Re: No Escape

[Tom Droege <tdroege@veriomail.com>]

Hmmm!  Seems like we are mixing up Time and Timing.

Time determines where we are pointing in the sky.  I think we can be off a 
lot and still recover.  Arne says his PC clock has a 6 second per night 
error.  This is 1 part in 3000 or so at the worst.  This is pretty bad for 
a crystal.  We should let the data tell us what time it really is.

Timing is our ability to measure time differences.  This, with the above 
clock is 1 part in 3000.  But we can calibrate this to another factor of 10 
or so with a simple calibration.  Even a cheap PC crystal will be 10 ppm 
per C.  So we only need a scale factor to get to 1 part in 10,000.  Now 
there is the question of when the shutter actually moves per our 
command.  I think Doug's fix has greatly improved this.  I need to figure 
out how to do a timing experiment, but I think the time between open and 
close can now be measured to the PC clock ability.  This has a resolution 
of 50-60 ms by experiment.

OK, we could do better by doing the shutter Timing in the BX24 or the 
Stamp.  Everything will be available for someone to improve this.  The BX24 
is plug compatible.

Until then, we can time to of order 0.1 second for exposures.  Since the 
shutter takes 0.15 seconds to open, and since there will be some variation 
in the motor opening speed, this is of order as good as we can really do.

I do not really see any good reason to be better than a few seconds for 
DATE-OBS and  0.1 second for EXPTIME.  I think we can do this now.

For those that care, DATE-OBS can be improved by the proper synchronization 
of their computer clock with an atomic clock somewhere.  Lots of people 
have suggested programs that will do this.

Improving EXPTIME will probably require moving the timing to the 
Stamp.  Since the Stamp probably does not have the program capacity for 
this, it may have to be done ing the BX24.  But it is a possible improvement.

I think I will stick with trying to achieve the above, and leave the 
improvements to others.

Tom Droege

PS, I just realized that I can measure the shutter time with my 
counter-timer.  So I can do the experiment.  But this is a later experiment 
for when I have nothing better to do.

At 09:36 AM 3/14/01 -0700, you wrote:
>Hi, Oscar!
>   The accuracy needed is often called into question on
>this list.  My feeling on time, as I continue to state,
>is that this is one parameter for which we can get really
>good accuracy.  So, you should do the best job you
>can within reason.  Certainly +/- one second at
>the time you set your PC clock is easily obtainable.
>You should know the drift of your clock
>so that you can continue to keep this one second
>accuracy at any time during the night.  If you have
>NTP or equivalent, then the time will be accurate
>to a few milliseconds or better.
>   Do we need this accuracy?  For the minute-class
>exposures, knowing time to +/- one second is probably
>overkill since you have +/- 30 seconds timing error
>just from the open shutter time.  For short exposures
>of a few seconds (possible with the Mark IV), you
>might want even better accuracy than a second.  So the
>bottom line is that, since you have control over time,
>make it as accurate as you can so that you can forget it
>as an error source in the future.
>Arne