Building an equatorial mount with autoguiding for astrophotography

[TheoP]

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

Theo

[happy-kat]

Very interesting project to follow.

[Ajohn]

I'll 2nd that.

I have seen lots of web shots of people making worm wheels like that but always wonder how easy it is to do and get the teeth to match up once the disc has completely rotated.

Not my work but an example of a home made worm wheel made in a different way if some one has the kit to do it.

These are made by cutting ordinary gear teeth at a slight angle to match the worm. The width of the teeth is around 1/4 of the diameter of the worm or less and the tops of the teeth are concave.

It needs a dividing head but people use a gear for indexing and make up a simple one. It also needs a hob to cut the teeth. The problem with that is buying one that can match a worm that can be cut on a lathe so people often make a hob. There is a decent run down on making one here and there will be plenty of others on the web.

http://www.helicron.net/workshop/gearcutting/gear_cutter/

The other way of cutting the wheel is with a fly cutter - a single toothed cutter with the same form as is used to cut the worm making sure that the two mesh on the flanks of the form and that there is clearance at the ends of the teeth of both.

Rather than cutting the teeth at an angle the worm could be angled instead. Accuracy can also be improved by lapping the 2 parts together with a very mild abrasive.

 It's the way I would do it but I have a lot of other things to do so cheated and bought a ZEQ25GT instead.    Weather has still prevented me seeing how good it is and lack of adapters means I haven't topped out how much load it can take yet.

When one off worms wheels are made professionally they part precut the teeth  with a slitting saw so that there is something to ensure that the hob or tap has an accurate guide as the blank rotates.

John

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[gajjer]

Have you seen this link?

http://www.mini-lathe.org.uk/making-telescope-worm-wheels-gears-mini-lathe.shtml

cheers

gaj

[Peter Drew]

I must have cut hundreds of worm and wormwheels during my professional career. It is quite straightforward to cut the wormwheel with a tap as shown in the earlier post photo. This of course allows you to use a thread form for the worm. It is possible to calculate the diameter of the wheel blank to produce the desired number of teeth but some elusive variables can add or lose a tooth. As mentioned earlier, gashing the wheel with a dividing head will ensure the correct number of teeth. Three things to bear in mind when making sets, 1) turn the O/D and bore of the wheel blank at the same operation to guarantee concentricity, 2) machine the worm and both journal ends in the same operation to avoid eccentricity and 3) truncate the thread crests of the worm so that the sides of the thread form contact the wheel rather than the tops.  

[Ajohn]

It's better than most gaj as it mentions gashing. The actual worm form he produces isn't as wide as the blank is thick which fits in the the home made one I posted.  I have made a tap like that and it was very difficult to clean up the cutting edge after it was hardened - I couldn't get it dead sharp before that, there was always a bit of a burr.  I also think that making the worm out of silver steel / drill rod is just making life more difficult. It isn't the easiest of stuff to get a decent finish on but cutting oil helps. I would make the hob separately and try to finish the worm off between centres so that if the diameter needed trimming down later or the thread recut a little deeper  it would be easy to do. I'd just use ordinary free cutting mild steel or free cutting stainless for that.

To be honest I think better results could be obtained by replacing the slitting saw with a fly cutter carrying a small tool with the form that is needed on it.  The rad of the fly cutter used ideally needs to be a bit larger than the worm's but simply cutting straight through the teeth on the worm wheel works well. Lapping will generate a more exact form anyway especially if the size relationships is similar to what I suggested. There is plenty of information on making and using single point gear cutters on the web. Either the worm has to be tilted or the blank for the wheel has to be tilted when it's cut this way but I feel it's far more likely to give accurate results.

John

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[Ajohn]

Might be of interest. While browsing ebay I noticed that there are some 1 mod gear hobs available at rather low prices for this sort of thing from China in HSS. I mod is pretty close to 0.125 circular pitch. It's actually 0.1237. I would have thought a bit of lapping would easily take care of that. 

For metric people there are  also some 1.5mm cp ones from the same source but I feel the teeth formed would be a little weak. 3mm would be a lot better.

John

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[TheoP]

I have now made a new worm gear and worm with a 10×2 ACME threaded rod (10mm diameter with a 2mm pitch). I 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 thread would. This meant that I would have had 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! 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.

The tracker will be mounted on a Meade wedge at first until I make a more solid wedge out of a few steel plates. The orientation of the tracker’s RA shaft is 90 degrees different to the normal way of mounting it on the Meade wedge. Fortunately the Meade wedge can adjust far enough and I can reach the southern celestial pole with this unconventional configuration.

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 test photos to see how the tracking and autoguiding works on the new mount. The Omega Centauri image is a heavily cropped section of a photo taken with a Canon DSLR with a Canon EFS 55-250mm lens at 250mm and a single 2 minute exposure at f/5.6 and ISO-400.

I am happy with the performance of the mount at this stage.  The stars are mostly round but I can definately do better under dark skies when I will spend more time to get the alignment right. The stepper motor has a nice quiet hum and the 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. I can’t wait!

[happy-kat]

The image looks very rewarding for the work you have put in.

Thank you for sharing I enjoy reading creative projects like these.

[Tomjo59]

If only I had the skills necessary to undertake a project like yours!

[JamesF]

Looks like a very nice piece of work.  Obviously you work with what you have; in my case I'd be tempted to swap the parallel port connection for a USB<->serial one, then run a single USB cable from the laptop to the Arduino with a small USB hub to split the Arduino control input and the stepper motor control inputs there.  Parallel port is probably easier, but they're not very common these days.

Did you consider direct drive or belt drive with no worm?  I guess if you're making it yourself you can create the worm wheel as large as your machinery will allow which will mitigate backlash issues somewhat.

Other than the Meade wedge most of it appears DIY, but did you buy in the castings for the RA bearing mounts?  They look really quite substantial.

James

[Ajohn]

The castings have self aligning bearings in them and they are a form of pillow block. They are basically a ball race with a spherically shaped outer diameter so that they can tilt in the housing to align with each other when a shaft is fitted. I did know that they were available but not what they were called but on ebay at least when the usual pillow block is searched for this type come up as well.

The other type were very popular for making home made table saws etc.

John

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[JamesF]

Ah, yes.  Searching on ebay for "self align pillow block" finds a selection with various internal diameters.

James