Making a Telescope - Part 3, The Mount

Hi frankd20,

This is a great series of articles and I have been following it with great interest. While I am not building a telescope I recently received a 1,900 x 125 mm Cassegrain telescope with a computerized mount for my birthday. I am interested in how you motorized you mount so if you do have the time I would be interested in seeing how you went about it.

I have been playing around with a digital camera and temporarily mounted it to the telescope with some blue tack and good old duct tape then focused it as best as I could by hand. I got some images that weren't too bad considering how quickly it was thrown together. Here is an image that I took of the moon on 12^th April.

I am currently building a proper mount and enclosure for the camera plus a digital focusing mechanism that will enable me to focus it better. If anybody is interested and you don't mind me posting it to your blog, I will write it up and add it to your series when I have finished.

I admire you for building your telescope from the ground up. It's something I have always wanted to do but have never gotten around to so well done and thanks for taking the time to share your experience with us.

Your picture looks great for the short time you spent on the mount. After you get the focus motorized and have it all built I think it would be a great addition to this series. I will eventually write up an entry about the tracking part in more detail, although as I do with the telescope project I will put this series aside for a while and then come back to it. A related project I did which I will also write about was modifying a web cam to take long exposure pictures for use with the telescope. Let me know when your done with your camera mount project and I then I could do another 3 part series with your focus my motorization and CCD camera. In the mean time I have plenty of other projects to write about.

I finished doing the drawings about an hour ago and I plan to start machining the enclosure and mount tomorrow. The big hassle I had was fitting it all in, when the telescope is vertical there is only 19 mm of clearance behind the back of the scope and the mount. The drawings say that it all fits with just over ½ a mm to spare so machining it is going to be super critical.

Hopefully I will have the mount finished within a week or so I will write that up and post it as soon as it's done. I will write the digital focusing up as a separate article. I have some 1.5° 4 phase 12 V stepper motors somewhere that I removed from some old Printronix line printers. There is a kit that you can get that uses the parallel port of a laptop to drive stepper motors so that might be the way to go but I plan to visit the local telescope shop as see what they have before I start making anything.

You can drive one of those steppers from the parallel port, although for focusing that sounds a little overkill. For your application you could make a simple circuit with a pic chip. You might also want to look into using a servo motor since the driver for those can be very simple for your application. Is your scope on an equatorial mount? if not might want to look into adding a derotator while your at it.

Good point, I actually have a PICkit 2 lying about that I purchased a few months back that I have been meaning to get around to playing with. With the addition of some driving transistors it could easily drive the focusing mechanism .

Being able to focus the telescope from the parallel port has it's advantages though. The telescope I have comes with a software package that can be used to drive the telescope via an RS-232 port so being able to drive the telescope, focusing and camera all from the same computer has its benefits.

I am very interested in stepper motor details. I am almost ready to make a tracking solar accumulating barbecue. It will have a section of a parabolic dish on an equatorial mount. All it needs now really is a motor to turn it at 15 degrees per hour on its axis. Perhaps a stepper motor would do? the reflector is going to be about 1 m2 in area and might get buffeted around a bit with the wind sometimes (it will be mostly under the barbecue though so perhaps not bad wind buffeting). Would that affect the motor? Anyways, any motor that can turn it a little bit every 10 minutes or so to do 15 degrees per hour would be awesome for the project. thanks Brian

Hi gaiatechnician,

You certainly could use a stepper motor to drive the reflector. The first thing you need to do is find out how much torque is needed to both drive the reflector and then hold it in position. You can then utilize these figures to pick a suitable stepper motor for the job.

You will more then likely not find one that can supply the required torque so you will need to use some kind of pulley or gear arrangement that will increase the torque at the reflector. This won't be as difficult as you may think as in a not insignificant portion of the applications that utilize stepper motors utilize a series of pulleys and toothed belts to transfer the torque to the final drive shaft.

The final step is fairly simple and once you have decided the pulley ratio you can calculate the angle that the final drive will step through by multiplying the stepper motor step angle by the inverse of the drive ratio. For example. If you had a 5 to 1 pulley system and a motor that stepped through 3° with each step the final drive will be 0.6° per step. In your case you need to have the final drive move by 15° every hour the motor will need to be stepped by 15/0.6 or 25 times each hour (1 step every 144 seconds).

