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Astrokits Quartz/Stepper barn door platform

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Pictures and comments on the Astrokits Quartz Stepper barn door style drive (the larger platform)



Sample full resolution astro images taken with this larger version platform:

>>Be sure to click on the Flickr images to view larger versions<<

Saggitarius region: http://flic.kr/p/9uFsHR

North America nebula region: http://flic.kr/p/9vsbCA

Comet Hyakutake: http://flic.kr/p/8zaviB

Comet Hale-Bopp http://flic.kr/p/dxjuUk


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This platform is as accurate as any Haig (barn door) drive can be.

My original Astrokits design (above right) took only 2 months from concept to production. The Quartz/Stepper model took nearly a year to refine the design, select parts and finish building and testing. It was intended to sold alongside the original. But it would take weeks just to build one unit. For example, the brass rod needs to be stars-on-film tested. Only 1 out of 20 regular rods qualified.

The drive is over 20 years old but works perfectly. It can be used with 400mm lenses for up to 15 minutes. Info on the original drive is HERE

More detail on each picture is available at my Flickr page for this platform  HERE . Mouse over images for detailed comments not shown here.

This image highlights the large size Stroboframe ball head
Astrokits Quartz Stepper Barn Door camera platform

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The motor output gear can be disengaged from the main gear allowing the platform to be hand-operated in the event of loss of power. The center of the bearing (and rod) is exactly 7.14 inches from the center of the hinge.

The bearing is an industry standard model 7R10 which is 1-3/8 inches in outer diameter. The mounting hole was made with a Fornster bit, which creates a flat based hole with a nice snug wall diameter. The Fornster bit stopped short of going through. An ordinary 3/4 spade bit was used on the other side to create a smaller hole. This created a floor for the bearing to rest on and a hole for the moving area of the bearing and of course the brass rod.

It should be mentioned some brands of bearing were problematic because of a small radius at the top of the inner hole. This would interfere with the hub of the gear. With some brands I needed to use a Dremel Moto-tool to make the radius larger. A close look at this bearing (on Flickr) will show barely noticeable grinding marks. It was easier to do than it sounds. The bearing spins up against the Moto-tool. If it is held at an angle, the curve can be reshaped by eye quickly.

Astrokits Quartz Stepper Barn Door camera platform

Astrokits Quartz Stepper Barn Door camera platform

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A view of the bottom of the 100 tooth main gear used in the Astrokits Quartz/Stepper camera platform. The center hole is tapped to accept the 8-32 brass rod. Only 3 threads of the rod are in contact with the gear, which allows for the rod being curved.

The gear is a custom design by Chicago Gear Works. The hub has a taper to allow the gear to approach the bearing on a curve. Only the top of the hub fits the bearing. Also, there is a 0.035 high surface around the hub. This allows the gears to rest on the bearing without touching the fixed outer ring.

The top surface of the bearing is at a height that is barely below the height of the center axis of the hinge. The center of the gear is exactly in line with the hinge axis. The gear was tapped a little loose to allow for slight mismatches.

Sadly, Chicago Gear Works is no longer in business.

Astrokits Quartz Stepper Barn Door camera platform

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This is the stepper driver I designed for this application.

View of the circuit board. The 8650A integrated circuit at center left was an item made by Saronix. It has a built-in quartz crystal and has DIP switch selectable output rates. The rate is such that the motor output shaft runs at 5 rpm. The circuit is designed to run the stepper motor at a minimum of torque, which results in low power consumption. The dip switch allows for high torque, reverse direction, other speeds as needed for other applications. A micro LED on the output flashes in tune with the Saronix's output pulses.

Astrokits Quartz Stepper Barn Door camera platform

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Power wiring on the underside of the Astrokits Quartz/Stepper camera platform. The power jack accepts outside DC power from 5VDC to 15VDC. The screws on the left are actually in the circuit box, the motor wires go to screw down header commection. The gray wire carries power to the micro LED that illuminates the polar scope.

Astrokits Quartz Stepper Barn Door camera platform

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This hinge took quite a while to find. I tried many other solutions but none offered the rigidity and simplicity of this hinge made by Stanley. It is a Model CB concealed bearing 3 knuckle design for heavy doors. I still needed to shim the center pin with brass shim stock  It now has essentially zero play with no noticeable friction. The two smaller holes on the right side are the polar alignment scope mount screws. The original hole left just enough room for the top screw hole.

