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About NEM

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    North Coast, N. Ireland
  1. If you use all three bolts for primary collimation, there's a danger that you'll repeatedly turn them all the same direction and eventually 'walk' the mirror off the bolts. I'm not sure if that applies to the secondary as well. Noel
  2. As I understand it we're only hurtling through space relative to our local group of galaxies. At the speeds necessary for red-shifts to be noticeable, our 'hurtling' becomes glacially slow.
  3. I don't know the specifics of this strip, but I doubt you need to connect + and - together at the end of the strip. I suspect that would damage your power supply or burn out the tracks on the strip. I'd try it without connecting them first, that shouldn't harm anything. You could add your red LED there, but remember you'll need a current limiting resistor in series with it. Noel
  4. So, you're saying I have to fill in the snake pit, remove the trip wire that fires the crossbow bolts and disable the pressure pad under the scope that releases the giant stone ball? You never let me have any fun! ?
  5. NEM

    Stepper motors

    You could use a micro-switch or slotted opto-switch at each end of the travel and get the pi to use them to limit the travel.
  6. That type of clip is known as a Terry Clip. Noel
  7. A wild guess here, from someone who's never even used an A4988, so it's probably not the answer, but... Are you sure there's a negative connection from the Pi to the driver chip ground? What if, when the DIR pin is at 0V it's providing the ground to the driver through internal leakage in the A4988, so the circuit works? Then, when the DIR line goes +ve there's no ground and the circuit stops. Noel
  8. NEM

    DIY power box ?

    It's the current through the fuse that matters. The voltage rating is only important to stop arcing to the outside. As long as the voltage rating exceeds the voltage you're usng., it's OK.
  9. A few years back, a friend, who's a farmer, asked me to have a look at the PC he used to run his milking parlour as it had stopped working. He kept it in a kitchen cupboard in the barn. When not in use, the doors were kept closed and a 25W or 40W (can't remember exactly, it's been a while) incandescent light bulb provided heat. The bulb had accidentally been turned off overnight and the PC stopped recognising the keyboard. A few minutes work with a hair dryer on the keyboard and all was well again. So, yes, I think your idea is a good one.
  10. Are the surface mount chips already installed on the board when you receive it? I can't imagine soldering U1 in place by hand.
  11. Does anyone know the thread size on the outer end of the counterbalance rod of a Meade LXD75 mount? I suspect it's 5/16" diameter, as an M8 tap screws about half an inch into it and then sticks. As for pitch, UNS (20 TPI) is closer to M8 (1.25 mm) than UNC (18 TPI) or UNF (24 TPI), but is UNS commonly used? I need to get an extension bar made up to cope with a Skywatcher 200P. If anyone else is using this set-up with the standard LXD75 weight, what length of bar do you use? Noel
  12. There's no need for a really high speed interface if you cache the data in external RAM and only load the parts you're interested in. A real time display can't be updated at 20MHz and the human eye couldn't follow it even if it could. Oscilloscopes are typically used to look at repeating patterns, so subsequent scans overwrite the first ones. For a 20Mhz resolution you could take 2000 samples at 40Mhz sample rate every 0.1 seconds and only have to input data at 20kHz. Noel
  13. Hi Chris, I'm looking for a USB hub and that one looks interesting. Am I right in assuming that the DC socket on the hub itself takes 5V? In the description it says Input: AC 110v-240v, 50/60 Hz Powered by bus-power, or by external power adapter (NOT INCLUDED) which is slightly confusing. If you have to supply your own adaptor I'd have thought you could power it from anything. Noel
  14. Have you tried this out yet? I've just ordered two of them to daisy chain together into a powered hub at the far end. Noel
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