Homemade Electric Telescope Focuser
Shown On A Celestron 8SE Schmidt-Cassegrain


Created by Brian Sumpter, Greeneville, TN February 06, 2008

INTRODUCTION:

This page will show you how to design an electric focuser for your telescope at a fraction of the cost of the commercial units. After spending what felt like massive amounts of money on a new telescope, eyepieces, CCD camera, etc. I was not looking forward to telling my wife that I needed yet another purchase. While I'm sure the Celestron and JMI commercial offerings are first rate products, they are unfortunately $140 - $160 plus shipping. As someone who is involved heavily in the radio control hobby as well as astronomy, the thought occured to me that I could probably build a focuser using an off-the-shelf hobby servo, and a few spare parts.

Video Demonstration

This video demonstrates the completed electric focuser working the focus knob on my Celestron 8SE. The rheostat at the top of the hand paddle controls the speed (voltage) of the motor, while the switch controls direction. By manipulating these two controls, you can achieve a very fine focus with no shaking of the telescope mount. Another advantage would be to focus remotely while doing indoor CCD capturing with your telescope outside. For a higher resolution H.264 version of this video, click here for a direct link.


PARTS LIST:

You will need the following:

1 x Hitec HS-81 Micro Servo
http://www.wholesaletrains.com/Detail.asp?ID=200402695
$14.86
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1 x 25-Ohm 3-Watt Rheostat
Radio Shack
Model: 271-265 | Catalog #: 271-265
$3.99
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1 x 1/8" Mono Panel-Mount Audio Jack (3-Pack)
Radio Shack Model: 274-251 | Catalog #: 274-251
$3.99
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DPDT Mini Toggle Switch (ON-OFF-ON)
Radio Shack
Model: 275-663 | Catalog #: 275-663
$4.99
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1 x 1/8" Mono Phone Plug (2-Pack)
Radio Shack
Model: 274-286 | Catalog #: 274-286
$2.99
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1 x Project Enclosure (4x2x1")
Radio Shack
Model: 270-1802 | Catalog #: 270-1802
$2.69
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Rubber O-Rings Variety of Sizes
Lowes, Auto Parts Stores, etc.
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Bracket - scrap metal (curtain hanger, etc)
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Telephone Coiled Handset Cord (12' coils nice and tight)
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Possibly rubber wheels from a toy, etc.
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CONSIDERATIONS:

Please note that this entire process is totally "up in the air". Most of this can be done differently, and probably better -- I just wanted to demonstrate the principle and give the basic concept some wings. There are things I would do differently if I were to do this again, so let me save you some trouble and go over that now.

The DPDT toggle switch I used is not a momentary switch. The only momentary DPDT OFF-ON-OFF switch I could find at Radio Shack was simply too large to fit in the hand paddle box. An ideal switch here would be a ON-OFF-ON rocker switch, sort of like the ones you see for a car's power windows. Now before you get any ideas, a car power window switch is not DPDT, so they will not work. You can find DPDT momentary rocker switches online at various parts supply houses for about $10 + shipping. I may yet order one of these and replace my toggle. In the interim, the toggle works just fine and is easy to get precise focus.

On a cosmetic note, I absolutely hate the white coil cord! I looked for black phone cords but was unable to find one today. I will be changing this as soon as possible, as I think it mucks up the looks of the project. Tiny thing to get worked up over I know, but still.

THE BUILD:

I used an HS81 micro servo, as the back end of a Celestron 8SE doesn't have a lot of clearance when the scope is pointed at zenith. If you have a rack and pinion focuser, or a little more room available I suggest using a standard sized hobby servo. Not only will the standard servo have more power available, but it will be easier to modify for continuous rotation. It may be possible to use a standard size servo on the SE series telescopes as well, but I had the HS81 handy and thought it best to take no chances. Needless to say, there is more than enough clearance with the micro servo in place.

