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Telescope Dome Controller

Posted: Thu Jan 26, 2006 8:05 pm
by WhiteFeather

My school is trying to build a telescope dome controller.

Can anyone describe to me the basics of shaft encoders? ie, starting with WHAT is a shaft encoder and how is it used?

Barring that, does anyone have any suggestions as to how I might be able to get any kind of, oh, say, TTL level feedback from the dome to know within x degrees where it's pointing?


Re: Telescope Dome Controller

Posted: Thu Jan 26, 2006 8:31 pm
by Chris Smith
One way the encoder works is to count pulses, and use a counter/divider which incorporates a Divide by chip [like the 4017] to add things up which = X or logic 1/0 as a final answer.

For example you might count how many rotations there are, divide by 100 or 1000, and at the given number you require, the device starts or stops or is = 10 degrees north for example.

Another simpler way is to use two sensory switches at either end of the driver to sense fully open, or fully closed for the cover of the dome or a set point in rotation such as 90/180/270/360.

If you trying to locate degrees in the sky, you need one convertor to send in pulses into a counter to equal X pulses = 1/4 y degrees location for the four quadrants, 90 degrees each, but you can also do it for 360 degrees with out a reset at each quadrant.

For example 10,000 pulses are required to do a full 360 so you place 360 into 10,000 and each 27.7 pulses are one degree.

You would have to round off, or use a more complex unit to decode such as a “Stamp” or micro processor to do this automatically.

A shaft encoder can be as simple as a pie wheel with one or many holes in it where light passes through to a sensor and is counted as 1 pulse for every revolution, each hole, or 360 holes for each one Revolution.

For Stepper motors you merely count the out going pulses and translate this into how many pulses were there and this equals how many degrees you have advanced. Each stepper has different amounts to make one full rotation called Degrees per step.

However to keep track of advancements and retreats this is where the Micro processor comes in handy.

A computer, The stamp, or basic micros should be able to be programed to point and shoot, and the rest happens from the pulse counts.

Re: Telescope Dome Controller

Posted: Thu Jan 26, 2006 9:35 pm
by rshayes
There are two general classifications of shaft encoders, incremental and absolute. An incremental encoder gives one count for each increment of motion. A toothed wheel is an example of this. You can tell that a tooth has gone by, but you can't tell which tooth it was. This type of encoder can give an absolute position if you count from a known reference position. However, if you lose track of the count, you have to move back to the reference position and begin a new count. A telescope dome is designed to move slowly, so finding the reference position could take a fair amount of time.

Absolute encoders identify the absolute position of the shaft. Usually these have several tracks, often formed optically on a glass disk or mechanically with conductive patterns on insulating disks. One track will have only two segments, each 180 degrees long. This will identify which half of a full revolution that the shaft is positioned at (first half or second half). The next track will have four segments and will identify wheter the shaft is in the first or third quarter or, alternately, the second or fourth quarter. The third track will have eight segments, and so forth.

Sometimes, the tracks will use a Gray Code, where only one track changes state at any given position. This requires code conversion for a binary coded output, but the process is not particularly difficult. (A Gray Code counts 000, 001, 011, 010, 110, 111, 101, 100.)

The problem with a telescope dome is mainly mechanical. You probably don't have a shaft that corresponds to the position of the dome. The drive shaft probably rotates many times for one revolution of the dome.

You might have an area where you could form optical tracks using black and white paint. These could be read using reflective sensors. Eight tracks would give you a resolution of 1/256 of a turn, and would be convenient to interface to a microprocessor.

An alternative would be to add a gear train to the drive shaft that duplicates the dome rotation and to use that to drive an absolute shaft encoder.

Absolute shaft encoders might be available as surplus, they can be quite expensive if purchased new.

