Building a GOTO Mount

First stab at a PCB.

I will need to increase the stand off a bit as 100% accuracy in lining up top and bottom hole isn't guaranteed.

Well, I went to make the PCB today - and sods law says I only have singel-sided board!

Now cramming it onto a single layer.

Single layer PCB:

Quite a bit of progress, but I haven't been posting it.

I have redesigned the PCB as instead if a 16x2 alphanumeric display, I will use a 64x128 green graphic display from an old project instead which can display a lot more information, including proper lists of targets.

I have a nice project box made up, with five colour-coded XLR socket on the back: RA, DEC and focus steppers, guide port and dew heaters. On the front the display, chunky brightness control (for ease of use in a hurry), power and an XLR socket for the control handset. The handset is a nice small project box with five coloured push bottoms - I found out why they were cheap on eBay - the bodies have a tendency to melt during soldering!

Found a lovely little unipolar stepper for focus and it had six solder pads, two joined together. I split them and it's magically become a bipolar stepper so I can run it from a polulu, but that's right down the road.

Not much more I can do with hardware until get some molex connectors - I don't want a board tied to the box with dozens of wires. The steppers are fitted to their 'gearboxes' and just await wiring. So for the time being I've started on the software side.

Here's a pic of my 'splash screen' for now.

Neil

Basic box is working now, solved a problem with excess current consumption - one of the Pololu stepper boards was swallowing 65mA extra following every reset. Swapped it out and all three boards only take about 7mA each when doing nothing. Mind you the display is greedy - but the LEDs will be 'turned down' for night use!

Some screen grabs of work so far. Coding for calculating sidereal time to an accuracy of about 3-4 seconds over the whole century and the position of Polaris to half a degree over the same period were challenging in 8-bit assembler.

I'll share some pics of the case inside when I've done a bit more wiring.

Neil

Impressive work, Neil

Just one little negative point, if I may ... It is generally accepted that Red is a better colour for use in a dark environment.

MichaelG.

I hesitate to use the word 'always', so I'll settle for 'generally' or 'usually' when I say that any connection between two or more points is called a net (some software even assigns a net name to a pin that has no connection but that's another story). The type of connection that you're referring to is often called a 'global net' or 'global' for short.

Apologies for digging up an older post.....

Brian

Try finding a RED 64 x128 LCD display! Swapping out the LEDs on these waveguide -type displays isn't a trivial exercise.

That said, there is a counter-argument that as the eye is more sensitive to green than any other colour, you can set a green display much dimmer than a red one and a display doesn't have to be very brightly illuminated at all to be readable at night - much dimmer than my red torch. My existing unit uses a 2x16 green LCD and it doesn't cause me any problems.

And having said all that... I have a QVGA full colour display that I might drop in instead, which would do a very good job of red text/lines on a black background. But then I will probably want to add an SD card to display colour pictures of the objects...

Neil

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The problem is more to do with Loss [vs. retention] of Dark Adaptation.

But; if dim Green works for you, who am I to argue.

[You may not be popular at a typical 'Star Party' though.]

MichaelG.

I don't know if you are going to get into actually making a mount as well Neil but another thread reminded me of something. Many people make their own worm wheels but Celestron used another technique on my first slightly larger scope, 8" sct. They used a spur gear and drove it with a worm. It worked rather well. They also used the same arrangement on larger ones.

There are several gear pitches that are very close to threads that can be easily cut. Even if there is a few thou difference it's probably going to require less lapping than a typical home made worm wheel. It would also be possible to lap the gear against others based on different primes. That's how pro telescope drives used to be made.

I never had to take it apart but what I would expect to see is a worm of larger diameter than the thickness of the gear angled to account for it's helix angle.

All but the most expensive mounts have lots of spur gears in them anyway so why not use one directly. There is also a fad for toothed belts at the moment but as anything will stretch to some extent they don't make much sense really. The cogs will have periodic errors anyway. Chain drive might make more sense.

John

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Mount is next year's project

I suspect periodic error is mainly an issue for the worm - a theoretical 10-minute sub on a 144 tooth gear would only use on tooth & space on the wheel, but a whole turn of the worm.

