Last Fall (2011) I took full advantage of the fantastic sabatical policy where I am employed. My wife and I spent a good portion of this time driving around the beautiful American Southwest and the many awesome parks on and around the Colorado Plateau. While driving hundreds of miles in desolate landscapes the clarity of the night sky inspired me to dream up a camera mount that would rotate to accommodate the polar rotation of the planet. Any long photographic exposures of the stars with a static tripod will result in star trails--which is cool--but precludes an astrophotographer from capturing faint details in the night sky. I calculated the gear ratios in my head (over many hours of isolation) while my wife slept in the passenger seat next to me and started to dream up the other mechanical requirements to build this tool. After returning from my sabbatical and after the new year rolled over I began spending time at the San Jose TechShop where I discovered all the awesome tools that can bring musings like mine out of your head and into the real world. With the access they provide to so many otherwise difficult-to-access tools I decided to make it at TechShop (www.techShop.ws.) The laser cutter and acrylic sheet were the media and method I chose to make this dream real. I also used Autodesk Inventor, which I learned to use at TechShop, to create the mechanical system and the drawings that would drive the laser to cut the acrylic with mesmerizing precision and accuracy. This instructable describes the process and steps I defined to create this equatorial mount.
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Take some time off of work and go somewhere. Spend many, many hours driving in remote and unfamiliar places. Go and explore the world. Without the distractions of work it is amazing how your mind can wander and dream up ideas. I included one of my favorite pictures from our trip of Monument Valley using a long exposure shot with a car's tail lights creating trails. The second photo is an example of how the rotation of the earth creates trails when taking even short (30 second) "long exposure" pictures of the stars. This was taken at F1.8 at 50mm on a Canon T1i. You don't have to look too closely to see the star trails. You can also catch a faint glimpse of the Milky Way across the image.
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thanks again for your exceptional piece of work :)
http://www.123dapp.com/stl-3D-Model/Equatorial-Mount-for-Astrophotography/667245
I would like to do something so, but much simpler and therefore cheaper.
http://www.astropix.com/BGDA/SAMPLE2/SAMPLE2.HTM
If you build it right you simply need to turn the screw at the same rate a watch's second hand turns. Not terribly difficult (and the photography is somewhat forgiving) and you can do exposures up to several minutes.
Alternatively, since a cooker doesn't have near the precision requirements, you might be better off just making your reflective surfaces more tolerant to differing sun angles, rather than having it movable at all. Really depends on the cost of a bigger reflector vs. motor + mount.
1) A wide angle lens is MUCH more tolerant to an innacurate mount. I'm using telephoto to super telephoto lenses which magnify the effect of vibration, bumps, even wind to the point where the photo will develop streaks. The snap shot in the example at the link you provided covers a ~70 degree diagonal angle (20mm on a 1.6 APS-C.) I'm using lenses with anywhere from 100mm up to 1120mm meaning my diagonal FOV is anywhere from ~20 degrees down to ~2 degrees. There is very little tolerance for vibration at this small FOV.
2) The screw is a linear device, but the rotation must happen around the polar axis at a constant rate. It won't take long to begin seeing see the effects of this if you rotate the screw at a constant rate. In order to maintain constant rotational velocity wrt to the polar axis you will need to accelerate the rotation of the screw following some trigonometric function. Not something I wanted to deal with :-). My design will rotate as accurately as the motor you use and not require any funky non-linear rate of rotation. The stepper motor should be extremely accurate and therefor enable "indefinate" long exposure photos.
3) The linked spur gear design, I figured, was more likely to absorb any vibration generated by the motor. On the barn door design the motor would be almost directly linked to the camera mount translating more of the vibrations to the camera itself.
http://web.archive.org/web/20101225122400/http://www.astronomy.net/articles/9/
I donated some money to them for the valuable service they provide.
Happy to answer any questions about the clock drive. It's very easy to build.
For the smoothest operation you should look into a linear stepper driver. Microstepping only approximates a sine wave. This is a link to a page to read about one such driver:
http://www.piclist.org/techref/io/stepper/linistep/index.htm
They're popular with the astronomy set.
Regarding cogging at low RPMs on the stepper motor--I do see this in the system, however I'm not seeing this vibration translate into any measurable impact on the photo. When I experiment with my 1120mm lens combination (400 * 1.4 * 2.0) I'll take another look. The shots from my experiement were taken at 100mm focal length with and APS-C sensor.