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INDI Library v1.8.9 Released (01 Mar 2021)

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Question about focuser module

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Hi all,

I don't know where I read it, but it seems that the focus module to work properly needs to start from a bit focused image, and then, fine tune it. That is, it cannot start from a completely unfocused image. Is it true? If so, why? What's the theory behind this? I can only think that in the worst case it would take longer to focus, and that's it. Sure I'm missing something.

Thanks!
1 month 1 week ago #68030

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Replied by jerry on topic Question about focuser module

Yes, that's my (naive) understanding of the situation is that each V curve series of exposures works better when constrained to near focus, mostly for timing and the scale of the issue. My V curve runs over about 400 motor count values and it's a steep short V within this range. Given the range of values for my FocusCube and focus tube is about 30,000 it would take a while to find that narrow dip over a larger range.
120 MM Skywatcher Esprit on Celestron CGX, ZWO ASI 224MC guiding, Pegasus FocusCube v2
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1 month 1 week ago #68036

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Yep, as Jerry says.

What the focus routine does is measure the size of the star images at different focus positions. Imagine focuser positions along the xx axis and star sizes (in pixels or arc seconds) on the y axis. Focus is best when stars are smallest. Near the right focus, the graph looks like a V with the best focus at the bottom of the V. This only works near focus.

In practice, when you get WAY out of focus the software can't even FIND the stars and even if there are big round unfocused blobs it's hard to measure how big they are. Also for many scopes out-of-focus stars turn into "donuts" because the middle gets taken out by the shadow of the secondary mirror. At that point the software gives up trying to measure the stars.
1 month 1 week ago #68038

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Thanks for the replies, guys. Yes, that was just I thought.
And that leads me to another question. Usually, focus motors are stepper ones, is that right? And they have a defined number of steps, let's say from 0 to 30000, for example. Is that right? So, it can happen that it runs out of steps while trying to find the focus. That is, it would need a number less than 0 or higher than 30000 to find the correct focus. Something is wrong with my assumptions, sure. I like to understand how these things work
1 month 1 week ago #68050

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mgutierrez wrote: Thanks for the replies, guys. Yes, that was just I thought.
And that leads me to another question. Usually, focus motors are stepper ones, is that right? And they have a defined number of steps, let's say from 0 to 30000, for example. Is that right? So, it can happen that it runs out of steps while trying to find the focus. That is, it would need a number less than 0 or higher than 30000 to find the correct focus. Something is wrong with my assumptions, sure. I like to understand how these things work


You should set the zero position with focus tube all the way in, and also set the maximum amount of steps it can travel too, with tube all the way out, this way it will not go over or under these values, and focus point will be somewhere in between...if you don’t set these numbers, and you have a rack and pinion focuser, you can cause damage....
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Pegasus focus motors on all scopes
1 month 1 week ago #68055

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AstroNerd wrote: You should set the zero position with focus tube all the way in, and also set the maximum amount of steps it can travel too, with tube all the way out, this way it will not go over or under these values, and focus point will be somewhere in between...if you don’t set these numbers, and you have a rack and pinion focuser, you can cause damage....

Thanks for the reply.
I think I got it. So, the motor itself does not have a predefined total number of steps, but does have a prefedefined number of steps per revolution; is that ok? So, the procedure should be: 1) put the focuser manually all the way in; 2)set position to zero via software, with ekos; 3) put the focuser all the way out via sofware, to count the steps; 4) via software (ekos), set that maximum steps that has travelled.
is that right?
1 month 1 week ago #68057

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mgutierrez wrote:

AstroNerd wrote: You should set the zero position with focus tube all the way in, and also set the maximum amount of steps it can travel too, with tube all the way out, this way it will not go over or under these values, and focus point will be somewhere in between...if you don’t set these numbers, and you have a rack and pinion focuser, you can cause damage....

Thanks for the reply.
I think I got it. So, the motor itself does not have a predefined total number of steps, but does have a prefedefined number of steps per revolution; is that ok? So, the procedure should be: 1) put the focuser manually all the way in; 2)set position to zero via software, with ekos; 3) put the focuser all the way out via sofware, to count the steps; 4) via software (ekos), set that maximum steps that has travelled.
is that right?


