University Lowbrow Astronomers

Kitchen Timer Mini Equatorial Trackers.

by Yasuharu Inugi
Printed in Reflections: April, 2010.

Kitchen Timer Mini Equatorial Trackers

Overview

I have created utterly “lowbrow” Poncet type equatorial tracking platforms for small tabletop telescopes. They are small, light weight, and extremely low cost. They are powered mechanically by a kitchen timer and require no electricity.

And they do work.

Rough specifications are as follows:

<Specifications (for unit #2) >

Concept

For Dobsonian telescopes, many of us use “Poncet” tracking platforms. They are very useful, especially when observing with higher magnification. I own two of those for my 8” and 12.5” scopes.

Many Poncet platforms are commercially available today. But as far as I know, they are all made for medium to large aperture telescopes, and fairly expensive.

I own a 3” Celestron FirstScope, a $50 tiny tabletop reflector on a Dobsonian mount. It is a nice little scope for the money (see my article in June 2009 newsletter [“This Little Scope” by Yasuharu Ingui, June 2009]). I use it quite often for quick observing from my balcony. And despite the short focal length (300mm), I often use 100 to 150X magnification. For that kind of power, you’ll start to want to have tracking.

But I didn’t want to put my 3” scope on one of those large trackers. What I wanted was a tiny “letter size” tracker that’s easy to carry around and set up, and would not cost much.

I searched around but I could not find such products, at least commercially.

So I decided to build one.

There were some specific goals I wanted to achieve for this project:

  1. It must work reasonably well with a small tabletop telescope. Tracking should be fairly accurate so there is no significant problem when observing with 100~150X power range.
  2. Easy to set up and use
  3. Small and lightweight, therefore easy to carry around
  4. Low cost
  5. I should be able to build with the basic tools I have. (saw, drill, cutter, etc.)

Being a “lowbrow” astronomer, goal #4 was very important for me. But I had doubts about keeping the cost low in the beginning. If I had to use an electric drive like all the other Poncet trackers, that alone would cost over $50. Plus you would need to add a battery. All that would increase the cost, make the size larger, and add more steps for set-up.

I was wondering if I could possibly replace the electric drive with a mechanically powered one. I knew a tracking platform, even a large one, wouldn’t consume much power. What I wanted was an inexpensive mechanical “motor” which would rotate slowly and steadily.

I looked for such a drive for a while before actually starting the project.

Then one day I walked into a dollar store, and viola! I found a kitchen timer for $1. It is a mechanically powered device which rotates with a slow steady rate of 360 degrees/hour. It is small, light, and cheap. After a 24 to 1 speed reduction, I could get the required 15 degrees/hour rate. It even “rings” at the end of the tracking. A perfect motor for the mini tracker!

It boosted my motivation, and I started to seriously work on this project.

Design

Many of you might already know how a Poncet tracker works, but I’ll explain briefly anyway.

Figure 1

Pictures are better than words, so please also refer to Figure 1 for details.

On a Poncet tracker, a telescope is placed on a horizontal plate (platform), which is designed to turn around an axis parallel to the Earth’s rotation axis. The rotation of the tracker is designed to be exactly the opposite of the Earth’s rotation, therefore cancelling the apparent movement of celestial objects.

A circular segment (I call this “platform wheel” in this article) is attached below the platform on the north side, at an angle equal to 90 degrees minus the latitude of the observing site. There is a pivot point on the south side below the platform, so that the platform can “swivel” around an axis parallel to the Earth’s rotation axis. The rotational speed of the tracker must be 360 degrees/day, or 15 degrees/hour, going from east to west.

The latitude at Ann Arbor is about +42 degrees, so the platform wheel must be tilted 48 degrees from the platform.

I did some geometry calculation and found that for the desired “letter size” platform, the wheel must be about 12” in diameter. To reduce the kitchen timer’s rotation rate of 360 deg/hr to 15 deg/hr, I needed a 24 to 1 gear reduction. This required the size of the wheel on the motor (I call this “drive wheel” in this article) to be about 1/2” in diameter.

Key Elements

Initially, I built a prototype with foam board and mostly scrap parts I had at home. I learned a great deal about the design and found some key areas that needed improvement. I changed the design and built one with wood board base. Then I tested and revised, tested again, etc., until it became reliable. So far, I have built two units: they are about the same size and have very similar design.

