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ATM-OBSERVERS-L  January 2007

ATM-OBSERVERS-L January 2007

Subject:

Re: PC Timing for Occultations

From:

"Tandy W. Carter Jr." <[log in to unmask]>

Reply-To:

Tandy W. Carter Jr.

Date:

Wed, 24 Jan 2007 12:05:37 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (83 lines)

Hi Howard,

    I guess we'll have to look into this at the ATM meeting with Dr Oliver.

Tandy W. Carter Jr.
[log in to unmask]
http://home.att.net/~tandy.carter/
----- Original Message ----- 
From: "Howard L. Cohen" <[log in to unmask]>
To: "Tandy W. Carter Jr." <[log in to unmask]>; 
<[log in to unmask]>
Sent: Wednesday, January 24, 2007 11:48 AM
Subject: PC Timing for Occultations


> The problem with PC clocks is not entirely due to errors in the displayed 
> time but the internal software clock, which is typically a poor 
> timekeepers since timing uncertainty is limited by the stability of 
> interrupt requests.  Changes in interrupt request rates cause the clock to 
> gain or lose time.  Also, if you leave your computer turned on for awhile, 
> the software clock might be off by possibly a minute or more.  Poorly 
> behaved software may also use the timer-counter for other purposes and 
> change the interrupt rate making the clock keep poor time.
>
> The display can also cause problems.  You can get software that will 
> display the time to the nearest second not just to the minute.  So this is 
> not a problem.  However, the software clock also has limited resolution in 
> that it can only display values that are even multiples of time interval 
> between interrupts.  So, a time as 00:00:01.00 might be displayed as 
> 00:00:00.98 and 00:00:01.04.  Of course, this may be accurate enough for 
> your purposes so the problem, again, is not with the display but with the 
> internal software clock.
>
> Thus, even if you synchronize right before observing, the software running 
> on your PC during your observing session might screw up the time unless 
> you constantly synchronize the clock.
>
> Note that this problem seems to be the reverse of many GPS units which may 
> have good internal times but inaccurate displayed times.
>
> John Oliver mentioned that cell phones can produce accurate times.  I know 
> little about this but some research suggested that some but not all cell 
> phones might give good times.  However, there is another problem.  Many 
> cell phones (like mine) only display the time to the nearest minute, which 
> makes them less useful as accurate time pieces.
>
> Howard
>
> At 10:34 AM 1/24/2007, Tandy W. Carter Jr. wrote:
>>Hi Howard,
>>
>>    I know the clock in my laptop doesn't keep good time. So, what I do is 
>> I synchronize the clock right before I go out to do my astronomy at home, 
>> or I synchronize it right before I load it up into the car to go to the 
>> star party. I also run only the programs absolutely necessary for my 
>> planned astronomy for the evening. Of course, I wonder if it is the 
>> displayed time that is bad and not the internal time. I would think that 
>> a timing chip running at the speed of modern computers would keep the 
>> time spot on. However, you're probably right that the displayed time is 
>> not accurate at all. Since the time on my laptop is only displayed to the 
>> minute. I don't know how SoftwareBisque handles the time it gets for the 
>> FITS header CCDSoft uses.
>
> 

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