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Difference between type 3 and 4 Barn Door


Annydrite
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Hi,

I am new to astro photography and am building a barn door tracker.  I was thinking of building a type 3 tracker.  I also see where a type 4 can achieve much longer tracking.  While I do not need tracking to last that long I have a question regarding construction.

I see plenty of pictures that show the construction of a type 3 drive with 2 inch hinge spacing and a beta of 2.  As far as I can tell type 4 is a beta of 2 and the layout of the boards is the same.  Is this true? 

The classic picture from Dave Trotts article shows them from the side view and dose not really make sense to me.  It seems type 3 and 4 are the same as the sliding point and drive arm are the same.  The 3d drawing seems to show a type 3 with a bata of 6.

Double-Arm-Types.jpg

Double-Arm-Iso.jpg

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No for two reasons :

- the contact stell/brass is almost frictionless

- the angle of contact between the cam and the stud bolt is always 90°

However, I was using a big 400s/r stepper motor, full step driven and it was generating a lot of vibrations...

In fact, buying a small motorized EQ1 is cheaper and gives better results...

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You might do better to think about a tangent arm drive rather than a platform. I've tried to do things with hinges and usually run into problems because they are loose. There are all sort of ways of making bearings and that sort of thing is easier on things that look like arms rather than a platform.

This gives and idea of what accuracy the twin arm can give. Use the link towards the top of the page to download it

http://adsabs.harvard.edu/abs/1995JBAA..105...65C

This might help

http://www.mikeoates.org/mas/projects/scotch/hdm/home.htm

An alternative is to drive the nut and and bend the all thread as shown here. This one is over the top for just a camera but it should be possible to include the sort of adjustments equi heads have for alignment. Longer possible exposure times need accurate alignment. I suspect that is why many simple trackers have been made just using one pivot and arm.

http://www.dv-fansler.com/Astronomy/portable_equatorial_mount.htm

Must admit I wondered about a commercial camera platform recently. The max payload is disappointing and they suggest mounting them on a Manfrotto geared head and a hefty tripod. Price - ouch. While I haven't handled a geared head like that I doubt if they allow easy fine adjustment as most equatorial heads do. This is why I drifted away from any form of camera platform but do feel most parts of a tangent drive could be made from mdf or even ply.

John

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The point was making about think arms rather than platforms is that it's easier to make a rigid pivot. One way for  instance would be to fit a ball race into a hole in one, few washers for spacers and and bolt it to whatever it pivots on. The 2nd arm needs a sliding contact -  ball race again. Another alternative might be the the bronze bushes bearing suppliers sell. The only problem with those is that they are meant to be driven into holes sized to shrink them a bit. It might be possible to buy say ones with a 6mm bore and ream them out to 1/4 in or some similar idea. Bright drawn metal bar is usually pretty close to the stated diameter. Silver steel these days or drill rod as it's called in the USA isn't as close as it used to be. Some supplier stock ground rod. That will be very close.

I would be inclined to use 18mm MDF even though breathing the dust isn't a good idea. I have also routed aluminium plate in the past. To add a camera all that is really needed is a small platform on the end of an arm with a ball head on it.

:evil:  I've been intending to do it for some time but ordered the light weight Ioptron mount earlier this week. I suspect I may get annoyed with the suppliers. If they sell it that should stock it / declare delivery times on their web sites.

There are probably some better web pages about on tangent arm drives than the ones I posted. It's an old idea but I would seriously wonder about alignment if really long exposures are contemplated and wonder if the complication of the 2nd arm was worth while.

John

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Edited by Ajohn
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  • 5 years later...

When I built my barn door (modified type 1) the size of the boards was driven by the size of the bolt which governed the distance to the hinge, I had used an M10.

This was the site for the calculation what I don't know is if the same applies for type 3 or 4, but I imagine it does as the aim is the same.

https://blarg.co.uk/astronomy/barn-door-tracker-calculator

If you are after reducing tangent error (the point I think of type 3 and 4 designs) then on my build thread, there is a link in my signature, is a contribution from a member on how to print a 3d part to use on a type 1 design to reduce tangent error.

Edited by happy-kat
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52 minutes ago, happy-kat said:

When I built my barn door (modified type 1) the size of the boards was driven by the size of the bolt which governed the distance to the hinge, I had used an M10.

This was the site for the calculation what I don't know is if the same applies for type 3 or 4, but I imagine it does as the aim is the same.

https://blarg.co.uk/astronomy/barn-door-tracker-calculator

If you are after reducing tangent error (the point I think of type 3 and 4 designs) then on my build thread, there is a link in my signature, is a contribution from a member on how to print a 3d part to use on a type 1 design to reduce tangent error.

Hey thanks for replying so quickly ..!

I'm planing to build a type 4, I found a PDF and need to recalculate the distances and speed of the threaded rod.
Not planning to build it this way, I'll be printing lots of parts(of course..😁), only a larger version.
Point is, looking at the drawing, How do I recalculate distances r, b and c when I want to make it twice as large.
image.png.07dd770f177a3a772860eba2fb27a2f4.png

heavy-duty-double-arm-barndoor-building-plans.pdf

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My thoughts

Type 4 is working like my modified type 1.

'r' is the one of note that is the hinge to the drive rod bolt.

Assuming the m6 bolt used on the PDF has 20 threads per inch then I make it 'r' would be 290.25 mm so I don't see how they have got 423.06 mm unless the rod used had just under 14 threads per inch or type 4 uses different calculation. The blurb did not as far as I could see include the calculation that had been used for 'r' which I had expected would rely on which bolt being used.

If you use the online calculator for the bolt you are using then you can calculate 'r' or do it by hand. If you need a much bigger overall size then the PDF is using an m6, changing to an m10 etc. would give a larger 'r' and from that given the original diagram scales you could work out a new 'b' and 'c' I think. But if a type 4 uses a different calculation for r then that calculator is probably wrong.

EDIT Of what I can see of the type 4 design is that the camera arm is free to slide against the drive arm, but the measurements used are key to the design. I found another website here where the 4 arm type was used but it did not give the raw calculations to do it from scratch. 

Looking at the size of your fabulous telescope build an m6 bolt is probably not going to cut it :)

Edited by happy-kat
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Following the building instructions from Dave Trott I drew this very basic version.
It is basically nothing more then three parts(in the drawing) Hinges are not included in the drawing. But the constraints(to simulate these hinges) are in place.
The driven part is the brown one, the bleu surface is where the camera(s) go.
Pushing the brown part upward, the blue part has to go with it. Notice that  both moving parts do not have the same speed. The difference is very little. But just enough to correct the error that is produced by a 'single arm Barndoor tracker'.

In fact the error is no more then 1 arc second (max) during a one hour tracking session, according to this list.
After two hours tracking the error gets smaller again.
image.jpeg.d8fd92908a0b7a88493017cfe05e9c1b.jpeg

image.jpeg.0fcc258349381eaaefedc1b8f585e806.jpeg

 

image.png.953bd863b713c7954900121ef0e6adfe.png

Fully opened the angle between the two moving parts is a little bit wider. Not much but apparently just enough to eliminate tracking errors completely.

image.png.fa66f5644992aedf3134a8d17823aa1c.png

 

 

 

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What's key from what I was reading was the length of the flat part on the blue piece before the hinge and DT did not share the calculation he'd used or for the other distance too of the bolt size tpi to the brown arm hinge.

I've come across using Arduino code to manage tangent error.

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1 hour ago, happy-kat said:

 

I've come across using Arduino code to manage tangent error.

These mechanical solutions are ingenious but isn’t this the better more modern solution? Or couldn’t you print or cut curved gear segment actuation levers @Chriske like the curved threaded rod in the other thread?

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Maybe it's a bias due to my lack of mechanical skills ;) but I am a great fan of minimising the number of moving parts in a machine. To do as much of the "clever" stuff as possible in software.

I am in awe of a lot of the skill and genius that went into Victorian mechanisms. But give me a digital clock, over a magnificanlty crafted timepiece, any time!

Edited by pete_l
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