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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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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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I think from reading your telescope specs, and using 2 arcsecs seeing and 7% focus tolerance, you actually have 62.4um CFZ size (full width). Now, using your 1600 motor steps/rev divided by 2000 um/rev focuser travel, you have 0.8 steps/um or 1.25 um per step. So, you have 50 steps through your CFZ (+/- 25). This is fine. Read those mount instructions carefully. I suspect you'll be using the slow knob, not the 1/10 fine focus. You're sampled sufficiently. You will enjoy the motorized autofocus that Ekos provides. If you have access to a temperature sensor, you may even benefit down the road from automatic focus adjustments between exposures via AFC, but that's a topic addressed by a different thread. ;-) 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 #68136

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sorry for the dumb question, Doug; but, with the slow motion knob I mean the 1/10 fine focus; is not the same? I have a dual speed crayford, you know, the typical one. The motor is mounted and engaged directly in the axis where the orange knob was connected.
Regarding the cfw, well, I used a cfz calculator that does not take into account seeing and focus tolerance (I guess that's the old cfz :) ) In any case, it seems I'm enough sampled, I hope!
And yes, I also have a sensor temp connected to the rpi I'm using and weather watcher configured to take the most of it :)

cheers
1 month 1 week ago #68137

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These aren't dumb questions at all. Most people are confused the first time they do this. I mentioned reading those instructions carefully because many focusers are slightly different in how they mount. I don't have your focuser. What I see from the picture is that there's a brass colored knob (likely the fine focus), and a black knob (likely the coarse / slow) focus. In my case, my motor vendor had me remove both knobs and attach the motor coupler to the slow focus mechanism. If you connect to the fine focus, it will take 10x the number of counts to move a revolution. I'm sure that once you git fiddling with it, you'll get it figured out pretty quick.

On the CFZ, note that the "new" way of calculating this is somewhat important. The reference is a good read for why. You don't want to get complacent and assume your CFZ is actually 2x as big as it really is....that leads to wasted exposures.....
RASA11, Celestron CGX-L, ASI183mc Pro, 60mm guider + ASI290mm mini, ASI EAF focuser, PPB, Rpi4-4Gb+SSD, Powered USB3 hub, hardwire Ethernet.
Last edit: 1 month 1 week ago by dmsummers.
1 month 1 week ago #68138

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Well, if you're curious, have a look to the assembly process:









There you can see the motor is engaged into the slow motion knob axis :)
Regarding the number of counts per revolution, well, actually, yes, it would take 10x more counts to perform a full revolution of the normal/big (I don't know how to say sorry!) focus knob. Indeed, during the measurement of the drawtube travel, I counted the number of revs of the slow (orange) knob. So I guess it's right: the motor needs 1600 steps to perform itself a full rev, which translates in 16000 steps to make the "big" knob perform a revolution cause it's connected to the 1:10 axis. Sorry, I know, I'm not using the correct terms properly. Hope you know what I mean.

And regarding the cfz, you're right. Maybe I have to count with the worst case to perform the calculation.

m
1 month 1 week ago #68140

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I think you've got it. Nice looking focuser BTW. About having gone through the learning curve to knowing your CFZ size, it's going to help you down the road.

FWIW, I highly recommend you begin logging your autofocus results (you'll need to turn on verbose focus logging for that). This will begin to create a record of temperature versus focus position (plus some altitude residual information) for your gear. You only have 50 steps in good seeing to stay in the CFZ. Getting a feel for where you normally reach focus and how much it changes (in steps) is a good step towards knowing how often you need to autofocus. Autofocus has some definite quirks, and these are being at least partially addressed by Adaptive Focus Control (temperature and altitude focus compensation). That feature when released will automatically analyze your historical data and add some sophistication and efficiency to focus management. Wasting open-shutter time on AF runs isn't where most of us want to be spending time. ;-)

Edit: One thing I forgot to mention is that you'll also want to measure your focus motor's backlash. There's an INDI/Ekos setting for this that you should set once you know what it is. I was surprised to see mine was so big. You can "eyeball" this measurement if you don't have a caliper. Just hook ekos up to the focuser, and do manual moves in larger and smaller increments until you can see where reverse motion kicks in (via a coupler move). Once you get it as close as you can, set the backlash control. That will definitely help you down the road too.
RASA11, Celestron CGX-L, ASI183mc Pro, 60mm guider + ASI290mm mini, ASI EAF focuser, PPB, Rpi4-4Gb+SSD, Powered USB3 hub, hardwire Ethernet.
Last edit: 1 month 1 week ago by dmsummers.
1 month 1 week ago #68141

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Thanks again, Doug. Very useful your help and tips :)
Need to learn more about focus processing. It has more details and quirks than I initally thought.

I will also have a look to backslash and its measurement; which I'm not really sure.

Kind regards, Doug!
1 month 6 days ago #68187

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The learning curve is part of the fun... This part will be over quite soon enough. If confused, just ask (lots of folks are willing to provide help when you need it).

About backlash, simply put, this is just some mechanical slop in the gear mesh of your focuser motor. It's normal. The effect of backlash is that when your focus motor reverses direction, some amount of steps are required before that slop is taken up and the motor actually causes shaft motion. You can eyeball this by carefully watching your coupler's set screw or anything else (as close to the motor shaft as possible) that moves as you push focus-in or focus-out controls.

The test is pretty simple. To set up for the test, move one direction with a reasonably small jog until you know you're causing motion in that direction. Once you know you have motion in that direction, you're ready to begin the test. Using something like 20 steps (progressively refining as you start to learn what the real value is), jog focus in the REVERSE direction. Keep jogging IN THAT DIRECTION, noting total steps as you go. When you first detect motion, the total number of counts it took is your backlash value. Repeat the same test now in reverse again (the other way). Do those tests repeatedly (both directions) until you convince yourself you have the smallest number of steps required to cause focuser motor shaft motion in either direction. That's the number you want to set in the Ekos/Indi backlash value. FWIW, my EAF motor has 95 steps of backlash. An educated guess might be that you're looking for something on the order of 25 < backlash < 100? Hope this helps. Good luck...
RASA11, Celestron CGX-L, ASI183mc Pro, 60mm guider + ASI290mm mini, ASI EAF focuser, PPB, Rpi4-4Gb+SSD, Powered USB3 hub, hardwire Ethernet.
Last edit: 1 month 6 days ago by dmsummers.
1 month 6 days ago #68207

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Thanks Doug. I've been reading about focuser backlash. I have some experience with dec backlash in autoguiding, but that was quite automatic. This is a manual task. I've been reading how to do it.
But I've just noted that the backlash option within ekos is greyed out and cannot set it. I will open a new thread.

m
1 month 6 days ago #68252

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