Yes correct.
The motor will just keep goi g as long as the focuser will move, so it’s all about setting the limits for the length of drawtube you have on your focuser, and some motors will move more steps per inch then others depending on there geared ratio...mine moves 20,000 steps in total for my 50mm long focus travel, so that equates to 400 steps per mm of tube movement..so very fine adjustment...
Stellarmate OS on Raspberry pi4b
Skywatcher EQ8 pro, on steel pier
Takahashi FSQ85, FS60cb & Meade 8” SCT (de-forked)
Starlight Xpress SXVR H18, SXVR M25c, Lodestar x2 Guide Camera
Pegasus Ultimate Hub V2 for all USB & Power
Pegasus focus motors on all scopes
1 month 1 week ago #68059

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AstroNerd wrote: Yes correct.
The motor will just keep goi g as long as the focuser will move, so it’s all about setting the limits for the length of drawtube you have on your focuser, and some motors will move more steps per inch then others depending on there geared ratio...mine moves 20,000 steps in total for my 50mm long focus travel, so that equates to 400 steps per mm of tube movement..so very fine adjustment...

Great, thanks for the explanation.
I've just bought (not received yet) an electronic focuser (16000 steps per rev) for my refractor (crayford focuser). It is mounted directly on the fine focusing knob so I guess it will have to count a huge amount of steps to cover the whole drawtube travel. Well, I will have to try and test several times to see how this model works.

Thanks again to everybody for the replies!

m
1 month 1 week ago #68061

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A couple of thoughts here. First, can you post your electronic focuser model? 16K steps/revolution is quite a bit! Could this be a typo? Second, it's been my experience that motor vendors typically recommend *not* using the fine focus shaft for attaching the motor. In your case, with 16K counts/rev, you may find this will be problematic (overkill on resolution). What you REALLY want is to match your motor counts per revolution and distance of focuser drawtube motion (commonly referred to as thread pitch) such that the number of motor counts per micron (motor + focuser) is well matched to your telescope's Critical Focus Zone (which is a function of your telescope's aperture and f-ratio).

You've got motor counts per revolution (but it should be verified). Your next unknown to resolve should be to know how far (microns) your telescope's focuser drawtube moves per knob revolution. Once you know this, you can detemine motor counts/micron. That's an important number because you can then evaluate it against your telescope's CFZ . You want enough "resolution" to have at least several counts through the CFZ. Too many is overkill, and too few will have you jumping over the CFZ.

If you're confused about CFZ size and you haven't already looked at this page, it should help:
www.goldastro.com/goldfocus/ncfz.php
Don't let the equation scare you.... If you use seeing of ~2 arcsecs, and a focus tolerance of say 7%, you can calculate a good value for CFZ. Finding your focuser drawtube distance per knob revolution will finally get to your counts/micron. Good luck...
Cheers, Doug
RASA11, Celestron CGX-L, ASI183mc Pro, 60mm guider + ASI290mm mini, ASI EAF focuser, PPB, Rpi4-4Gb+SSD, Powered USB3 hub, hardwire Ethernet.
1 month 1 week ago #68078

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dmsummers wrote: A couple of thoughts here. First, can you post your electronic focuser model? 16K steps/revolution is quite a bit! Could this be a typo? Second, it's been my experience that motor vendors typically recommend *not* using the fine focus shaft for attaching the motor. In your case, with 16K counts/rev, you may find this will be problematic (overkill on resolution). What you REALLY want is to match your motor counts per revolution and distance of focuser drawtube motion (commonly referred to as thread pitch) such that the number of motor counts per micron (motor + focuser) is well matched to your telescope's Critical Focus Zone (which is a function of your telescope's aperture and f-ratio).

You've got motor counts per revolution (but it should be verified). Your next unknown to resolve should be to know how far (microns) your telescope's focuser drawtube moves per knob revolution. Once you know this, you can detemine motor counts/micron. That's an important number because you can then evaluate it against your telescope's CFZ . You want enough "resolution" to have at least several counts through the CFZ. Too many is overkill, and too few will have you jumping over the CFZ.

