Trying to modify for sidereal rate

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rschmit
 
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Trying to modify for sidereal rate

Post by rschmit »

I've built the clockk and all is well, but I'm hoping to change the rate at which the clock runs. I would like it to track sidereal time: the time it takes for the stars to move across the sky. To do this, I have to make the clock run at a rate where 1 day is 23:56:4.091, so 1 sideral second = 0.997270 regular seconds. Could someone suggest a fix for changing the rate?

I've got my USBTiny and everything is reading/writing properly, but not sure what to do in the firmware that would allow me to change the rate like that. I've read the fourms and found minor adjustments (for accuracy), but this is a pretty big change. Any suggestions?

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Re: Trying to modify for sidereal rate

Post by adafruit »

which clock?

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rschmit
 
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Re: Trying to modify for sidereal rate

Post by rschmit »

Sorry... The Ice Tube Clock.

I've looked for clocks that would do this, and while there are PC-based alternatives, I'm hoping to find it in an actual clock.

When I called about the Ice Tube as a possibility, the guy I chatted with said the clock could be programmed to do that. Sounds like exactly what I'm looking for! "Oh, PROGRAMMED programmed. So, not like 'I programmed my VCR' programmed... Hmmm Well, I can do that... Let me get my FORTRAN card puncher out, and... D'oh!

Been a while since I coded (can you tell?)

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Re: Trying to modify for sidereal rate

Post by adafruit »

first thing you'll need to do is get it to run on a more precise clockbase. the built in clock is not very precise. search the forum for people who have connected a chronodot to the ice tube. thats a temp compensated clock.

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Re: Trying to modify for sidereal rate

Post by pstemari »

A WWVB receiver or maybe a GPS would be even better, since those will give you the UT1-UTC correction, especially if certain groups in the ITU get their way and let UTC drift without leapseconds.

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rschmit
 
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Re: Trying to modify for sidereal rate

Post by rschmit »

Thanks, but for this one, I'm not going for high precision. If I can get this first one to work, I'll look to the GPS mod to make the next one more precise. To get started, I just want to dial in that sidereal rate.

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Re: Trying to modify for sidereal rate

Post by adafruit »

the rate is less than the error of the built in RTC, you -do- need precision!

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rschmit
 
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Re: Trying to modify for sidereal rate

Post by rschmit »

OK. I'll take another look at the chronodot.

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stinkbutt
 
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Re: Trying to modify for sidereal rate

Post by stinkbutt »

How does this rate differ from ordinary time by less than the accuracy of the Ice Tube Clock? According to his definition of Sidereal time, he's running fast by 3 minutes and 56 seconds a day. The stock Ice Tube's more accurate than that.

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Re: Trying to modify for sidereal rate

Post by adafruit »

he wants
23:56:04.091
note the 3 extra digits -after- the seconds. that is a lot more precision than we have!

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rschmit
 
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Re: Trying to modify for sidereal rate

Post by rschmit »

Oh, good point. Let me clarify: 3 min 56 seconds faster each day would be adequate for my application. I've read the other posts regarding accuracy over a week, and I'm fine with that. Thanks!

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stinkbutt
 
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Re: Trying to modify for sidereal rate

Post by stinkbutt »

OK, in that case the next question is: What do you want the time to be?

Do you want:

a) The time to read hours, minutes, and seconds accurately, and then flip over from 23:56:03 to 00:00:00

or

b) The time to read hours, minutes, and seconds inaccurately, skipping a second every ~6:06, a total of 236 times per day, and then flip from 23:59:59 to 00:00:00?

Either way is doable, it's just a question of how you'll need to handle the math.

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rschmit
 
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Re: Trying to modify for sidereal rate

Post by rschmit »

It's b). The celestial clock is a 24 hour clock, it just runs at a different rate. Just have to change the rate so 1 sidereal second is faster than 1 solar second.

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Re: Trying to modify for sidereal rate

Post by stinkbutt »

rschmit wrote:It's b). The celestial clock is a 24 hour clock, it just runs at a different rate. Just have to change the rate so 1 sidereal second is faster than 1 solar second.
In that case, check these two posts out:

http://forums.adafruit.com/viewtopic.ph ... 764#p63721
http://forums.adafruit.com/viewtopic.php?f=41&t=12720

The first one is the implementation of an idea discussed in the second. Originally, this was intended as a way to make the clocks more accurate, and you'll probably need to refactor the code to make the calibration factor add more than just the couple of seconds per day it was designed for in the posts, but that's how you'd do it.

Check out iv.c in Digisage's firmware and find the relevant code. He doesn't do too bad a job commenting his BANNED, so it shouldn't be that hard.

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rschmit
 
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Re: Trying to modify for sidereal rate

Post by rschmit »

Yep, I looked at that post previous to writing, and I thought this would be a show stopper:
-128 <= cal_factor <= 127
If that's the limit to the range of values for cal_factor, then a full range value only buys me 0.366 seconds per day. At least that's how I was reading it.

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