Figure 2: Mini Tracker Key Elements

Here are some key design elements I would like to describe. Please also refer to Figure 2 for details:

  1. Motor: A kitchen timer, or the “motor”, is attached to the base at an angle parallel to the inclined platform wheel.
  2. Drive wheel and speed adjustment: I picked up a 1/2 inch “nylon spacer” from a hardware store for the drive wheel. I then wrapped a small strap of duct tape several times to adjust the speed (the more tape you add, the faster the tracker goes.) It was a trial and error process but after several tests I was able to adjust the speed fairly accurately.
  3. Coupling of Motor: I initially mounted the drive wheel directly on the motor, but the weight of the telescope gave too much stress to the motor. So I changed the design to “couple” the motor to the drive wheel so that most of the weight of the load would be absorbed by the wheel support, but not by the motor. For unit #1, I used a screw driver head and a bolt for coupling. This worked, but it had a large amount of “play” that tended to cause problems. On unit #2, I changed the coupling to a cut hex nut and a bolt (thanks to a suggestion by Mike Radwick) for much less play.
  4. Platform wheel: To prevent slipping, I glued a piece of rubber band (used to bind broccoli) on the left bottom of the platform wheel where it contacts the drive wheel.
  5. Pivot point: I placed a cut bamboo skewer stick below the south end of the platform. Then the stick is placed in a small hole on the base. This enables the platform to swivel.
  6. Latitude adjustment: I placed T-nuts and bolts at the south ends of the base for latitude adjustment.
  7. Power support: Kitchen timer’s power was limited and initially I often had a problem of premature stopping. To prevent this, I added a piece of elastic band (the same kind used for your underwear) to pull the platform in the driving direction. This power support enables the tracker to work without stopping, up to about 8 pounds of load.
  8. Platform surface: To prevent slipping, I glued a non-slip sheet (for kitchen cabinet) on the surface of the platform.
  9. Tracking position indicator: To indicate the tracking position in the dark, I drew lines and applied glow in the dark paint on the front side of the platform wheel and the base.

Field Test

I have done tests and made improvements repeatedly, but here are the latest test results I have.

Tests were mostly done on the balcony of my apartment. I used a Celestron FirstScope 76mm mini Dob (f=300mm) with a 6mm Orion Expanse eyepiece (66 degrees of FOV) and a 2x barlow on the tracker. This gave me a 100X magnification, with a FOV of 0.66 degrees. I tracked a star near the celestial equator, mostly either Rigel or Sirius. I set a star in the center of FOV and watch how it drifted out.

The tracking was not dead accurate and the star slowly drifted. But the speed and direction of the drift were pretty steady for the most part, and after adjusting the latitude and the speed, I was able to keep the drifting down to a very low level. When properly adjusted, Sirius stayed in the FOV for the entire tracking period of 60 minutes. This means the drifting was kept under 0.33 degrees/hour (Without tracking, it drifted out in about 80 seconds.)

I also noticed the stars “jiggle” slightly in the FOV. It was noticeable but not uncomfortable for visual observing. (I’ve seen similar kind of jiggling with larger trackers.) I suspect the cause of this jiggling is either imperfection (rough surface) of the wheels, or possibly from the oscillations of the kitchen timer. I also noticed that when someone was walking around or when it was windy in the star could bounce around. This I believe is mostly because the table on my balcony is not sturdy, but also the light weight of the tracker could be a factor.

I believe placing the tracker on a sturdier table, or directly on the ground, will reduce the jiggling and bouncing, but I have not tested yet. Building the tracker with heavier material could also reduce jiggling and vibration.

Final Remarks

Are these mini trackers useful after all? Yes, well, at least for me. Even for a small 3” scope, I found the tracker to be useful, especially for planetary and detail lunar observations where I spend longer time to look for details. Since the mini trackers are so easy to use and carry around, I almost always use one whenever I use the 3” scope. Also tracking is nice to have when you are showing an object to other people. I have never used a camera on the mini tracker, but I would like to try one someday, to take long exposure photos.

I have enjoyed making these trackers and learned a great deal about Poncet platforms. It was a good winter spent, and now it’s spring again! But these trackers are no way near perfect and I’m still making improvements here and there.

Tabletop Scopes on Mini Trackers

Links

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This page originally appeared in Reflections of the University Lowbrow Astronomers (the club newsletter).
This page revised Sunday, March 9, 2014 4:30 PM.
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