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How to Build an Equatorial Mount for Long Exposure Astrophotography

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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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adarsh_ananth2 months ago

How long can this be used for tracking?

Stan1y6 months ago
how much torque does a quartz clock movement produce a 2:1 drive from the hour hand sprocket would give your 1 revolution per day
i add a few pics here i hope it helps a bit
1.jpg2.jpg
that was an absolutely brilliant effort :) however i have a problem designing my own gearbox using Autodesk inventor. i have designed and linked the gears following your steps but the trouble begins here: after defining a plain for sketching the contour, i followed as you said: projected the holes and sketched the contour but it doesn't allow me to extrude the contour saying:"create extruded feature: problems encountered while executing this command" maybe i should've asked this in a CAD forum but please give me a clue. it's been three days or so I'm trapped in this part. i've checked that the contour is closed and also there's no possible intersection between the extruded sheet and the gears.
thanks again for your exceptional piece of work :)
Applebohn1 year ago
Absolute fabulous!
ridiculously.awesome (author) 2 years ago
I just added the CAD files for folks interestsed in building this unit for themselves.
jeffbot2 years ago
I love this article and is a great project for us sky watchers. Could you provide (add) the DWG files so I could have the gears cut?
ridiculously.awesome (author)  jeffbot2 years ago
Thanks for the compliment. I'm happy to add the DWG files and will update the post with this. In the meantime, I did export the STL files and posted them to the 123Dapp site:
http://www.123dapp.com/stl-3D-Model/Equatorial-Mount-for-Astrophotography/667245
ridiculously.awesome (author) 2 years ago
I added a build of materials list with cost and sources for the components. The total cost for the build was just under $200.
rimar20002 years ago
WOW, what a good work!

I would like to do something so, but much simpler and therefore cheaper.
A very inexpensive alternative is the hand-turned barn door tracker like this one:

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.
I considered the barn door type design, but there are a few problems with this design.

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.
I gotta ask then, why not just jury-rig a standard telescope equatorial mount (or build one in that style). One of those has gotta be less complicated gearing-wise. Most telescopes on mounts like that are way more intolerant to vibration than your camera lens and they do just fine. Just curious (I totally get it if you simply wanted to build something of your own design, that's very cool in and of itself).
This all came to be on our trip to the Southwest and my imagination got the best of me :-). I learned as I was going through the design process that there are mounts for telescopes that you can buy but I really wanted to go through the process of drafting up, fabricating and building what I had dreamed up. I did have some concerns over these mounts not being designed to tolerate vibration since they are built for real-time observations, not long exposure photos. Doing this project provided me a fantistic experience in learning how to take an idea and make it real. It's been a gateway to other projects too like a custom fit glass rod holder designed to fit precisely into the rod warmer we have. (We are also flameworkers, another recent hobby my wife and I acquired.)
Thanks for the info, it is very interesting. But I want to make an unattended solar cooker.
Then all you need is a 1rpm motor and a gear.

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.
ridiculously.awesome (author)  rimar20002 years ago
The most time consuming part was learning as I went. CAD was new to me and I'm not a mechanical engineer (I am a computer engineer by training.) I was considering including a BOM cost for the project and will update the Tools and Materials section with this information. I estimate that the total cost of parts was around $125 including the electronics, materials and parts.
Zachrey2 years ago
I wrote the article for the human powered clock drive for astronomy.net a while back! The site is no longer up, but I found this Waaaayy cool website called the 'Wayback Machine' which archives stuff from the web. Praise Jesus Hamashiach, they have an archived copy of my article:

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.
Zachrey Zachrey2 years ago
Hmmm... I am unable to delete this comment. What I meant to say is that I wrote "an" article years ago about a clock drive I built. It is quite different from this design. Sorry for any confusion. I thought I responding to a comment from an astronomy website. That's what I get for having too many tabs open!! :-p
pfred22 years ago
I noticed you put 1/4 I.D. bearings right onto 1/4" threaded rod. Didn't that misalign the bearings axially due to the pitch of the threads? Also, I've always found stepper motors to cog a lot at very low speeds.

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.
ridiculously.awesome (author)  pfred22 years ago
I did not observe any misalignment of the bearings resting on the threads. I already have a little misalignment in the system due to a need to enlarge the hole and using a jig to recut them. I'm sure there is a better way to assemble the cogs onto an axle, but I was impatient and anxious to get it assembled :-).

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.
I didn't notice it myself either until I built this. It has bearings on both ends and the misalignment is very pronounced running it. I actually had to loosen the mountings up in order for it to run.