Neil

I like fork mounts Neil. No counterweight so far more rigid and lighter. The 8" SCT I mentioned was sold due to aperture fever. I went for 10" which is still around but have regretted selling the 8" many times so recently bought another. I'm encouraged by astro shots I have seen when used with a focal reducer.

I've also added a good quality 6" newtonian as a coma corrector will sort that out. It's fairly happy on an EQ3 with an EQ5 tripod but as I mentioned before if I added weight to the end of the newtonian I'm pretty sure it could be fork mounted and be even more stable - on day maybe and then look at directly driven spur gears.

I've recently sold a couple of other scopes, both apo's so the money went into more workshop gear and another lens for my Nikon V2, the long zoom for it. I'm aiming to try M 4/3 on astro but the local council recently messed things up for me. LED street lighting. It was lp sodium and a simple filter killed that completely even just being a couple of miles south of the centre of B'ham. Not much good looking north but south was fine.

John

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Despite lack of posting, I have done a fair bit recently.

The mount now appears to be performing it's GOTO moves (just a minor issue of clockwise and anticlockwise getting mixed up, but that's a minor tweak.

More detail to follow soon...

Neil

I've fettled a few mounts that use a friction drive rather than the traditional wormwheels. No backlash, but a small chance of slipping under acceleration, which can be ramped up and down of course.
One chap in Ireland has used a large diameter small pitched toothed belt. Then there are the 'caterpillar' drives, sectors, drum/wire etc...
So many solutions to the same problem

I find the worm bearings on mass produced mounts a major source of PE, better quality bearings or angular contact bearings can make a measurable improvement. Also worm end float needs better control.
A lot of worms have their cetre holes so a conical set screw on one or both ends can help.

The belt conversion kits are a good idea too, doing away with the spur backlash.

Edited By john carruthers on 24/06/2016 08:12:48

Well, more or less of the final straight now. Tomorrow I will take a few pics of the control box and gear'boxes'.

Some little tweaks needed to make the 'user interface' friendlier.

Neil

Neil - apologies in advance

Having just had ALL the (overloaded) shelves in my new workshop/garage collapse (DON'T use normal rawlplugs in Celcon blockwork!!!), I'm bored and grumpy so read this thread ... understood very little but absolutely fascinated by one of your earlier contributions - "a nice small project box with five coloured push bottoms - I found out why they were cheap on eBay - the bodies have a tendency to melt during soldering!" ... the mind truly boggles!!

and thank you for the broadcast scam warning.

rgds

Bill

Neil- just seen your picture of the pcb you made and I am truly impressed with the quality. Did you use a printed mask and UV exposure on coated board or some other technique?

I have been making printed masks for years but have found the quality of print very variable. Getting a good dense solid black image can be a real pain and I have had to resort to overlapping more than one layer to achieve this.

If you have a good technique please spill the beans.

Doug

If you leave the soldering iron on the tabs too long, they melt. While I suspect the plastic was chosen because something with a bit of 'give' was needed for the waterproof design, I suspect that in practice it makes them a bit too fiddly for mass production, and that's why they have ended up being sold off cheap. Soldered with care they seem to work perfectly well and you can get five nice colours.

Neil

I suspect you over-estimate the 'design' process. The plastic would have been used because it was cheap, and cheap to mold, with little or no consideration of fitness for use.

It is not a new problem. It must be 40+ years since I discovered that cheap toggle switches melted when you tried to solder to the tags. Quality switches, which use thermosetting plastics for the body, don't have the same problem. Cheap connectors can also have the same issues.

Andrew

They probably melt as they have been designed for use by people who solder all day using a very hot iron.

Very hot in my view that is. I spent some time working in a place that had no facilities to remove flux so they used a special solder and flux along with by normal standards a rather cool iron. Later when I was sold yet again I worked with the "hot iron" crowd and soon realised that the tip I normally used made life more difficult for me. They could solder way way more quickly than me as well.

Mount gearboxes ? More expensive mounts have always generally used rather large worm wheels. Way way bigger than Skywatcher for instance. There are many attempts to get round that including very very high resolution direct encoding but personally if making one myself I would stick to a big worm wheel - ideally no other gearing of any sort at all.

John

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Edited By Ajohnw on 25/06/2016 09:50:25

Edited By Ajohnw on 25/06/2016 09:51:01

Edited By Ajohnw on 25/06/2016 09:51:31