Another way of measuring shaft angle is to use a synchro to get an analog shaft position and to convert the synchro signals to digital form with a S/D Converter (Synchro to Digital converter). At one point these were made by companies such as Burr Brown and Analog Devices. The conversion can also be done mathematically using A/D converters to measure the values of the synchro signals. Resolvers can also be used. (Synchros and resolvers are similar devices. Synchros give three signals that are from windings that are mechanically 120 degrees from each other. In a resolver, the windings are spaced 90 degrees apart.)

The outputs of resolvers correspond to the sine and cosine of the shaft position. This makes them useful for coordinate conversion systems, such as converting from Declination and Right Ascension to Altitude and Azimuth. There is a fair amount of material on this in the MIT Radiation Laboratory Series published in the late 1940s. Texts and handbooks on Radar will probably also discuss this.

<small>[ January 26, 2006, 09:38 PM: Message edited by: stephen ]</small>

Re: Telescope Dome Controller

Posted: Thu Jan 26, 2006 11:07 pm
by philba
There are a number of sites that have information on motorizing scopes. In particular, the Dob scopes would have exactly the same issues that your dome has though your scale is a lot bigger.

I agree with Chris that you should consider stepper motors.

Here's a decent tutorial on encoders - ... 0700573BC0

By the way, you can also find plans on the net to use the meade autostar which just outputs step pulses for RA and Az stepper motors. With stepper motors you don't need an encoder. Typical steppers are 1.8 deg/step and you can gear down for finer resolution and higher torque. The autostar has built-in calibration and the newer ones have gps. Depending on the size of your dome, you might need a larger stepper like a NEMA 34 if you want more speed (er torque). The steppers in my Meade etx125 are suprisingly small, though. There are several pc programs that work with the autostar so you could wind up with a very nice system. Autostar controllers typically go for $200 new and you can find them for less on ebay, sometimes a lot less.

Re: Telescope Dome Controller

Posted: Fri Jan 27, 2006 11:04 am
by rshayes
It sounds like a little bit of clarification is needed. Most of the small telescope domes that I have seen were at least ten feet in diameter and probably weighed several hundred pounds. The motor was usually in the 1/4 horsepower range with a fair amount of gearing. The actual dome rotation was probably about 1 revolution per minute or less.

This doesn't sound like a job for a stepper motor.

On the other hand, a small telescope may only weigh 40 or 50 pounds and will be geared to operate at a much slower rate, so very litttle motor torque is required. The basic rate is 1 revolution per day, but there is usually a high speed rate also. Stepper motors are a possibility here, but might have a vibration problem unless they are driven with some form of microstepping. Telescopes are amazingly sensitive to vibration.

Re: Telescope Dome Controller

Posted: Fri Jan 27, 2006 11:40 am
by philba
A nema 34 stepper can be quite powerful and the dome doesn't need to move that fast. But it would be good for the OP to understand what his requirements in that area are. It should be pretty easy to figure out RPM requirements. With a bit more work, torque needs can be deduced. Then it's no longer a debate issue. Also, while the dome is heavy, it should run on a track with decent bearings and not be that hard to turn.

I used to think steppers were a bad choice for scopes but they are used all over the place, even for astrophotography. microstepping and gearing down means that each step is a tiny fraction of the rotation and thus isn't an issue. I'm still amazed how good my stepper driven meade etx125 is - even when fast slewing, I see straight star tracks.

Re: Telescope Dome Controller

Posted: Fri Jan 27, 2006 12:03 pm
by Sambuchi
Hello WhiteFeather

You asked "the basics of shaft encoders? ie, starting with WHAT is a shaft encoder and how is it used?"

They are real simple devices that can keep track how far a shaft has turned..

I made one from a old mouse that was laying around my house one day.. ... r_home.htm

How big is your telescope?

