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Electric focuser advice please......

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#1 tbone0168

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Posted 25 December 2014 - 04:07 PM

I'm thinking about getting an electric focuser for my 102mm scope but have no idea what qualities to look for.   :help: please!  Thanks! 

Terrance



#2 coopman

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Posted 25 December 2014 - 04:16 PM

Jim's Mobile has a growing line of their highly rated MOTOFOCUS units:
http://www.jimsmobil...lore_Scientific

#3 airbleeder

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Posted 25 December 2014 - 06:36 PM

I use an Orion accufocus that works very well. About $80.



#4 oo_void

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Posted 25 December 2014 - 11:51 PM

First off, if you're wanting to motorize for imaging you're going to want a stepper. I'd highly recommend Rigel Systems, http://rigel.datacor...s/rigelsys.html. Not only does he make and design the controllers that most other manufacturers use, he makes a set of nice after market motors and mounts that play nice with most focuser types. What I like is that they can either be easily disengaged, or even removed for those times when you want to just go visual, unlike others. To top it off, they're relatively cheap too.

 

Send him an email and he'll help identify what mounting and motor will work best with your scope.  



#5 jbalsam

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Posted 26 December 2014 - 09:42 AM

If you want to be able to do autofocusing, get a focuser solution that reports absolute position: FLI Atlas ($2500), Optec TCF ($1800), Starlight Feathertouch + microtouch (~$1200ish), Moonlite focuser + stepper (~$1000), Rigel systems ($185-$550), Robofocus ($500)... maybe others that I don't remember. The top two options there (FLI and Optec) are suitable for very heavy imagine loads (25 lbs for the Atlas), and are overkill if your imaging rig is 5lbs at its heaviest. 

 

If you just want to have a control box that lets you move your focuser without you needing to touch your telescope, get a DC focus motor to attach to your existing focuser: JMI MotoFocus (~$180), Orion AccuFocus (~$70), or a DC motor solution from one of the above companies. 

 

Side note: You can do autofocusing with a DC motor, but that method is not as robust as an absolute focuser, so it's not as suitable for unattended imaging. I used one of JMI's heavy duty DC motors (they sell them as Cassegrain focus motors, they are far more powerful and robust than their "refractor" models) to drive the coarse focus knob on several telescopes, and I was able to autofocus reasonably well. The system would occasionally drift, and obtaining good focus with a system like FocusMax took some playing around. It can be done, but I don't recommend it for remote imaging, or for all-night unattended sessions. Also: never attach a focus motor to the fine focus knob of your scope if you want repeatability (many of JMI's adapters connect to the fine focus knob). The way most of those fine focus knobs are designed allows for a lot of slippage to occur. The coarse knob is the better solution, and most focus motors have fine enough resolution that you can still focus successfully using the coarse knob. 



#6 Jeff Struve

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Posted 29 December 2014 - 02:30 PM

Yep... I swear by Rigel Sys... I have 5 motors and brackets and 2 handcontrollers....Leon is great to work with...



#7 WebFoot

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Posted 29 December 2014 - 02:58 PM

Just a few words of caution.

 

I agree with others that, if you want to be able to focus precisely for imaging, you will need an absolute position system, like Robofocus.  Otherwise, you just end up chasing your tail.  My first good one was a JMI Crayford-style focuser, with the Motofocus replaced by a Robofocus (the Motofocus simply wasn't accurate enough for precise focusing).  It worked like a champ.

 

Related to that, I find a motorized focuser for viewing to be an exercise in frustration.  To me, there's no substitute for having a fine focuser with my fingers on it, getting it just right.  But if you want a motorized focuser for viewing, I caution you against focusers that run on a bit after you tell them to stop (that's essentially all motors that aren't absolute position ones).

 

Mark



#8 oo_void

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Posted 29 December 2014 - 04:59 PM

The other thing that I'd like to add about Rigel Sys is that once you have the motor and the controller, you can most likely move it between similar focuser types by purchasing additional mounting brackets. I can use my motor and controller on the basic GSO crayford on my AT6RC, or easily move it to the rack and pinion focuser on my AT65EDQ. 


Edited by oo_void, 29 December 2014 - 05:00 PM.


