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Oh What a Silly Billy


Stub Mandrel

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A few nights ago, I had my first try with my home-made GOTO unit. Unfortunately the DEC was too stiff and the stepper current to low, and anyway lots of cloud appeared.

Sunday night I tried again. I managed to rough focus on Vega, but every try with the Bahtinov mask just gave a long streak. It just wouldn't track :-(

I spent ages looking at the code and couldn't see a problem.

Some reflection and looking at past photos showed the error was large, about the same as no tracking at all. Was it backlash - no?

Perhaps it was jamming. I ran it for half an hour with just the counterweight to impose a big load. it ran for half an hour, perfectly :-(

Perhaps just one of those things? I set up earlier and pointed it at a TV aerial as a test, in a place where I can leave it for a go at imaging later.

Looking through the finder, its tracking. Yes it's definitely racking. Zero the RA scale, a few minutes later yes the speed seems to be about right.

Perhaps I had the RA and DEC steppers swapped over., Move up and down, look at movement relative to cross hairs. No, it was set up OK the other night, anyway the star was streaking in the RA direction as if the tracking was slow or stopped.

Look at aerial through the finder again.

Oh. The crosshairs are moving towards the chimney. They should be moving towards the end of the aerial...

Perhaps if I set the scope to track clockwise instead of anti-clockwise it will work better...

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Well its working, imagine my joy when I told it to slew from Vega to M11 and it was there in the same place in the frame as the star had been! So excited I took 30 subs even though the sky was still blueish!

I've realised the GOTO needs to flip the DEC movement when I do a meridian flip :-)

Plus some errors in my little database. The GOTO worked fine for most targets, but I made the mistake of wandering off after telling it to find the Wild Duck Cluster - and came back and it was pointing at the ground.

Found a few minor bugs, mostly data errors - any object I chose was either spot on or way off. I need to tweak the menus and my 'find object by RA DEC doesn't work as it didn't let me change the hours - should be easy to fix.

Lots of short runs in my not very dark sky, but who cares even as a debug session I managed twice as many targets as normal.

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That sounds like real engineering practice.

This reminds me of a story I heard long ago.

Engineers had built a device for doing testing in a magnetic field. To cancel the earth magnetic field they made two measurements, rotating the magnet 180 deg between measurements. But they just couldn't figure out why every second test came out with a sign error. The right value, but the sign was wrong.

Until they finally figured out they didn't reset the magnet to initial conditions after the second measurement of each test.

Back to topic: could you post some images of your creation? Would love to see it.

 

Cheers,

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19 minutes ago, wimvb said:

That sounds like real engineering practice.

This reminds me of a story I heard long ago.

Engineers had built a device for doing testing in a magnetic field. To cancel the earth magnetic field they made two measurements, rotating the magnet 180 deg between measurements. But they just couldn't figure out why every second test came out with a sign error. The right value, but the sign was wrong.

Until they finally figured out they didn't reset the magnet to initial conditions after the second measurement of each test.

Back to topic: could you post some images of your creation? Would love to see it.

 

Cheers,

:-) Nice story, not far from my error!

Some pictures here, but I need to add a couple of the Heath-Robinson 'gearboxes'

 

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Many years ago I made a simple GOTO mount. Part of the software was calculating the local sidereal time and I was testing my result against that of the U.S. Naval Observatory, who at the time conveniently made it available on the internet.

Annoyingly there was this 4 minute discrepancy between our results and I spent many hours analysing the algorithm to find the problem. Living in a bubble at the time with nobody to turn to for help, I eventually came to the conclusion that it is definitely not me who is getting it wrong.

Thankfully I checked the date my pc was set to before dropping them an email.

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2 hours ago, Icosahedron said:

Many years ago I made a simple GOTO mount. Part of the software was calculating the local sidereal time and I was testing my result against that of the U.S. Naval Observatory, who at the time conveniently made it available on the internet.

Annoyingly there was this 4 minute discrepancy between our results and I spent many hours analysing the algorithm to find the problem. Living in a bubble at the time with nobody to turn to for help, I eventually came to the conclusion that it is definitely not me who is getting it wrong.

Thankfully I checked the date my pc was set to before dropping them an email.

LOL! I had my own trials and tribulations with calculating sidereal time, but I must admit I didn't get quite as far as that!

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