Re: Telescope Dome Controller

Posted: Fri Jan 27, 2006 12:26 pm
by Dave Dixon
I'm confused. Are we discussing moving a typical dome structure? I'm invisioning a "slotted" dome, like this one... telescope dome photo
Driving one of these would be quite a bit different from a telescope drive system. What am I missing here? Surely those are totally separate items. (okay not "totally", I can see a link, as part of a system maybe. But still separate drive mechanisms)Thanks,

<small>[ January 27, 2006, 12:29 PM: Message edited by: Dave Dixon ]</small>

Re: Telescope Dome Controller

Posted: Fri Jan 27, 2006 1:08 pm
by WhiteFeather

Thanks all. It's a dome, a slotted dome in fact. It's about 6 feet across and made from wood (heavy).

It wouldn't be too similar to a scope controller since all I really want is a way to tell the dome to rotate and in some way be able to tell which direction the scope is pointing. There is, of course, no reason to account for right ascnension and declination since I can handle the software programming.

I'd probably want to create some code that sends and receives a signal via usb or ? ... but the real issue does seem to be the heavy mechanicals...

Re: Telescope Dome Controller

Posted: Fri Jan 27, 2006 2:21 pm
by haklesup
I would construct the encoder around the perimiter of the dome (probably inside) It would consist of a row of photodetectors (probably at least 16 to get the resolution you need of about 1.4 deg per count). YOu will also need (an 8 foot disk) to drill a sequence of holes around the perimiter of the dome. The photodetectors would be mounted to shine through these holes. At each 1.4 deg increment around the dome you would drill 16 holes radially (oriented in line with a radius to the center) well not 16 holes at each spot but a binary representation of the 16b count at each spot.

In operation the photodectors will shine through some of the holes decoding a 16 bit binary number. The positioner would know the angle of the dome simply by making note of what binary result comes from the detectors. The system would simply start the motor right or left and run until the correct binary count is detected.

The same can be acomplished using magnets and a magnetic pickup as well.

YOu could also use a row of lever switches activated by screw heads like a giant music box.

The encoder and detector are largely electromechanical. The next step is to get the signal into a circuit that can count and activate the motor. USB being serial would need a shift register function to convert the data but that is easy using one of the OEM USB modules available from many vendors.

Edit, Oops, 8 bits for 1.4deg

<small>[ January 27, 2006, 04:02 PM: Message edited by: haklesup ]</small>

Re: Telescope Dome Controller

Posted: Fri Jan 27, 2006 2:43 pm
by rshayes
If the dome is 6 feet in diameter, the perimeter will be about 19 feet or 226 inches. An 8 bit digital word encodes 256 states, so an 8 bit system would allow positioning to about 1 inch. That should be adequate. It makes life simpler since you don't need to process multiple precision numbers.

The simplest position pickoff might be an electronic compass mounted at the top of the dome. That would avoid a lot of mechanical work, and it may be possible to get one with a digital interface.

Re: Telescope Dome Controller

Posted: Sat Jan 28, 2006 12:57 am
by Dean Huster
It would seem that one of the problems you need to consider is the fact that a degree of rotation will allow a decent telescope to sweep across a lot of sky. If you're using the scope with a camera and letting the dome rotation take care of azimuth while the scope itself is geared for elevation, a stepper motor will screw up your photos. The entire system needs to be slaved to an analog clock drive or a digital drive with really fine resolution so that the jumps in the dome rotation are smaller than the resolution of the telescope.

If your application is to just use the circuit for a "coarse" positioning of the scope while all the clock drive is applied to the telescope mount for shorter-term exposures, then you'll be OK with nearly any rotational system.

If you're wanting absolute position from an optical source, I know of very few that don't require Gray code vs. binary, for the electronics will otherwise get very confused with the asynchronous multiple-bit changes that would otherwise result. The nice thing is that with a circumference like that, the perimenter of the dome would provide a perfect place for a high-resolution encoding system that could easily resolve down to at least a tenth of a degree with some careful encoder design. A six-foot dome would give you about 0.62 inches of perimenter rotation for each degree.

The down-side of using a relative vs. absolute system where you know where you start and count your steps is that any slippage or backlash will mess things up. In addition, you'll always have to start with an accurate reference point. However, an absolute positioning system, in the mechanical sense here, is very involved and probably not the best solution.