If you're confused about CFZ size and you haven't already looked at this page, it should help:
www.goldastro.com/goldfocus/ncfz.php
Don't let the equation scare you.... If you use seeing of ~2 arcsecs, and a focus tolerance of say 7%, you can calculate a good value for CFZ. Finding your focuser drawtube distance per knob revolution will finally get to your counts/micron. Good luck...
Cheers, Doug


Hi Doug,

To be honest, I'm not completely sure.
The focuser in particular is this one: en.rbfocus.net/product-page/motor-de-enf...nico-rb-focuser-v1-0
As you can see, it is based on a nema 17 motor but it seems it has 1:10 reduction gear embedded and the motor working at 1600 steps. Not sure about it. I will have a look to the provided link. Thanks!
1 month 1 week ago #68087

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From the specs you posted, you've got it right.....16K steps/revolution. Nice motor resolution! Ok, now you want to measure your telescope focuser drawtube motion (unless you already know it). If you don't know this, just rack the focuser some measurable distance while paying attention to the number of revolutions. Divide the travel distance by the number of revolutions, and convert to microns. Finally 16K cnts/rev divided by N microns/rev focus travel = Y motor cnts per micron travel. Now compare to your CFZ size, and you'll know how well your motorized focuser will sample your CFZ. I suspect you're going to have plenty (maybe too many) counts through your CFZ. Being oversampled isn't really a problem other than it works the motor a bit more. Better oversampled than undersampled!
RASA11, Celestron CGX-L, ASI183mc Pro, 60mm guider + ASI290mm mini, ASI EAF focuser, PPB, Rpi4-4Gb+SSD, Powered USB3 hub, hardwire Ethernet.
1 month 1 week ago #68099

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dmsummers wrote: From the specs you posted, you've got it right.....16K steps/revolution. Nice motor resolution! Ok, now you want to measure your telescope focuser drawtube motion (unless you already know it). If you don't know this, just rack the focuser some measurable distance while paying attention to the number of revolutions. Divide the travel distance by the number of revolutions, and convert to microns. Finally 16K cnts/rev divided by N microns/rev focus travel = Y motor cnts per micron travel. Now compare to your CFZ size, and you'll know how well your motorized focuser will sample your CFZ. I suspect you're going to have plenty (maybe too many) counts through your CFZ. Being oversampled isn't really a problem other than it works the motor a bit more. Better oversampled than undersampled!


Hi Doug,

First of all, thanks for the links and info. I had completely no idea about those concepts (cfz, etc.). In fact I need some more time to digest :) but I think that more or less I get the point.
First. The focuser has just arrived. Now I think I misunderstood what the manufacturer said and what is published in their site. The motor is engaged into the low motion knob of the crayford focuser: that's the, I guess, reduction 1:10 gear he meant. So, it is not 16k steps per motor rev, but 1600, and, that's it, 16k steps per revolution of the main focuser knob. I don't know if I'm explaining correctly and, if so, is ok; but I guess. In fact, I've connected the focuser to ekos (perfectly detected, by the way) and a 1600 steps motion order sent to the motor makes it a full (360 degrees) revolution. So, the motor itself has (if I'm not wrong) 1600 steps per full revolution.
Second. According, Doug, to your explanation (thanks again!), my TS102 (f7) refractor has a (more or less) 120um of cfz. I've also measured the drawtube travel per low focuser knob revolution and after the calculations it results in ~2000um per rev (more or less, the drawtube travels 1cm per 5rev, that makes 2000um per rev). So, 1600/2000 ~ 0.8 counts per micron travel (that is, 1.25um per count). Since I guess we can only send discrete movements (steps, counts) to the motor, every step "action" movement I send to the motor, makes the drawtube travel 1.25um.
Third. As stated, my scope has a ~120um of cfz. As I have understood, the cfz is basically the amount of um I can move the focuser maintaining a sharp image. So, under a perfect focus (that is, I'm just in the middle of the cfz), I can send the motor up to (120/2)/1.25=48 steps and still keep a perfect and sharp image. And this would be the resolution I would expect from the motor and the focuser system. I don't know, however, if consider I would be under or oversampled.

Doug, I would really appreciate your comments about these thoughts. I'm still quite newbie and these terms and concepts are not still completely clear to me and all of this reasoning is, well, just that, reasoning from what I have understand from the info I've read.

Thanks for your help!
1 month 1 week ago #68133

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