#9 Rocketman4992

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Posted 29 December 2014 - 08:13 PM

Lots of stepper motor options to choose from--Rigel, JMI, Moonlite. How does one make an informed decision? I'm guessing increasing costs relate to weight of your imaging setup and desired focus precision. I've been looking at a Rigel system for my Atik 314 and four position filter wheel. Moonlite looks like a great product but I'm not thrilled about having to replace my WO focuser to work with their stepper motors.



#10 Ed Wiley

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Posted 29 December 2014 - 08:44 PM

I use Moonlite and like their focus units a lot. But if your WO focus unit works for you, try an add-on product. In my case (four Moonlites) I replaced the stock focus units on my Schmidt-Newtonians and added one to my C11.  I replaced the Rigel unit that came with my Moonlite on the Royce with a motor unit made by Moonlite. One noice thing about the new Moonlite is that the motorized units now have a clutch. Don't know if the competitors have one or not, but it is convenient.

 

Ed



#11 frolinmod

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Posted 31 December 2014 - 12:49 AM

Yep... I swear by Rigel Sys... I have 5 motors and brackets and 2 handcontrollers....Leon is great to work with...

 

Yup, I have a bunch of 'em too and I agree about Leon. :-)



#12 Jeff Struve

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Posted 31 December 2014 - 09:29 AM

With my limited experience, an electronic focuser... like the Rigel Sys... makes a marginal focuser much better... I can adjust the amount/speed of the motor movement much more finely than i could ever consistantly do by hand... also, the Rigel, since uses a bracket and gear mechanism under your current focuser, does not require a different focuser... you can lossen the motor from the braket a bit and manually focus if you want...

 

I am at the point of using my laptop to auto focus.. because i wear tri-focals... sometimes it is dificult fine tuning focus for AP... with the Rigel and  my laptop, I expect consistant results... 

 

I have them on my Orion Dob... Celestron SCT... Explore Scientific Triplets... and Lunt



#13 TonyMcGrath

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Posted 01 January 2015 - 07:46 AM

I recently purchased the Rigel nFocus system for the William Optics 2 speed focuser attached to my SCT.  While my exact focuser was not listed on the Rigel Systems web site, a couple of emails with Leon Palmer of Rigel quickly sorted things out.  Leon was most helpful and always prompt to reply.  When the focuser arrived, the installation was simple and I was up and running very quickly.  The focuser works exactly as advertized.  The focuser controller sends  a train of 12 volt pulses to the focuser motor when the focus button is pressed.  You can quickly adjust both the pulse width and pulse rate via two knobs on the controller, making it a snap to tune the system to your requirements.  Switching between high and low speed is also very easy.  There is a video on YouTube where you can see the system operate...the entire experience exceeded my expectations.  Highly recommended.



#14 KD58

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Posted 01 January 2015 - 08:18 PM

I have placed Moonlite focusers on an ES 127 APO, AT65EDQ, WO 81, SV105T, TS65 Quad, and have replaced the stock focuser on my Takahashi 130 FNB with a Moonlite CXL focuser. All have been motorized. It's tough to beat Ron's service and the quality of the focusers. They are aesthetically very pleasing too. I'm sure the Rigel system focusers are great too so the choices are there to be made. Just wanted to point out my experience with the Moonlite focusers which has been extremely positive. Feathertouch focusers are great too and I have had FT focusers on my SV105, and on my C11 Edge HD (microfocuser and crayford) but have become so used to the Moonlites that I replaced the FT crayford with a Moonlite. It certainly adds weight and the capacity of the mount will have bearing on the focuser to be used. The FT focusers are certainly lighter than the Moonlites. I cannot comment on the Rigel system focusers.

 

khavar



#15 tbone0168

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Posted 14 January 2015 - 04:23 PM

Thank you all for your responses!.......

 

Clear, dark skies!

 

Terrance :)



#16 eoverstreet

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Posted 11 April 2015 - 09:35 AM

Can a focuser, like the JMI Motofocus, move the focuser to far to the left or right and harm the focus system (perhaps mirror) in a reflector scope, SCT



#17 jbalsam

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Posted 13 April 2015 - 12:00 PM

I've never seen a focuser design that could do this.