How you drive the stepper motor is up to you and how much of the process you wish to take on. Personally I would suggest purchasing a kit from one of the many electronic component suppliers that have the motor and electronics to drive it. But if you wanted to go deeper into it you can purchase microcontroller prototype board that are designed to drive stepper motors or go all the way and design the electronics yourself.

That's an overview and I hope it has helped. If you need more detail including how stepper motors work please don't hesitate to ask no matter how basic it seems and I will do my best to help.

Regards MASU

Hi, I would not be using the motor to drive the reflector, the reflector gets too much wind stress and I would have to make it too sturdy. Over the last month, I have been playing with a water clock. (The 2 bucket dripper tracker. which is at http://solarcooking.wikia.com/wiki/Tracking) This handles the wind really well so far but I worry about the accuracy of the water clock. I see a possibility for control by a slow stepper motor instead of the waterclock and I will upload a new pic soon. Could steppers release a string or rod at a rate of 2 or 3 cm per hour? And with say, 12 steps per hour? Thank you Brian

Hi Brian,

The link you supplied refers to a water clock that is used to position a solar collector. It also opens with the statement,

• Solar box cookers and solar panel cookers normally do not need to be turned to follow the sun unless you are cooking beans or an especially large amount of food.

which implies that you really don't need that much accuracy.

My telescope has a 1,900 mm focal length so even with a low powered eyepiece with a 25 mm focal length the field of view is only about 40 minutes of arc. That means that to even get the target within the field of view you need to position it within ±20' or ±0.093%. For doing things like astronomical photography or greater magnifications you need at least an order of magnitude better resolution of ±0.009 3% (95 parts per million)

You have about as much chance of getting a water clock to work with a 95 ppm accuracy as getting a politician or lawyer to tell the truth. Personally I would ditch it as you are more than likely wasting your time, effort and money.

• Could steppers release a string or rod at a rate of 2 or 3 cm per hour? And with say, 12 steps per hour?

The simple answer is yes, but before we go any further it's probably a good idea to review the concept of a stepper motor, how they work and what the various specifications mean and relate to selecting a stepper motor for your application.

The image below shows a 4 phase stepper motor that will move by 9° per step and 36° per complete cycle.

So, what does this mean and how does it work and relate to your application?

Like other types of electric motors that produce a rotating force a stepper motor consists of two primary components a stator which is the stationary part of the motor and a rotor which is the part that rotates and is used to drive whatever it is connected to.

The Rotor: The rotor is the part of the motor that rotates. In a stepper motor the rotor is a consists of a cylindrically shaped permanent magnet that has multiple magnetic pole pairs arranged in a radial manner around its axis. As the rotor turns a fixed point adjacent to its circumference will see a sequence of alternating magnetic poles The rotor can be though of as a series of horse shoe magnets arranged so their poles form a circle with equally spaced around its circumference

In this case we have 10 pole pairs labeled a-j and represented by the Grey U shapes arranged at 36° increments with their polarity shown by the RED N for the north poles and BLUE S for the south poles.

Something that is worth mentioning is that while each of the poles is separated by 18° it is the 36° angle between the pair of poles or next pole of the same polarity that is important.

The Stator: This consists of a series of coils that are equally spaced

In our system we have 8 coils, but they are interconnected so that electrically there are only 4 coils A-Red, B-Green, C-Blue, and D-Purple that are spaced at 45° intervals.

When there is no power to the stepper motor the rotor will rotate fairly freely but it will try and stop so that one of the poles aligns with one of the coils. In this instance the rotor will try and settle in increments of 9° or 40 separate points.

Step 0: In the image on the left shows what would happen when power was applied to the stator coils labeled A and drawn in red. The magnetic fields these coils produce will then cause the rotor to turn till the North pole of magnet a aligns with the South pole of the red coil A. On the opposite side of the rotor the North pole of magnet f would align with the South pole of the other red A coil.

Something worth noting is that provided the holding torque is not exceeded the rotor will stay locked in this position until the power is removed from the red A coils. Unlike other forms of electric motor a locked rotor will not result in damage to the motor or burnt out coils.

Step 1: If we now remove the power to the red A coils and apply it to the green B coils the rotor will rotate till the North of rotor magnet b aligns with the South of stator green B coil. On the other side of the rotor the North of magnet g will align with South of the other stator green B coil.

The important thing to note here is that even though the magnetic field of the stator has rotated through 45° the rotor has only rotated through 9°.

If we not continue to energize each of the coils red-A, green-B, blue-C and purple E in sequence the motor will step through 9° each time a coil is energized.

