The barn door is coming along. I am playing around with construction methods and processes to get a feel before I make my final unit.
A question I have is, is the perpendicular alignment of the hinge in relation to the longitudinal axis of the arm critical? In my experimental version and with my limited woodworking skills I can get the hinge to within 1 deg of perpindicular to the longitudinal axis of both arms of a single arm design. I can get the threaded rod arm to the correct distance from the centre of the hinge without a problem, am I making myself clear here? I plan on using arms of about 19mm x 100mm.
My motor will be 12VDC, speed controlled with a PWM controller.
Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,478
Quote:
Originally Posted by Steve_C
Ok,
The barn door is coming along. I am playing around with construction methods and processes to get a feel before I make my final unit.
A question I have is, is the perpendicular alignment of the hinge in relation to the longitudinal axis of the arm critical? In my experimental version and with my limited woodworking skills I can get the hinge to within 1 deg of perpindicular to the longitudinal axis of both arms of a single arm design. I can get the threaded rod arm to the correct distance from the centre of the hinge without a problem, am I making myself clear here? I plan on using arms of about 19mm x 100mm.
My motor will be 12VDC, speed controlled with a PWM controller.
TIA
Steve
Steve,
The errors I worked out for you in an earlier post apply here.
Quote:
"A 1mm error in the radius, that is where the radius error results in a 1mm greater radius than the ideal radius will result in a drive rate error of 4 arc sec per minute. This is not significant for short exposures on wide angle lenses. Up to 2 min exposures up to 100mm lenses. Accurate polar alignment will present a bigger problem than this error."
If it isn't quite perpendicular a 1 deg error shouldn't displace the thread by 1mm so it will be less than the error quoted above
I also used a control knob (think its called a potentiometer) to fine tune the speed as I found that the 2rpm motor from China wasn't very accurate, might have something to do with my voltage supply? I
An update on my project. I fiinally received the 2rpm motor from China, it does 2 1/2 revs/min on a fully charged 12v battery. I have decided to change the gearing to 1:1 to give me a bit more speed control a pwm controller. I also figured out a way to ensure the mount from the bottom arm to the tripod was very strong, a 1/4" t-nut held in place by a short 1/4" bolt that allowed enough thread left in the t-nut for the tripod screw.
I'll post some photos when complete, if we ever get some clear skies in Sydney again I can run some photo tests!
Hi all, not sure whether to post this here or start a new thread, but as it is barn door related............
I have come across a method of aligning a finder scope on my BD to the hinge, by looking at a fixed object, moving the top plate and seeing if the object moves in the finder. Would someone please explain why this works?
It seems to me that moving the top plate would cause the fixed object to move out of view. What am I missing?
Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,478
If the optical axis of the finder and the rotational axis of the platform are parallel. For a distant object they are both pointing at the same point.
However, when viewing a nearby object with the finder, parallax will make the object leave the field.
Many years ago, I had an old mount that couldn't have a polar finder attached, solid 2" shaft. I tried attaching it to the outside of the shaft housing but the mount was so heavy, If I bumped anything, I lost alignment.
So I used to set up an 80mm shorty refractor on the telescope and during daylight, point it at a distant object and line up the mount with the scope by rotating the polar axis. Worked a treat.
Further update, BD is now complete. Images attached.
The left switch is power, the other switch is motor direction (not really needed). Red leds indicate motor direction green led power on. Powered by 8 AA cells. So far everything looks good, the BD top arm moves approx 0.25 deg /min which is correct, still need to fine tune that. The motor and gearing do not stall with a camera and heavy lens attached, so that is good.
A simple sight tube might help in getting the first 'eyeball' alignment going.
If you are going to be using somewhere consistently like your backyard you could do a solar noon & plumb bob line on the ground. That will give you a good Nth/Sth line to start with. Probably accurate enough for widefield. I find the digital compasses on phones\tablets can be somewhat inaccurate.
Another method is to get onto Google Earth and zoom in on your site beforehand and then look for a horizon landmark as a reference point either due Sth of it or offset by some measurable amount. I used this first at home to set a line on the driveway for repeated use.
A proper hiking compass can then let you establish a pretty good Nth/Sth line using it as a protractor.
The inclinometer applications on tablets and phones seems to work quite well as long as the phone has a good straight edge to sit on.
Hey Presto SCP !!
If you are going to be using somewhere consistently like your backyard you could do a solar noon & plumb bob line on the ground. I find the digital compasses on phones\tablets can be somewhat inaccurate.
A proper hiking compass can then let you establish a pretty good Nth/Sth line using it as a protractor.
The inclinometer applications on tablets and phones seems to work quite well as long as the phone has a good straight edge to sit on.
Hey Presto SCP !!
Thanks.
I will be working in different locations, but I do have a proper hiking compass and a good inclinometer app.
T
I
