BarnDoor Star Tracker

[axle01]

I thought i'd have a go at making a BarnDoor Star Tracker to track the stars as the earth turns hoping to be able to get somewhere between 5 to 10 minutes shutter speeds.

First i did a lot of research to work out length, motor speed, gear ratio etc. then i got to work i started with 2 pieces of 350mm x 100mm x 10mm alloy plate and 2 pieces of 120mm x 100mm x 10mm alloy plate.

I then went to Bunnings and got 2 of the best hinges with the least amount of side play i could find.

I drilled and bolted the 2 long plates and the 2 small plates to the hinges.

I then bolted the small plate to the large plate, this is to adjust for our latitude which in my case is 19*16S.

I then mounted the ball head towards the low end of the wedge to allow the least weight resistance to the motor drive.

I had worked out that the distance from the hinge pivot point to the threaded rod mount would be 29cm so i got a length of 1/4' SS threaded rod which i bent in a 29cm radius.

The reason i use a curved rod is if i used a straight rod it would introduce tangent error which means as the angle increases the speed will change.

The gears are 80T x 20T which is 4 to 1 and the motor spins at 4RPM which on the 1/4' x 20 will give me 15* per hour which is what the earth turns at.

I got most of the motor mounting and gear mounting hardware etc from ServoCity in America, was a real pain waiting for the stuff to arrive then having to order more as i needed it.

I had a forward/reverse controller which i mounted on the bottom plate to control the speed.

Now i have it all together i did a test and it rises at 15* an hour which is what i am aiming for.

[axle01]

More photos

[Averton]

Hi Alan,
Looks great. High quality workmanship. Good luck with the first pics.
P&C

[axle01]

Appreciate that P&C I’m going to scrap this one and make another one, I know I can improve on it for my first attempt.

Al

[Billyboy78]

Nice job

[axle01]

Ta mate.

Al

[Averton]

Hi Alan,
If you are going to make a new improved version and your new design hasn't progressed too far yet it might be helpful to incorporate extra hinging for the tracking hinge. That is put two or even three hinges side by side to increase the length of the hinge and therefore reduce the unwanted side to side movement. We have a design base on the "barn door" and the quality of the hinges is an issue. With your skills you could maybe construct a hinge using bearings and far less side play. Just a thought. Hope it goes well.
Peter & Clare

[Billyboy78]

Or even use a piano lid hinge. IIRC I used a hinge with a bearing race, but my downfall was using bits of 6 x 1"wood.

[bojan]

Bronze bushing (https://www.ebay.com.au/itm/10x-SF-1-Bronze-Bushing-Bearing-8mm-x10mm-x10mm-Bush-Motor-Sleeve/271733237908?hash=item3f44902894:g: g0YAAOSwlgNa~sZY) + stainless rod (https://www.ebay.com.au/itm/Stainless-Steel-304-Round-Bar-Solid-Rod-Dia-3-4-5-6-8-10-14-mm-125mm-500mm/223103528608?var=521917357162&hash=item33f201aea0:m:mGKe185x3m92I sTRJDcswAg)

[axle01]

Thanks guys I’m looking at these pillow blocks and a 1/2” SS rod.

https://www.servocity.com/500-bore-pillow-block

Al

[bojan]

Yep..

[Merlin66]

Well done, looks great!
For Gen2 I'd seriously look at a double arm Trott design - even more capable.
I wouldn't worry too much about bearings.

[axle01]

Thanks Ken,,,,,I haven’t looked at that design,,,I’ll do it now.

Al

[iborg]

Another option - tweak the motor speed on the fly!


https://barndoor.space/index.php/2018/04/11/the-tangent-error-in-a-barn-door-tracker/


Philip

[bojan]

No need to tweak motor speed in Alan's original design, as the threaded rods is curved - so motor speed needs to be constant.

The only issue may be the quality of the thread (it's uniformity and smoothness).

[axle01]

Yeah this is for a straight threaded rod but I did contact him but haven’t got a reply yet.

Al

[iborg]

Quite right - I didn't notice the curved rod :shrug: