I finally motorised the StarTracker!!

[middy]

It happened. I finally finished motorising my home built StarTracker. I built this thing months ago out of scraps of wood and had been hand cranking it until now. I took it out for a test spin last night and it worked great. I was so excited

The attached pics show the original hand driven model with the milo tin lid handle, and the new improved motorised model with a stepper motor that I ripped out of an old dot matrix printer about 4 years ago (I knew it would come in handy one day!!). I still need to tweak the timing yet as it runs about 2 seconds slow every 60 seconds.

Now the StarTracker is motorised I can do other things while the camera is taking pictures. I just have to walk over to it every 2 minutes to press the shutter again for the next shot.

[Dennis]

Hi Andrew

A very nice piece of work - well done. In search of continuous quality improvement, maybe marine ply is more dimensionally stable, a polar scope would help achieve better polar alignment and a curved drive screw would track better?

Look forward to seeing your first light images.

Cheers

Dennis

[joe_smith]

What are you using to drive the motor? Looks like a good set up

[middy]

Originally Posted by joe_smith

What are you using to drive the motor? Looks like a good set up

I bought a stepper motor driver kit from Oatley Electronics in Sydney. This is the kit .....
http://www.oatleyelectronics.com/kits/k142.html

I bought K142D because my stepper motor was a 6 wire motor.

I cobbled together a simple pulse generator on a breadboard using a 555 timer IC. Power is supplied by my benchtop power supply.

At the moment it is circuit boards and wires dangling everywhere. I need to tidy it all up now and fit it into a box.

I will try to take a picture of the electronics tonight but I must warn you it isn't pretty.

[middy]

Here is a star test which is a crop from the original full size image.

The left one is the first 64 sec exposure at the start of the run.
The second one is the last exposure at the end of the run. (21 images total)

As you can see there is some star trailing near the end which is due to the tangential error of the StarTracker mount. I had to discard the final 7 images. Once I tweak the timing to get it correct I will hopefully only have to discard the last 4 or 5.

StarTracker MkII will fix this problem as it will be a non-tangential system.

[middy]

Originally Posted by Dennis

A very nice piece of work - well done. In search of continuous quality improvement, maybe marine ply is more dimensionally stable, a polar scope would help achieve better polar alignment and a curved drive screw would track better?

Steady on there old chap, I've only just got around to motorising it after 6 months of hand cranking. Maybe by the year 2020 I will have incorporated your suggestions.

[Dennis]

Hi Andrew

I'm sure you'll achieve these design goals more quickly once you see how good the images are with your current system.

I once read a book that suggested there is little difference between a straight and curved drive bolt for the first 20 to 25 mins of a single exposure. After that, the straight bolt needs to be turned a little faster due to the tangent error.

Here is the formula for calculating the distance between the hinge centre and bolt drive for a 1 rpm drive:

D=228.6/tpi

where tpi = threads per inch.

Cheers

Dennis

[middy]

Spent last night tidying up the electronics a bit. The stepper motor driver now has a box to live in and there are proper cables for power and to the stepper motor itself. The pulse circuit is still on a breadboard though. Once I get that onto its own circuit board it will all fit in the one box.

[Dennis]

'tis wonderful to see amateur telescope making is alive and well on Ice In Space.