I'm not going to go into detail about how to modify a servo for continuous rotation in this tutorial. There are many websites on the internet that go into detail on how to modify your servo for continuous rotation. As you read these websites, please keep in mind that many of them will want you to either a) center the potentiometer, or b) remove it and replace it with some resistors. This is not necessary for our application! We will be gutting the circuit board from our servo, so the last steps of continuous rotation modification do not need to be followed. Here are few websites to get you started with this first critical step:

http://www.acroname.com/robotics/info/ideas/continuous/continuous.html
http://www.uscoles.com/kapcontinuous.html
http://www.robotstore.com/download/Servo_Mod_Notes_1.0.pdf
http://www.natrium42.com/wiki/Hacking_servo

Briefly, modifying the servo will involve removing the stop tab that normally prevents a servo from rotating all the way around, and drilling out the servo horn gear so that it can turn on the potentiometer shaft rather than with it. It sounds more difficult in words than it really is, so you should have no trouble with this step. The one thing we are going to do differently than the websites above is actually remove all of the circuitry from the servo. All we want is the plastic case, the gearbox, and the motor itself. Once you have modified your servo for continuous rotation, simply remove the circuit board leaving you with the motor as your only remaining electric component. So once again, ignore the sections in the links above that discuss centering the potentiometer, or replacing it with resistors. Just make it so that the primary output gear can rotate a full 360 degrees and you're done.

One other thing to note is that different brands of servos are set up differently for the stop tab. On my Hitec HS81 servo, the stop tab that prevented the gear from completing a full rotation was molded into the top case. A few minutes with a dremel took care of it. Other brands of servos have the stop tab molded onto the output gear itself, and can probably be cut off with an X-Acto blade (carefully!). If you're unsure, do a quick Google search on your servo brand combined with the words "continuous rotation", and you may find someone explaining how to do it for your particular servo. If not, just take the servo apart and have a good long look at the gears - it will quickly be obvious what is preventing it from turning all the way around. You'll also note that in all servos the output gear is a tight fit over the potentiometer. This is so that in normal operation the gear turns the potentiometer and tells the servo circuitry it's position. This needs to be a looser fit, so that the gear can turn on the shaft and your potentiometer basically becomes an axle. I handled that problem by reaming the output gear slightly with a drill bit.

One last thing and I'll leave the continuous rotation topic alone. Please note that not all servos can be modified for continuous rotation. On some of them, the primary output gear is not a complete gear - it's only geared for about 90 degrees, and then it is "cut off". If this is the type of servo you have, you cannot modify it for continuous rotation. I had an Airtronics servo here that I looked at before the Hitec, and it was made with "half a gear" and could not be used. I do know that most all of the Futabas can be used for this purpose, and of course the Hitec I am using here. If you are unsure, once again, Google is your friend. Now on to the actual servo focuser creation...


Servo Motor & Circuit Board

Here the motor is removed from the servo. The little circuit board hanging on the end is what we will be removing, leaving only the motor and the positive / negative wires.



Motor With 1/8" Plug Attached

Here is our motor with the circuit board removed, and replaced by the 1/8" mono jack. Polarity doesn't really matter here, so just solder the wires up whichever way they fall under your soldering iron.



Servo With 1/8" Jack Mounted In Case

The motor with a 1/8" mono jack attached reinstalled back into the plastic casing. It's a really tight fit with the micro servos, so take your time. If you use a standard servo you'll have much more room to work. Note that the jack fits perfectly into the spot where the circuit board used to reside.



Completed Servo Ready To Go

Here we have our completed servo. The "pulley" on top is simply two normal round servo horns (usually included with the servo) glued one to another. Notice that I also cut the mounting ears off the servo case for a cleaner look overall, as they are not needed for my particular bracket. If you fashion your bracket in such a way to utilize the mounting ears, leave them on the case.



Project Box (Hand Paddle) Interior

This is the inside of the project box upon initial assembly. Simply mount the rheostat at the top by drilling a hole for the shaft, the switch somewhere in the middle, and the 1/8" mono "headphone jack" at the bottom of the case. Other than the wiring and the battery, this is all there is to the hand control paddle.