 

The focuser needs to be designed so that it is mechanically constrained from doing this. Meaning that there is some physical stop to prevent it from contacting an optical element. For example, with Newtonian systems this is done by making the focuser draw tube short enough that it couldn't run into the secondary mirror if you rack it all the way in.

 

With SCTs, if you are using the built in focus mechanism that moves the primary mirror, that mechanism should be designed such that the mirror can't be damaged by moving the mechanism too far in or out.

 

This type of mechanical design has nothing to do with a focuser being motorized or not. When you add a motor, the worst case scenarios would be moving the focuser too far in or out so it hits one of these mechanical limits. If the focus motor doesn't know to stop trying to move in that direction at that point, it could burn out the motor or possibly damage a mechanical component of the focuser if the motor is too strong. Many designs will have a mechanism that allows for slip in the case of a mechanical stall like this. With crayford focusers, the design can naturally slip. For rack and pinion focusers, there needs to be a slip mechanism somewhere else in the system if mechanical stalls are expected. I don't know exactly how SCT focus mechanisms work, so I don't know if they have a slip mechanism or a "weakest link" component.

 

With the JMI Motofocus, you are just telling the motor to turn in one direction or the other. It has no knowledge of where the focuser draw tube actually is, so you can make it go too far in or out. If you have a rack and pinion focuser and you have the motofocus attached to the fine focus knob of your focuser, that's okay.... because the way fine focus knobs are designed they naturally allow slipping (all the ones I've seen at least). If you have it attached to the coarse focus knob, then there may not be a good mechanism to allow slip, and you risk burning out the motor if you force it in one direction continuously. The JMI Motofocus units that they market for SCT focusers are quite strong and might be capable of damaging a cheap focusing system if there was no slip mechanism built in. The ones marketed for use with fine focus knobs on other scopes are very weak and easily stalled (this makes them safer - the motor stalls before it damages a mechanical element).

 

More advanced electronic focusers (those with stepper motors instead of DC motors) will try to stop the focuser from moving before they reach a hard mechanical stop. However even these devices aren't fool-proof. I don't know of any focuser that uses a fully closed-loop feedback control system to monitor its position. Even the FLI focusers use a mechanical home position and a known stepper motor step-size to estimate their movement (they happen to be fairly good at this).



#18 rigelsys

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Posted 14 April 2015 - 01:27 PM

I've never seen a focuser design that could do this.

 

The focuser needs to be designed so that it is mechanically constrained from doing this. Meaning that there is some physical stop to prevent it from contacting an optical element. For example, with Newtonian systems this is done by making the focuser draw tube short enough that it couldn't run into the secondary mirror if you rack it all the way in.

 

With SCTs, if you are using the built in focus mechanism that moves the primary mirror, that mechanism should be designed such that the mirror can't be damaged by moving the mechanism too far in or out.

 

This type of mechanical design has nothing to do with a focuser being motorized or not. When you add a motor, the worst case scenarios would be moving the focuser too far in or out so it hits one of these mechanical limits. If the focus motor doesn't know to stop trying to move in that direction at that point, it could burn out the motor or possibly damage a mechanical component of the focuser if the motor is too strong. Many designs will have a mechanism that allows for slip in the case of a mechanical stall like this. With crayford focusers, the design can naturally slip. For rack and pinion focusers, there needs to be a slip mechanism somewhere else in the system if mechanical stalls are expected. I don't know exactly how SCT focus mechanisms work, so I don't know if they have a slip mechanism or a "weakest link" component.

 

With the JMI Motofocus, you are just telling the motor to turn in one direction or the other. It has no knowledge of where the focuser draw tube actually is, so you can make it go too far in or out. If you have a rack and pinion focuser and you have the motofocus attached to the fine focus knob of your focuser, that's okay.... because the way fine focus knobs are designed they naturally allow slipping (all the ones I've seen at least). If you have it attached to the coarse focus knob, then there may not be a good mechanism to allow slip, and you risk burning out the motor if you force it in one direction continuously. The JMI Motofocus units that they market for SCT focusers are quite strong and might be capable of damaging a cheap focusing system if there was no slip mechanism built in. The ones marketed for use with fine focus knobs on other scopes are very weak and easily stalled (this makes them safer - the motor stalls before it damages a mechanical element).

