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FORUMS Photo Sharing & Visual Enjoyment Astronomy & Celestial 
Thread started 27 Jul 2014 (Sunday) 11:09
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Barn-Door tracker test

 
davidmtml
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Jul 27, 2014 11:09 |  #1

I've been thinking for quite awhile about building a small barn-door tracker to improve my Milky Way shots. I got it built a couple days ago, and was able to get just a couple shots in last night. We had gone on a night float, and I had two other people with me who didn't want to spend much time out at 2:00 in the morning! I set up an extremely rough orientation with the North Star (as in, I couldn't even find it and took a wild guess) and fired away.

Here is the first shot I got.
Canon 5d, Samyang 14 2.8 @ 2.8, ISO 800, 191 seconds.


IMAGE: https://farm4.staticflickr.com/3881/14571418960_c4edf50e24_b.jpg
IMAGE LINK: https://flic.kr/p/ocCk​co  (external link) 20140727-022401-IMG_4452 (external link) by David Rabenberg Photography (external link), on Flickr

And a 100% crop.

IMAGE: https://farm4.staticflickr.com/3892/14571418580_7d42561478_b.jpg
IMAGE LINK: https://flic.kr/p/ocCk​5Q  (external link) 20140727-022401-IMG_4452-2 (external link) by David Rabenberg Photography (external link), on Flickr

For comparison's sake, here is a 100% crop from a few weeks ago without the tracker. ISO 3200, f2.8, 30 seconds. All of the photos have the same amount of noise reduction added (20 luminance, 15 color in LR5). I know I could push them a little further to reduce noise, but wanted to keep the NR minimal for this example.

IMAGE: https://farm6.staticflickr.com/5588/14754993671_5eda780b62_b.jpg
IMAGE LINK: https://flic.kr/p/otRc​yB  (external link) 20140704-004847-IMG_2315 (external link) by David Rabenberg Photography (external link), on Flickr

HUGE difference in the amount of noise in the image, and almost no star trailing at over 3 minutes! Consider me happy!


Here's a look at the setup (more details in next post)

IMAGE: https://farm3.staticflickr.com/2906/14571450259_f52c1767b8_b.jpg
IMAGE LINK: https://flic.kr/p/ocCu​v2  (external link) 20140727-093949-IMG_3892 (external link) by David Rabenberg Photography (external link), on Flickr



  
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davidmtml
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Jul 27, 2014 11:13 |  #2

IMAGE: https://farm3.staticflickr.com/2917/14735266156_930bef059e_b.jpg
IMAGE LINK: https://flic.kr/p/os76​g3  (external link) 20140727-093949-IMG_3892 noted (external link) by David Rabenberg Photography (external link), on Flickr


2x 3" hinges. A 1x6 is piece of wood is not actually 6" long, but about 5.5", so my hinges overhang by a 1/2" or so.
IMAGE: https://farm4.staticflickr.com/3878/14758105935_b6c5079bea_b.jpg
IMAGE LINK: https://flic.kr/p/ou89​Jk  (external link) 20140727-094221-IMG_3899 (external link) by David Rabenberg Photography (external link), on Flickr


This is really the piece that does the magic. Minds smarter than my own figured out that if you have a screw with 20 threads per inch, place it exactly 11.42 inches from the hinge, and make 1 rotation per minute (in this case, 1/2 rotation every 30 seconds, and you must turn more frequently the longer your focal length), the camera will track the movement of the stars.
IMAGE: https://farm3.staticflickr.com/2923/14777983253_6976900611_b.jpg
IMAGE LINK: https://flic.kr/p/ovT2​yH  (external link) 20140727-094026-IMG_3896 (external link) by David Rabenberg Photography (external link), on Flickr



  
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rfdesigner
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Jul 28, 2014 04:52 |  #3

davidmtml wrote in post #17059169 (external link)
This is really the piece that does the magic. Minds smarter than my own figured out that if you have a screw with 20 threads per inch, place it exactly 11.42 inches from the hinge, and make 1 rotation per minute (in this case, 1/2 rotation every 30 seconds, and you must turn more frequently the longer your focal length), the camera will track the movement of the stars

Good show.. even a little crude tracking makes a big difference.

But focal length doesn't change the speed the earth rotates. What you're trying to do is:

Camera rotation = earth rotation - mount rotation.

You want to make mount rotation such that camera rotation = 0.. so focal length doesn't change anything.

Well done anyway.


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pwm2
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Jul 28, 2014 05:31 |  #4

rfdesigner wrote in post #17060844 (external link)
Good show.. even a little crude tracking makes a big difference.

But focal length doesn't change the speed the earth rotates. What you're trying to do is:

Camera rotation = earth rotation - mount rotation.

You want to make mount rotation such that camera rotation = 0.. so focal length doesn't change anything.

Well done anyway.

Focal length affects magnification. More magnification means a smaller rotation of the earth will be visible. So more magnification - longer focal length - requires that his barn door needs to have the screw tweaked more often. Not rotated faster, but more often.

So instead of one turn every minute, he needs to switch to a half-turn every 30 seconds or a quarter turn every 15 seconds. Remember that he hasn't a motor that keeps the screw constantly rotating.


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rfdesigner
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Jul 28, 2014 05:54 |  #5

pwm2 wrote in post #17060877 (external link)
Focal length affects magnification. More magnification means a smaller rotation of the earth will be visible. So more magnification - longer focal length - requires that his barn door needs to have the screw tweaked more often. Not rotated faster, but more often.

So instead of one turn every minute, he needs to switch to a half-turn every 30 seconds or a quarter turn every 15 seconds. Remember that he hasn't a motor that keeps the screw constantly rotating.

Ah.. that wasn't what I understood him to mean. Yes, in that case absolutely correct... re-reading I see this now.

I always think in terms of constant rotation, just trying to keep the rate approximately correct. For some reason I'd never considered stop and go.


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davidmtml
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Jul 28, 2014 15:39 |  #6

Yep, you are both right haha. I see now how my original wording was confusing. 1rpm could be half a turn ever 30 seconds, a quarter turn every 15 seconds, 1/6 turn every 10 seconds etc...The ideal setup is to use a motor at 1rpm so that it IS a constant rotation. But luckily at wider angles, it is pretty easy to just make half a rotation every 30 seconds. As you get to longer focal lengths, it would be much easier to have a constant rotation.

Obviously at some point I would love to get it set up with a motor, but for now I was just going for cheap and simple!!




  
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dlos
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Jul 28, 2014 17:54 |  #7

What about this setup prevents "camera shake" or vibration reaching the camera when turning the screw?

Pretty cool idea and implementation!


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pwm2
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Jul 28, 2014 18:06 |  #8

dlos wrote in post #17062367 (external link)
What about this setup prevents "camera shake" or vibration reaching the camera when turning the screw?

Pretty cool idea and implementation!

Only the long exposure time. Same result as when you walk infront of the camera while using a 10-stop ND - the camera doesn't pick up walkers but only people who have stopped for a while.


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davidmtml
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Jul 29, 2014 00:14 |  #9

pwm2 wrote in post #17062390 (external link)
Only the long exposure time. Same result as when you walk infront of the camera while using a 10-stop ND - the camera doesn't pick up walkers but only people who have stopped for a while.

Yep, basically this. However, I do my best to be as gentle as possible while turning the screw.




  
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pwm2
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Jul 29, 2014 00:17 |  #10

davidmtml wrote in post #17063029 (external link)
Yep, basically this. However, I do my best to be as gentle as possible while turning the screw.

The most critical part is that the tripod stands on stable ground, and the head doesn't slowly sag - the long exposure times can handle quite a bit of vibration but not a changed position/orientation of the camera.


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pkim1230
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Jul 29, 2014 21:45 |  #11

why do you need a rubber band there?
do you have the link to the instructions?



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pwm2
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Jul 29, 2014 21:59 |  #12

pkim1230 wrote in post #17065034 (external link)
why do you need a rubber band there?
do you have the link to the instructions?

The rubber band shouldn't be needed, but reduces risk of bumping the upper part. And extra load might also reduce issues with play in the hinges.

But depending on where you live (how aggresive angle you need to use) and how long the screw is, you might need the rubber band to make sure the camera weight doesn't overhang the hinges too much.


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seres
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Jul 30, 2014 00:07 |  #13

Nice example of the construction. Thanks!


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losangelino
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Jul 30, 2014 00:19 |  #14

Wonder if this could help automate things. https://www.sparkfun.c​om/products/12219 (external link)



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pwm2
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Jul 30, 2014 00:36 |  #15

losangelino wrote in post #17065241 (external link)
Wonder if this could help automate things. https://www.sparkfun.c​om/products/12219 (external link)

A problem with it is that it is a "stupid" motor that gains more torque as you apply more voltage - so it also increases speed with the voltage while losing speed with the weight of the work it has to perform.

This means that you need to add some feedback sensor detecting how far it has rotated to decide if the voltage needs to be increased or decreased to actually maintain that speed of 1 turn/minute.

It's way better to use a stepper motor solution where a microcontroller issues step pulses at the required frequency. As long as they have enough hold torque and the step frequency isn't too high or the step acceleration too high, they will guarantee the requested turn rate.

I got inspired yesterday so had to make a quick check at the local "toy store" but they had no stepper motors home. So I have to decide if I have enough time to mail-order some gear and build. Or look at what older projects I can scrap to reclaim a suitable stepper motor.


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Barn-Door tracker test
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