Barn door star tracker

[Russman (Russ)]

Just an update; I'm making some mods to the mount for my barn door so it is easier to setup when i'm out in the dark. Currently need 3 hands to set it up accurately. Its probably going to look bulky but it will be very sturdy, I hope.

Also Im going to have to re-make the arms on the tracker as the plastic I used is bending and twisting!

I'll try and post more pics later tonight!

[OzEclipse (Joe Cali)]

Russ,
Why don't you build a double arm tracker? Much more accurate and much longer working time than a single arm.

Joe

[Russman (Russ)]

Originally Posted by OzEclipse

Russ,
Why don't you build a double arm tracker? Much more accurate and much longer working time than a single arm.

Joe

Hi Joe. Thats the next project. I'll get the mount sorted and start making one.

[Russman (Russ)]

Just some progress shots of the mount I was making over the weekend.

[Russman (Russ)]

A few more progress shots of the mount I've been working on for my barn door tracker.

I've used a ball lock bolt for azimuth adjustment (pic 2). By tightening the star grip (pic 4) a holding force of 5.3 kN can be achieved! Massive!!! By loosening the star grip the ball lock bolt can be removed and the mount then separated from the base plate (pic 1 & 2). The mount does not yet have a screw for fine azimuth adjustments, something I may work on soon if needed.

It also has an articulated joint which the tracker is mounted for elevation adjustment (these are usually used with aluminium profile). There is a star grip for fine adjustment (pic 3). The articulated joint can be locked into place with the indexing lever on the side after adjustments have been made.

Now I need some clear skies so I can put it into action with my single arm barn door tracker. Don't know how I will go without a polar finder scope, I may need to purchase one to help when making fine adjustments.

I'll be working on a double arm tracker soon and will post up progress shots.

[OzEclipse (Joe Cali)]

Hi Russ,

What are those two pieces you've used for the elevation adjuster? Are they aluminium and are they off the shelf or custom made?
Look like a great build. Who needs a commercial drive.


Joe

[Russman (Russ)]

Originally Posted by OzEclipse

Hi Russ,

What are those two pieces you've used for the elevation adjuster? Are they aluminium and are they off the shelf or custom made?
Look like a great build. Who needs a commercial drive.


Joe

Hi Joe. They are an off the shelf item. They are an accessory for aluminium profile modular systems. If you Google aluminium profile joint you'll see. I can get some prices if you are interested. I can order them through work. I was lucky to get mine for free off a local engineer though so not sure how much they are.

[OzEclipse (Joe Cali)]

Russ,

Thanks, I've been looking for something like that for my lightweight eclipse mounting and my Polarie. I've been thinking about custom making one for some time. Thanks for the hint. I found a possible supplier (Profilium) on line. I'll give them a try first rather than put you to the trouble of having to freight the stuff to me.

When you make that double arm drive, get in touch. I have an excel spreadsheet for optimizing the dimensions and modelling the error profiles of the type 4 isosceles platforms. It isn't very user friendly. I didn't make it for distribution so it needs a fair bit of driving.

Many years ago, I came up with a modification to the type 4 that greatly increases the drive time to almost 4 hours with theoretical ±1 arc sec drive error, plot also attached. I have my doubts that it would be quite that accurate unless you were very precise with the construction but even if you fall a bit short it would be pretty amazing.

I would be happy to optimize a set of construction dimensions and produce an error plot for you. I've attached a couple of examples of the sort of performance you can get from a type 4.

cheers

Joe

[Russman (Russ)]

Originally Posted by OzEclipse

Russ,

Thanks, I've been looking for something like that for my lightweight eclipse mounting and my Polarie. I've been thinking about custom making one for some time. Thanks for the hint. I found a possible supplier (Profilium) on line. I'll give them a try first rather than put you to the trouble of having to freight the stuff to me.

When you make that double arm drive, get in touch. I have an excel spreadsheet for optimizing the dimensions and modelling the error profiles of the type 4 isosceles platforms. It isn't very user friendly. I didn't make it for distribution so it needs a fair bit of driving.

Many years ago, I came up with a modification to the type 4 that greatly increases the drive time to almost 4 hours with theoretical ±1 arc sec drive error, plot also attached. I have my doubts that it would be quite that accurate unless you were very precise with the construction but even if you fall a bit short it would be pretty amazing.

I would be happy to optimize a set of construction dimensions and produce an error plot for you. I've attached a couple of examples of the sort of performance you can get from a type 4.

cheers

Joe

No probs!

