

A DIY Astro Tracker in Two Nights - skazka16
http://kukuruku.co/hub/diy/diy-an-astro-tracker-in-two-nights

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Florin_Andrei
There are many projects like this, but a lot of them are done wrong: they use
a straight main bolt and they don't compensate for the error in the driver
motor.

This one is done right - the main bolt is curved, so no error (or very little
error) is introduced while tracking. Good job.

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cgearhart
"Wrong" is a strong word. There are compromises in every design. As pointed
out by another poster, accurate pole alignment will be the dominant source of
error for any reasonably accurate barn door. The short focal lengths, wide
field of view, and relatively short "long" exposures of DSLRs (5 minute
exposures will induce thermal distortion on my D60 vs >15 minute exposures for
film) common in DSLRs don't require extreme accuracy, especially if stacking
the final images.

Barn door trackers can have very low error without curving the rod. Modern
microcontrollers also allow digital compensation compared to older barn doors
that often used analog synchronous motors, and variations on this design
(e.g., the double barn door [1]) can provide tracking accuracy at the limit of
hobby applications.

[1] [http://davetrott.com/inventions/double-arm-barn-door-
drive/](http://davetrott.com/inventions/double-arm-barn-door-drive/)

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Florin_Andrei
Well, curving the rod is a pretty simple way of eliminating one source of
errors - why not do it when it's so easy. Of course you still have to do polar
alignment and so on.

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jhallenworld
The overly high tech methods used in the project amuse me: design it with a
CAD system, use a laser cutter to cut the wood, design your own gears, write C
program for microcontroller to driver stepper motor- (advantage: easily adjust
for astrophotography from other planets). It's too much meta-work.

So here is my new challenge: make it using only parts and tools available from
Home Depot.

(even so, I have to get one of these laser cutters..)

~~~
lutorm
I made one a long time ago: two planks, a hinge, two nuts, a threaded rod, and
a camera mount. Operation was by turning the threaded rod 1/4 turn every time
my HP48SX beeped. :)

It's actually advantageous to be manual when you're doing this at -25C on a
cold winter night in Sweden. Batteries tend to not work well at all at those
temperatures.

(And no, you don't _need_ a curved threaded rod, you can just calculate what
the correct rate is as a function of position.)

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theophrastus
Astro-know-nothing question: why wouldn't it be more directly 'tracking' to
forget the constant clock drive (which we know will always be off by some
amount) and set up a suitably long sight tube on some bright star with a photo
sensor at the bottom of it. When the star's light moves out of the tube, a
fine-stepped stepper tracks west until it re-appears ..?

~~~
lutorm
That's how automatic guiders work, but they only provide extra signal above
and beyond the normal tracking rate. Rather than pulsing on and off
completely, you want to start with something you know is almost right and then
just provide small perturbations on top of it.

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codezero
Can't wait to see the results of this, good job!

