length call it L1.

Then tell the focuser to got 5000 and measure that and call that L2.

Lastly, tell the focuser to go back to 1000 and measure again call that L3. If there is backlash L3 will be less than L2  - the difference being the backlash.

You could then tell the focuser to go in with 10 steps at a time until L3 = L2

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1 hour ago, StevieDvd said:

Good to hear.

I read up a bit moe on autofocusing and how the settings in NINA worked as well as looking for details on the Pegasus Focuer.

On average it seems the adjustment is around 100, for the NINA setting you add a safety margin to that so about 130 would be a figure to use if it works as expected.

A youtube video showed someone measuring the backlash in downtime (cloudy or during day)

Basically you start at zero position and tell focuser to goto 1000 - that sets the gears ready to start.

Next you measure the focus extended length call it L1.

Then tell the focuser to got 5000 and measure that and call that L2.

Lastly, tell the focuser to go back to 1000 and measure again call that L3. If there is backlash L3 will be less than L2  - the difference being the backlash.

You could then tell the focuser to go in with 10 steps at a time until L3 = L2

Great! I shall try this. Thanks

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3 hours ago, StuartT said:

Yes!! I can't tell you how pleased this makes me

Absolutely perfect….So what backlash setting was that with..?

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35 minutes ago, Stuart1971 said:

Absolutely perfect….So what backlash setting was that with..?

Backlash and step size both at 150

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On 22/09/2021 at 18:55, StuartT said:

thanks so much! super useful!

Ok, so I have loosed the grub screws and wound the focuser all the way in, then a little way out (as I wanted one of the grub screws to be on the flat part of the shaft - that seems important!)

And I opened the Pegasus app and reversed the motor direction.

Now... I have set the focuser position in NINA to zero. Is that what you mean by syncing it?

I think this is being over complicated you did not need to ditch the focuser. There is a stand alone Pegasus astro app (focuser setup v4.4.5)  that you get from here:

It will allow you to reset zero, remove motion limits, reverse the direction of movement, set limits of motion and everything else you need to do. You really don't need to be removing grub screws and should not be trying to set it up via Nina.

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3 hours ago, Adam J said:

I think this is being over complicated you did not need to ditch the focuser. There is a stand alone Pegasus astro app (focuser setup v4.4.5)  that you get from here:

It will allow you to reset zero, remove motion limits, reverse the direction of movement, set limits of motion and everything else you need to do. You really don't need to be removing grub screws and should not be trying to set it up via Nina.

Why not…?

I have never used the pegasus software, I have always set my focuser limits in the software that I am going to use to control the focuser and do my imaging with, and besides the settings are all saved in the motor eprom board in the controller, so if you do open the Pegasus software the settings are all identical anyway….

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3 hours ago, Adam J said:

I think this is being over complicated you did not need to ditch the focuser. There is a stand alone Pegasus astro app (focuser setup v4.4.5)  that you get from here:

It will allow you to reset zero, remove motion limits, reverse the direction of movement, set limits of motion and everything else you need to do. You really don't need to be removing grub screws and should not be trying to set it up via Nina.

Thanks Adam. I did eventually get it sorted out. It was in fact a mechanical problem. My fault for not tightening the screws enough. No app can fix that!

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