The hinge can be found at:  http://www.stanleycommercialhardware.com/index.asp?Mode=SAH007_1

Aligning the hinge to the accuracy required at every step of making this platform took about a week. The hinge axis MUST be exactly 7.14 inches from the center of the curved rod.  I created a jig to accomplish this with the original platform's plastic hinge. With the metal hinge I used ordinary shop measuring tools such as a micrometer and steel rule. I don't remember the exact methodology but it was nothing special. Ultimately I fine tuned the hinge distance by direct sky observation and film exposure http://youtu.be/TeDxJ7suqVM

Wood being wood means I needed to  place shims under the hinge leaves to compensate for inevitable minor warping found in all wood . However over 20 years of time have introduced no additional warping. I do store the drive in a sealed case away from temperature and humidity swings however and have found no variation over the years. I chose wood over metal or plastic for a few reasons. It was easy for me to work with. It was low cost and locally available. I felt I would have problems getting plastic or metal looking halfway decent.
This is what was then called furniture grade birch plywood with a coating of red maple stain. I'm not sure how it is graded by today's standards. It was very consistent in appearance and dimensions except for some slight warping in the big sheets I would get. I was always able to easily compensate for it in all the platforms I built.


Astrokits Quartz Stepper Barn Door camera platform (Hinge)


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The stepper motor is a A. W. Haydon A82478 but it is also known as an Airpax A82478. In this application it draws low current and does not warm up even if run for hours. A set of batteries will several nights of imaging. The motor is running at a small fraction of it's full rated torque. It is basically being used as a timing motor. Usually motors like this are used in applications that need as much torque as possible. Most stepper schematics found online do exactly this. I decided to do the opposite and designed the circuit to run the motor with just enough torque needed for my barn door. The circuit has an dip switch option to run at high torque but I leave it on low. A light two finger grip will stop it. What I like is there is no middle ground. Either it's running correctly at 5 RPM shaft outut or it stops. There is no variable control at all. The circuit dip switch can set the motor to run at a number of other commonly used speeds needed for other applications.

Astrokits Quartz Stepper Barn Door camera platform

Astrokits Quartz Stepper Barn Door camera platform

I have to post a picture on an unpopulated board to show how the holes were drilled. Every Astrokits platform was shipped predrilled. I determined it could be a pain for most people and I was able to do it in just a few minutes. The wing nut hole is curved to allow the motor to swing away for no power situations. The small gear hole is curved on the motor side to accommodate the motor hub. The main gear hole is partially drilled to a precise depth with a forstner bit, then drilled on the back side with a spade bit. This creates a circular ridge for the bearing to rest on.

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The best polar alignment scope I could find at the time is the Carton Optical model S-2411 polar alignment scope. It is known in the US as the Astro-Physics model PASILL.
The scope was modified to allow it to be illuminated with a low power micro LED powered by the platform circuitry.
Alignment is done by adjusting the tripod and rotating the reticle housed in the scope barrel where the LED wire goes.  Without this scope, the precision of the other components would be meaningless.

Astrokits Quartz Stepper Barn Door camera platform

Astrokits Quartz Stepper Barn Door camera platform

Astrokits Quartz Stepper Barn Door camera platform

The concept of how this scope works is pure genius on Carton Optical's part.. Other polar scopes require charts or fussy time and date settings. The idea behind this reticle is once you put Polaris and another nearby star within their respective marked circles, two corners of a celestial triangle are formed with the third corner(the small center circle) being the polar axis. Later versions even take the Earth's precession into account.
 The scope is intended for large telescope  EQ mounts but  discovered I only needed to nudge the tripod legs a bit and rotate the reticle barrel, which takes only a couple of minutes.  Place Polaris in the small left circle (in this particular image) and place the dimmer star delta Ursa Minor, in the small circle on the right. That's it. The true (invisible) polar axis in now in the small center circle. Frankly speaking, the small center circle and the large circle are not needed at all. You have then attained precision polar alignment. 

BTW the above discussion of left and right is merely intended to match the picture below. The actual rotation angle depends on what's in the sky at the time you set up. The key point is there's no need to look anything up before hand and you can quickly check alignment before each exposure.  It's intended to provide rough alignment by pointed the line towards beta Cassiopeia and help mistakenly switching the two stars.

Carton Optical S-2411 - AP PASILL polar scope reticle

I may be contacted at  distar97@bestweb.net  or  distar97@gmail.com



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