Long Exposure Limit? - Script Writing - CHDK Forum
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Long Exposure Limit?

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Offline jmac698

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Long Exposure Limit?
« on: 27 / September / 2016, 11:04:51 »
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Hi,
I've been trying 17 minute exposures, or 1024s, but it's not working right.  The stats read like a very quick exposure (1/64th or less).  Have you ever found a limit to the exposure time?  512s worked fine, also 1024s is -10*96.  So it fails between -864 and -960.

Actual time taken was 17:06 with saving. 1024s is 17:04, so 2 second saving.
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Offline jmac698

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Re: Long Exposure Limit?
« Reply #1 on: 27 / September / 2016, 13:25:27 »
Update:

I found the limit
Code: [Select]
tv96   time   time(s)
-893    10:31    631
I can make no sense of this value, except that it's the limit of a 32bit timer with 6.8MHz clock.

The range of values from 1/2000 to 631s is about 21 bits, and you could easily handle this with some accuracy with a 1MHz clock and 21 bit counter.

If I were to guess at some of the system clocks, I could look at how fast the adc is, knowing how many columns it has to read out in at least 1/1500s(?).  A 6.8 MHz column clock would be a normal value.

Does anyone else have limits for their camera?
This was sd1200.

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Offline reyalp

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Re: Long Exposure Limit?
« Reply #2 on: 27 / September / 2016, 23:54:04 »
Quote from: jmac698 on 27 / September / 2016, 13:25:27
The range of values from 1/2000 to 631s is about 21 bits, and you could easily handle this with some accuracy with a 1MHz clock and 21 bit counter.
Hmm... maybe relevant
https://chdk.setepontos.com/index.php?topic=11342.10
https://chdk.setepontos.com/index.php?topic=10114.0

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Offline jmac698

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Re: Long Exposure Limit?
« Reply #3 on: 28 / September / 2016, 12:21:21 »
Not like that unfortunately. The value 631s on a 1MHz count needs a 30 bit number.  1/2000th is 500 counts. I meant the range of values was 21 bits, just to store with a base of 1/2000 (2000*631).  But there's no doubt the timing wraps around, at least part of it, the shutter part, and the waiting part doesn't.  I wonder if I could wait long enough to see where the waiting part wraps around :)

Also realized that if I'm taking dark frames just to get bias frames, I can go as quick as I want down to basically zero, where it will be dominated then by the readout rate, since the time of the mechanical shutter closing is just some min value for the widest aperture in all conditions. It does get a lot quicker in smaller apertures.

There might be a way to guess the readout rate though.
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