The Completed Hand Paddle

This is the front of the completed hand paddle. It is a good size to fit in your hand, and the toggle can be controlled by your thumb even though it is not a momentary switch. As mentioned above, a momentary rocker would be even better, so if you can find one I suggest that.



Hand Paddle Interior All Wired Up

The inside of the hand paddle with all wiring completed. The wiring looks a mess, but is very, very simple. There are only a few connections to make and I will include a schematic at the bottom of this page. One thing to note here - DO NOT USE A 9V BATTERY! I used this battery during the testing phase, but it is simply too much voltage for the little servo motor. A servo motor is rated from 4.8 - 6.0 volts, but honestly you don't even need that much power for our purposes. I ended up using a NiCD battery designed for a cordless telephone, and it was a 3 cell (3.6v) battery. You can probably fit a couple AAA sized batteries in here, or even a AA sized 3.6v lithium camera battery. I'll leave it up to you to decide how you want to power your creation, as I have chargers available from the radio control hobby side that you might not have access to.



Everything Ready To Mount

The completed controller and servo. Note the ugly white cord!!! :D



Assembly Mounted On A Celestron 8SE

The above photo shows the completed assembly mounted to the back of my Celestron 8 SE. The bracket in my case was a simple "L" bracket that I picked up from Lowes. I'm not sure what it was originally designed for, but I found it in the aisle with curtains and blinds. I mounted the bracket using a single (existing) screw on the Celestron focuser. Double sided servo tape under the bracket keeps it from twisting. Double sided tape and several O-rings are holding the servo to the other side of the bracket. It doesn't take much to hold this all together, and you'd be surprised at the strength of the servo tape. Of course if you have a rack and pinion focuser, your bracket is going to look considerably different.


The above photo shows a wider view of the installation.



Modification Made To Prevent Belt Slippage

The Celestron focus knob is rather small, and I found that the belt slipped on both the pulley and focuser side. To alleviate this issue, I found an old foam rubber model airplane wheel that fit very snuggly over the focus knob (push fit). I chucked this wheel up in a drill, and using a piece of sandpaper while it was turning cut out a groove in the foam. In essence, I created a soft foam rubber pulley that increased the diameter of the Celestron focus knob. The texture of the foam rubber prevented slipping on the focuser side of things, and the larger gear ratio from the big focus knob demanded less from the servo pulley and solved the slippage problem there as well.


Yet another shot of the completed assembly.


And another ...


And one more!



Hand Paddle Schematic

A rough schematic courtesy of my good friend Harold Belcher, to illustrate how to wire the hand paddle. Note that the "dots" in the middle are the lugs of the DPDT ON-OFF-ON switch. The "squiggle" at the top is the rheostat, and "B" stands for battery, "M" for motor. Use practically any gauge wire for this, as there is very little amperage being pulled. A few pieces of Cat-5 wire, speaker wire, etc. should work just fine.



IN USE:

The focuser works great in the field! It's extremely easy to achieve a very fine focus by varying the speed of the focus motor, and using quick blips on the switch. With the large wheel I have in place over the focus knob, the movement even at rather high motor speeds is extremely controllable. I plan to build a longer 25 foot cable with mono plugs at each end to use while the telescope is sitting outside, while I'm inside nice and warm. Once this cable is built, along with a computer and USB cable, I should have a complete remote "observatory" set up in my front yard, which was the goal of this project to begin with. An added benefit is that all "shakes" are removed when using the telescope in the field, as I no longer have to touch the focus knob. All in all I'm very happy with the way everything turned out. My wife is happy too - I saved over $100 by building this myself!

PARTING THOUGHTS:

As mentioned in the beginning of this article, it was not my intention to give a step-by-step how-to of this project. There are so many variables at play with all of the different focusers, servos, etc. I didn't want to get that detailed. Use you imagination, and build your focuser the way *YOU* want it to be. If you decide to take this project on, please snap some photos during the process and send them to me!



All photos and content property of Brian Sumpter, 2007. This project may be freely used and modified at will by the amateur astronomy community. If you have any questions or comments, please email me.