I am working out what materials I should use, I might use aluminium. Do you have optimised dimensions for M6 x 1.0P threaded rod? Running @ 1RPM? That would be awesome if its not too much to ask!

[elfinke (Trev)]

Thanks for keeping this updated, Russ.

[OzEclipse (Joe Cali)]

Originally Posted by Russman

No probs!

I am working out what materials I should use, I might use aluminium. Do you have optimised dimensions for M6 x 1.0P threaded rod? Running @ 1RPM? That would be awesome if its not too much to ask!

I am working out what materials I should use, I might use aluminium. Do you have optimised dimensions for M6 x 1.0P threaded rod? Running @ 1RPM? That would be awesome if its not too much to ask! [/QUOTE]


No probs! Happy to help Russ.

The parameters I'm giving you assume that you are building the type 4 Trott, with the two boards starting parallel and pushing apart. It's not for my mod. My mod involves building something that starts at a negative angle, passes through parallel, then heads out the other way. Easy to mathematically model, easy to say not so easy to make.

You can also use the type 4 principle but make a double tangent arm drive instead of a hinged board type. When I designed the bisymmetric double arm , it was to build it as a high precision 4hr tangent for the RA axis on my old mount. It would end up looking like a complicated Astrotrac arm.

I never built it, I was getting more and more into eclipse chasing and instead my time went in to build a very lightweight EQ mount to carry in my luggage to solar eclipses around the world. Eventually I bought an iEQ45 mount for home observing.

Assuming you are going to build the easier Trott Type 4 with the boards starting parallel and pushing apart, I've optimized the design parameters for that. I have guessed because you asked for M6 and 1rpm that you have a fixed 1 rpm motor/gearbox- perhaps a synchronous motor?

If you saddle yourself with a fixed motor speed, the dimensions need to be very very precise or the errors will blow out really quickly.
An error of 0.025mm on the radius arm will push the error out to 8 arc sec over 2 hrs.
An error of 0.25mm will push the error out to 80 arc sec over 2 hrs.

A variable speed drive will let you fine tune to cancel out any minor construction errors.

If you have a variable speed stepper driver, then you can make fine adjustments to the motor speed to trim out any construction errors at the end.

The construction parameters and error curve are on the excel zip file. The diagrams from the original articles in S&T and show which parameters r, b & c are.

cheers

joe

[Steve_C]

Hi all,

Just joined the site. I am planning on building a barn door tracker as per
http://www.garyseronik.com/?q=node/52.

Does anyone know where I can get similar parts in Australia?

I can get all these parts from the supplier indicated, but would prefer to buy in Oz

Regards,

Steve

[mswhin63 (Malcolm)]

Hi Steve, will be able to buy most if not all parts from various suppliers, Wood hinges from hardware. For the gears etc, www.smallparts.com.au comes to mind although there is quite possibly more around. Just a bit of digging.

[bojan]

Sometimes the best supply of such parts are thrown away printers, photocopiers, scanners..
Just pay attention to what pople are leaving on the streets over weekends..

[Steve_C]

I am not familiar with how to drive stepper motors, although I do have an extensive knowledge of general electronics. I would assume a stepper motor is not ideal for a barn door as the earth does not rotate in steps but smoothly. Is my assumption correct?

Steve

[Merlin66 (Ken)]

If the geared stepper ratio is sufficient, the "steps" won't be seen in the image.

[Steve_C]

What would be considered sufficient?

Also, in a two arm design, how do you stop the threaded rod drive gear from moving vertically?

Steve

[Merlin66 (Ken)]

Assuming you have the "usual" 1rpm requirement to give sidereal rate.
This gives a rate of 15 arc sec/ sec time with an input of 1 rev (360 degrees/ 60 sec) = 6 degrees of screw rotation every sec. Giving 2.5 arc sec/ degree rotation.
For the standard camera lens normally used on the Barndoor mount, a resolution of 10 arc sec would be allowable. (Users to verify)

The stepper motors are usually (basic) 1.8 degree per pulse.
if attached directly to the screw you need 6/1.8 (3.33 pulses) per sec to maintain the tracking rate and each pulse would only move the mount 1.8 * 2.5 arc sec = 6 arc sec. Well below the 10 arc sec limit.

Hope this makes sense.

[Steve_C]

Is there any advantage using a stepper motor vs a constant speed motor that is powered by a voltage regualted source?

[Merlin66 (Ken)]

No, not really.
Comes down to availability....
Frequency control of small motor drives was very popular with amateurs in the 70's but fell out of favour.