<p>noctilove.co.uk has beautiful images taken using his barn door tracker. It is amazing what it is possible to achieve with just the tracker and a DSLR camera. He gives <a href="https://noctilove.co.uk/barn-door-tracker/" rel="nofollow noreferrer">designs for his tracker</a> where he uses a hinge to pivot distance of a=274mm. I would like to build his design but when i calculate the distance i get a=286 mm. Is there something special about his design that makes the distance different? I calculated for an M8 bolt with a pitch of 1.25mm and assumed <span class="math-container">$\Theta=2\arcsin(a/2r)$</span> i.e. an isosceles barn door design.</p>

CC BY-SA 4.0
2024-01-30T17:00:32.170Z
You might need to show your work on how you got to 285mm
Darth Pseudonym
CC BY-SA 4.0
2024-01-30T18:23:05.203Z
http://www.questions4steveb.co.uk/html/Astronomy/photos/Isosceles-drive.gif. 
My calculation: for the isosceles triangle $\theta=2arcsin(a/2r)$. Therefore $r=\frac{a}{2(sin \theta/2)}$. Where a=1.25 mm/min for a M8 bolt rotated at one turn per minute,  $\theta =0.25$ deg/min is the rotation of the earth and r is the pivot to hinge length. This gives r=286mm. Noctiloves barn door design is here: https://noctilove.co.uk/barn-door-tracker/. I wondered if he had made a mistake or if there is something about his design that means the isosceles model is not correct.
dkmax
CC BY-SA 4.0
2024-01-30T19:48:24.603Z
The difference for such wide field, short exposures will be negligable.  The errors in polar alignment will dominate.  There's also a fine thread M8 with a pitch of 1.
Greg Miller
CC BY-SA 4.0
2024-01-30T20:19:45.387Z
@GregMiller Don't think a pitch 1 would help here. That overshoots a long way and gives r=229mm.  If dkmax's calculation is correct (and it looks right to me) then you'd need a pitch of ~1.20 to get r=274mm
Darth Pseudonym
CC BY-SA 4.0
2024-01-30T20:30:17.820Z
@dkmax Okay, I think you typo'd in your original post, because you said you had a=286 mm but that's definitely supposed to be an r based on your full explanation. I don't have any helpful input at this point -- your numbers look right to me. Best guess is there's some slop introduced by the way the barrel nut sits inside the plank that means the bolt is not exactly 90 degrees to the plank.
Darth Pseudonym
CC BY-SA 4.0
2024-01-30T21:33:44.050Z
yes, sorry i did change my calculation. the way noctilove uses the barrel nut is slightly different from most of the designs i have seen. it maybe that the easiest solution is to build it according to the given design and then measure the rotation as the bolt rotates – 
dkmax
dkmax
<tracker>
is the design of Noctilove’s barn door tracker correct?
dkmax
# is the design of Noctilove’s barn door tracker correct?

(Asked by: username_0 on 2024-01-30)

noctilove.co.uk has beautiful images taken using his barn door tracker. It is amazing what it is possible to achieve with just the tracker and a DSLR camera. He gives [designs for his tracker](https://noctilove.co.uk/barn-door-tracker/) where he uses a hinge to pivot distance of a=274mm. I would like to build his design but when i calculate the distance i get a=286 mm. Is there something special about his design that makes the distance different? I calculated for an M8 bolt with a pitch of 1.25mm and assumed $\Theta=2\arcsin(a/2r)$ i.e. an isosceles barn door design.

**Tags:** tracker
> **Comments:**
> **username_1**: You might need to show your work on how you got to 285mm  
> **username_0**: http://www.questions4steveb.co.uk/html/Astronomy/photos/Isosceles-drive.gif.  My calculation: for the isosceles triangle $\theta=2arcsin(a/2r)$. Therefore $r=\frac{a}{2(sin \theta/2)}$. Where a=1.25 mm/min for a M8 bolt rotated at one turn per minute,  $\theta =0.25$ deg/min is the rotation of the earth and r is the pivot to hinge length. This gives r=286mm. Noctiloves barn door design is here: https://noctilove.co.uk/barn-door-tracker/. I wondered if he had made a mistake or if there is something about his design that means the isosceles model is not correct.  
> **username_2**: The difference for such wide field, short exposures will be negligable.  The errors in polar alignment will dominate.  There's also a fine thread M8 with a pitch of 1.  
> **username_1**: @GregMiller Don't think a pitch 1 would help here. That overshoots a long way and gives r=229mm.  If dkmax's calculation is correct (and it looks right to me) then you'd need a pitch of ~1.20 to get r=274mm  
> **username_1**: @dkmax Okay, I think you typo'd in your original post, because you said you had a=286 mm but that's definitely supposed to be an r based on your full explanation. I don't have any helpful input at this point -- your numbers look right to me. Best guess is there's some slop introduced by the way the barrel nut sits inside the plank that means the bolt is not exactly 90 degrees to the plank.  
> **username_0**: yes, sorry i did change my calculation. the way noctilove uses the barrel nut is slightly different from most of the designs i have seen. it maybe that the easiest solution is to build it according to the given design and then measure the rotation as the bolt rotates –  dkmax  

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## 0 answers