 

More advanced electronic focusers (those with stepper motors instead of DC motors) will try to stop the focuser from moving before they reach a hard mechanical stop. However even these devices aren't fool-proof. I don't know of any focuser that uses a fully closed-loop feedback control system to monitor its position. Even the FLI focusers use a mechanical home position and a known stepper motor step-size to estimate their movement (they happen to be fairly good at this).

Stepper motor controllers do that, in "absolute" mode, you run the focuser all the way in set the position as 0 and then run to the other end, set as max position and software will keep the focuser in between the two end positions. For example in the usb-nSTEP

 

"Absolute: Select Absolute Focuser Type, rack focuser all the way in (to 'Home') and then press Reset Position. The gcusb-nSTEP software will will set the racked-all-the-way-in position to 00000. Absolute Focuser Type limits focuser position to between 00000 and Maximum Position in ASCOM applications. Maximum Position: Value is used by the Absolute Focuser"



#19 jbalsam

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Posted 16 April 2015 - 07:01 AM

Rigel -

 

My point was that the stepper-based focusers that I have seen don't use a fully closed-loop control system. They are open-loop controllers. You can establish a zero and a max position, and that usually works well if the focuser is well designed, but while the focuser is moving from zero to max, it doesn't know absolutely that it is where it thinks it is. It is operating under the assumption that 1 step command issued equals one step command properly executed by the focuser. If something happens in the focuser mechanism that causes it to slip (for example - if a drive belt slips or breaks), the focuser doesn't know that something has gone wrong. It has no independent way of verifying that when the stepper motor made one step, the focuser face moved the correct amount.

 

In a fully closed-loop control system, you would be actively measuring the displacement of the focuser, or you would be actively measuring some feature of the motion that was directly tied to the movement of the focuser. This might be something like having an encoder that reads the rotation of the lead screws that are used to advance the front face of the focuser, or having a linear encoder that directly measured the displacement of the focuser draw tube. That position would be fed back to the focuser controller.

 

The lack of a fully closed loop system can be seen in all of the focus automation software I've seen. They all have some sort of optional backlash compensation, where you can tell the focuser to compensate for backlash by adding a certain number of steps to the sequence after a direction switch. If there was a closed loop feedback system, this wouldn't be necessary because the focuser would know that those last 10 steps it issued didn't actually move the focuser face because of backlash in the gear mechanism.



#20 rigelsys

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Posted 17 April 2015 - 01:29 PM

 Televue uses a DC motor plus digital position indicator  for a closed loop system.  Doubles the cost, and that's with a DC motor. 

 

http://www.televue.c...77#.VTFQD-GVRnE

 

And the range of motion is limited, have to be close to focus because position guage only has maybe 10-20 mm of travel.



#21 rgsalinger

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Posted 20 April 2015 - 12:01 AM

I went from an Optec TCF-Si to a Rigelsys system for both my OTA's and there is fundamentally no difference in what I achieve in terms or FWHM using the same software- MaximDL 6.08. The beauty of the Rigelsys system is that it uses up no back focus which was a real problem with my refractor. It's also hundreds less. The downside is that if the stock focuser isn't up to snuff then you have a real problem as others have noted by just motorizing it. BTB on both focusers - a Moonlite 2" and a Televue 2.4" inch I managed during testing to go too far in both directions with no motor burnouts. I do want to say again that I started with quality focusers which I think is why the system works so well for me.

Rgrds-Ross



#22 jbalsam

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Posted 20 April 2015 - 06:52 AM

 Televue uses a DC motor plus digital position indicator  for a closed loop system.  Doubles the cost, and that's with a DC motor. 

 

http://www.televue.c...77#.VTFQD-GVRnE

 

And the range of motion is limited, have to be close to focus because position guage only has maybe 10-20 mm of travel.

 

That's really neat, Rigel, thanks for pointing me to it. They say on the site that it's "the only closed loop system on the market." That makes sense, given that I've never seen a closed loop system like that before (even from FLI and Optec). I read through the description of the software, not sure if you can interface it with something like FocusMax, but it seems like you should be able to (it has an ASCOM driver). You wouldn't need a stepper motor if you have a fully closed loop system like that.

 

EDIT: apparently you can use that system with focusmax, according to the results here: link.


Edited by jbalsam, 20 April 2015 - 06:55 AM.



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