Are we putting a little 3" Meade refractor in this thing or is it good-sized reflector? And with a dome size of only six feet in diameter, you must be using a camera on the scope vs. having someone actually inside. Or are you really tiny people there?


Re: Telescope Dome Controller

Posted: Sat Jan 28, 2006 5:14 am
by Mike6158
It seems to me that you would somehow want to key the location of the dome opening and rotation of the dome to the direction that the telescope is pointing. I suppose you could manually slew the opening to somewhat line up with the telescope opening and then rotate the dome at a rate that corresponds to sidreal time.

Since this is a school project I can't imagine that they are going to have a 19' circumference gear to attach to the dome so I suspect that they plan to use some sort of friction coupling(s) to connect the motor(s) to the dome? Could you use some sort of optical encoder arrangement? Like a ring of 3mm LED's around the dome (about every .633 inches would = 1 degree per LED) and have a single sensor to count light pulses? Positioning would be relative but you could account for that in your program. Just manually position the dome, plug in the "hey I'm pointin this way" factor, and turn it on. Or you could get real gee whiz and read the direction that you are pointing from the telescope... erk... but there is no point in doing that... the dome will be lined up with the scope and unless you've got some kind of oddball astro event thing going on the scope and the dome will rotate in the same direction :D so the dome doesn't care where it is just as long as it rotates with the scope and the hole is lined up with the telescopes "anti-hole"

Hmmm.. If you don't mind being locked down to one rotation direction, absolute position location could be determined by installing a 0° stop, rotating the dome back to this stop, and setting the 0° point in the software when a limit switch located at the 0° stop closed. Or a 0° light sensor lines up with a 0° led located off the centerline of the other led's if you don't want to use mechanical switch. From then on it's a matter of rotating the dome to position X° relative to 0°. That value could be read from the telescope. The reality is that for the dome opening a resolution of ± 1° is plenty good enough.

Time for more coffee...

<small>[ January 28, 2006, 05:22 AM: Message edited by: NE5U ]</small>

Re: Telescope Dome Controller

Posted: Mon Jan 30, 2006 5:13 pm
by philba
For those of you that don't believe steppers make sense for a telescope, please read: ... Drives.htm ... altaz.html ... eriorrack/

note that those links were just from the first page of google hits. note also that several of these scopes are used for astrophotography. You can also see plans for stepper based field derotators (to keep the camera orientation correct). And I'd also note that many people use the meade ETX series for astrophotography and it uses 2 wimpy little steppers. Proper gearing and microstepping is the key. It's a bit frustrating to see people shoot from the hip.

Secondly, take a look at motorized dobsonian telescopes. there are many, many out there. tell me the base motor issue isn't exactly the same issue as dome rotation. Maybe it's a bit larger but same issue.

Now, if one chooses to go the feedback route then toss all those ideas about encoders on the rim and just use a quadrature encoder on the motor's shaft (or a later stage gear shaft encoder). You will want a reasonable level of accuracy, even if the scope and dome are driven seperately - knowing fractions of a degree of rotation is important. If you want some sort of absolute encoding, then I'd use multiple index holes, say one every 5 - 10 degrees to sync-up. The problem with wiring the dome track with sensors is that you will have to be careful not to damage the wiring or sensors as a result of regular operations (lots of people stumbling about in the dark is not a good thing). By the way, I'd consider using hall effect switches since dust and dirt can plug led/phototransistor setups.

I believe that as long as you have a motor that can drive well above the dome's requirement (to avoid slipping), you should get decent accuracy and repeatablity from a simple quad encoder.

I would also spend a lot of thought and effort on making the dome rotation as smooth and low friction as possible. Inline skate bearings can be had relatively cheap on ebay. Each is good for 30+ lbs. I'd bet 20 to 30 of them will do the trick - they are used for homebuilt cnc machines all the time. It's suprising how small a motor could do the job if you get the friction low.