Building an equatorial mount with autoguiding for astrophotography

[TheoP (Theo)]

I am in the process of designing and building an equatorial mount for tracking and guiding of a DSLR camera + lens and/or a small telescope (perhaps up to an 8" SCT optical tube). My iOptron Skytracker is great for shorter lenses and exposures but I cannot load it up too much before it runs out of steam. I want a tracker than can manage an 80mm refractor or up to an 8" SCT optical tube assembly without throwing its toys out of the cot. I know I can go out and buy an EQ6 but where is the fun in that?

I don’ have a final design yet, it's a work in progress but you might find something of interest. I have built and tested a few versions already and assembled the electronics for a basic RA tracking mount using a stepper motor drive that uses an Arduino to control the stepper motor. I have also implemented autoguider functionality on the RA axis that is working quite well. All details are available on my blog.

I am machining my own worm gears and worms as well and it’s a "journey of discovery". I am busy on my third iteration of the mechanical design and I am now trying to cut a new 130mm worm (~204 teeth) gear pair with an ACME thread. I have used M10 stainless steel threaded rod before but have found that it is important not to cut the M10 thread too deep into the aluminium worm gear disk. It causes the M10 worm to bind very easily when the worm/gear assembly alignment is not absolutely perfect. I am now trying out a 10×2 mild steel threaded rod with an ACME thread (10mm diameter with a 2mm pitch). The ACME thread is much more suitable for a worm gear than the M10 type thread.

You are welcome to take a look at my progress at https://starsinphotos.wordpress.com/

Advice and comments will be appreciated.

Cheers

[nebulosity. (Jo)]

That's a great project Theo!

You wouldn't believe it but I came across your website the other day and was very interestedly checking out this mount build. I really love what you have done so far, and will be keen to see how you progress.

I love your way of doing lathe work with the mill, I have made a few worm gears for my horse shoe mount on a mill as well.

How will you make the ACME thread gears? Do you need a special ACME thread tap or will the ACME thread worm mesh with with the gear cut with a normal tap?

Cheers
Jo

[TheoP (Theo)]

Thanks Jo,
I have milled a few slots into a section of the ACME threaded rod to try and make a tap. It works quite well but this "ACME tap" cannot start the aluminium worm blank in the same way as the M10 tread would. This means that I will have to manually cut or saw the starter slots into the blank for the ACME thread to lock onto. It's quite a bit more work with about 200 odd slots to cut!
Unfortunately the M10 has a 1.5mm pitch and the ACME is 2mm so I can't use a metric tap to cut the first slots on the worm blank. I don't know if it is possible to buy a ACME thread tap, I suppose it is but I will try and use my own first.
This is just another lesson learnt and I will find a solution for it....

[TheoP (Theo)]

Eventually I got a M14 tap with a 2mm pitch and I started the worm with that until there were well defined teeth in the worm wheel. I then changed to my home-made ACME tap and continued until the worm gear was done. A new ACME worm was then mounted on the guider with two pillow blocks that I used previously.

The assembly is quite heavy but very solid.

There are more photos on my blog....

[Terry B]

Great job.
Have you seen these plans?
http://nsa.kpu-m.ac.jp/gijutu/kousak...sh/eqnotes.htm

[Willow127mm (Brad)]

Loved the Maggie story mate we had one growing up great pets.
Good to see the parents took it back.☺

[TheoP (Theo)]

Hi Terry,

I remember looking at this mount a while ago. It's a work of art! Unfortunately my skills do not stretch to the point where I will attempt to build something like that.
I will be happy if my attempts to build a RA mount will allow me to take long exposures with a heavy load resulting in round stars. I love the challenge of "figuring it out" while I build it. I don't plan ahead too far and when I get stuck I try to make a plan to solve the issue. I know it's a cliché, but for me its about the journey, not the destination. If it was about the destination I would have had a G11 or a EQ6 already.

[TheoP (Theo)]

I finished the construction of the new tracker to the point where I could test it under a suburban night sky. I mounted a piece of angle iron onto the RA shaft and mounted two camera ball heads on it; one for the imaging DSLR and one for the webcam autoguider.

To align the mount I use a green laser pointer that is mounted on a bracket that sits directly on the RA shaft. This allows me to to align the mount very accurately to the SCP. I was very pleasantly surprised with the stability of this new mount. There is almost no vibration or oscillations in the mount, even when touching the mount or adjusting the settings on the camera. It is reasonably heavy but it will also be able to carry quite a payload. At this stage I have only tested it with a DSLR but the next step will be to load it up with an Orion ED 80mm to see how it performs. Maybe it will take a 8″ SCT optical tube as well …..

Due to the very light suburban conditions I only took a few photos to see how the tracking and autoguiding work on the new mount.

The wide field view of the Southern Cross overhead was taken with a Canon 600D with a Tokina lens at 16mm and a single 4 minute exposure at f/3.5 and ISO-100.

The Omega Centauri cluster image was taken with the same Canon DSLR with a Canon EFS 55-250mm lens at 250mm and a single 2 minute exposure at f/5.6 and ISO-400. These images are not great but they show the initial tracking ability of the mount.

I am happy with the performance of the mount. The stars are mostly round. The stepper motor has a nice quiet hum and the ACME worm and worm gear seems to hit it off quite well without binding. It all works quite smoothly. I must also mention that the autoguiding software from Stark Labs; PHD Guiding (http://www.stark-labs.com/phdguiding.html) works beautifully and I could focus on the imaging and let the software and webcam look after the tracking.

I guess the next step is to take this new setup out to a dark spot under a clear sky and see what it can do.

[mswhin63 (Malcolm)]

Hi Theo,

Try PHD2 as this has an excellent drift alignment feature. Your tracking might be good but to completely remove slight egg shapes the mount will need to be aligned more accurately.