How do stepper motor specifications apply to the motors?

In our case we have a four phase 9° stepper motor but there are a few other things that you may need to use:

1. Phases: This is the number of separate coils that make up the system.
2. Step Angle: This is the angle that the motor steps through every time the next coil in sequence is energized.
3. Holding Torque: This is the amount of force that is needed to cause the rotor to turn while being locked in position by an energized coil.
4. Driving or Dynamic Torque: This is the amount of torque the motor can supply as it steps from one step to the next.
5. Voltage: This is fairly obvious and is the voltage that is needed to operate the motor.
6. Holding Current: This is the current that the coils draw when in the locked position.
7. Dynamic or Peak Current: This is the current the motor draws as it steps from one step to the next.

What are the advantages of using stepper motors?

In a control application like this there are several advantages:

1. Simplified Feedback: There are several advantages related to feedback:
1. Position: Since the motor can only be in one of the positions defined by the coils and permanent magnets in the coil finding the position of the system. All you need to do is start from a know position and then count the steps in either direction to calculate the position.
2. Speed: Since the speed the rotor turns at is governed by how rapidly you step from phase to phase you don't need a feedback mechanism to calculate the speed the motor is rotating at.
2. Locked Rotor: Unlike with other motors a locked rotor will not result in the current through the coils causing them to overheat and burn out. It can also be very helpful in situations where there needs to be some sort of breaking mechanism that can hold it in a desired position.
3. Simplified Drive Electronics: Unlike other types of motor where the current and voltage being applied to the motor need to be controlled through a range stepper motor coils only need to be either on or off. This makes the driving circuit much simpler and consequently more reliable and less expensive.
4. Reduced Maintenance: Since there is no commutator, brushes, etcetera that are prone to wear and contamination stepper motors require less maintenance and have longer life expectancies than other DC or servo motors.

What are the disadvantages of using stepper motors?

Like anything there are plusses and minuses and the following can be problematic when using stepper motors.

1. Step Induced Oscillations: Because stepper motor move in a sudden jerky manner between each step the steps can set up vibrations in the drive train that can be detrimental to the process and equipment.
2. Fine Control: The staccato or jumping motion of stepper motors can be a serious problem. As the motors can only be in specific positions as designated by the geometry of the rotor and stator you can have problems in applications where a smooth or continuous drive is required. To a certain extent this can be overcome by having steps that are around an order of magnitude smaller than required in the application, however, it is something that engineers need to be aware of.

This is only a summary of stepper motors and is not the full story but it will hopefully give you an idea of what they can do and how they can be applied.

There are a myriad of ways stepper motors can be used to drive and position a telescope but generally you will need at least 2 motors mounted on two axes that are at right angles to each other. As for the type of motor required I would suggest a 12 V 4 phase motor with the smallest step angle available.

For a 12 V 4 phase stepper motor the smallest step angle I could find was 0.9°. It we look at my telescope where the widest field of view is 40 minutes I would suggest that you would need steps that were about 1% of that or about 24 seconds. Since the motor has a .9° or 3,200 seconds you would need a drive mechanism that had around a 135 to 1 reduction. Of course that also means that the torque the motor produces will be magnified by a factor of 135 and therefore reduce the problems of the telescope being moved by external forces like the wind.

You can actually purchase off the shelf stepper motor packages that come with three motors and drivers that can plug into a lap top or notebook PC. That would make driving the whole thing considerably easier and I believe there are some free or at least very inexpensive software packages that will do all the calculations that are required to drive stepper motor telescope mounts.

It all depends on how far you wish to go with the hands on part. Personally I would go for purchasing a commercially manufactured mount that comes with all the mechanics, gear systems, electronics and software. The price of these has come down dramatically over the last few years and I doubt you could build such a mount for less than you could purchase one off the shelf. However, price is not the only thing to consider and constructing a system yourself can be just as rewarding as using it so it's up to you how hands on you get.

Hopefully that has been some help. I there's anything else that you think I can help with drop me a line and I will do what I can.

I am a big fan of PIC chips, driving a stepper motor at a constant speed is a rather trivial task with a PIC chip, a few darlington's, some diodes and a 7805.

You also might want to look into the planetary gearing found in the cheap electric screwdrivers. They have some backlash but you can't beat them for size and cost.

Is that your own design? Wow. You are good! Thank you for being my inspiration to make a telescope to glimpse the outer space!

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