[time-nuts] Sidereal time
bruce.griffiths at xtra.co.nz
Fri Jan 15 21:06:46 UTC 2010
You have missed the point entirely.
Autoguiders etc are only useful for tracking an object once it has been
Pointing is the ability to point the telescope at a desired object or
location starting from home or another position in the sky.
With large telescopes (or with inexperienced operators) one doesnt have
the time for manual correction.
It is desirable to place the object of interest close to the centre of
Once it is acquired then use of autoguiders and periodic error
correction (an open loop process) can be used to track the object.
When cloud cover permits, a wide angle (8 to 15 degree FOV) camera fixed
to the telescope can be used to implement a high accuracy (1 arc sec or
J. Forster wrote:
> These are all OPEN LOOP corrections. They are better than nothing, but
> nowhere near as good as properly implemented auto-guiding which is closed
> They are certainly not as good as even the simplest Adaptive Optics
> utilizing only a Tip-Tilt tracker.
>> One needs to know the local apparent sidereal time to aid initial
>> acquisition of the target.
>> Periodic error correction and using an autoguider are of little or no
>> use for this operation.
>> However correction for encoder error axis non orthogonality, encoder
>> eccentricity and bending of the telescope tube an the mount may be
>> A pointing model for the telescope is derived from the pointing (not
>> tracking errors) errors for a set of target objects uniformly
>> distributed over the sky.
>> For further details see:
>> Jim King wrote:
>>> I'm not sure your friend needs the time at all. Telescope tracking
>>> mounts have a periodic error due to non-perfect mechanical parts. The
>>> usual way to remove this error is to "train" the mount by manually
>>> guiding it through one or more periods, or - probably more common
>>> these days - use an optical autoguider.
>>> On 1:59 PM, Brooke Clarke wrote:
>>>> Hi Brian:
>>>> Why? Do you just want to see the sidereal time on a display or do
>>>> you need a digital output?
>>>> The Spark Fun "serial enabled" displays use what's called a "back
>>>> pack" that has the PIC 16F88 uC and it's used to do serial data to
>>>> LCD parallel data can control lines. I've made some clocks using
>>>> that chip.
>>>> A friend is setting up an observatory where the pointing accuracy of
>>>> the telescope mount is specified as " 7 arcseconds or less
>>>> peak-to-peak periodic error before correction". Much better after
>>>> correction. That implies he needs to know what time it is within
>>>> tens of milliseconds.
>>>> We looked into different ways to get the time into his computer to
>>>> that accuracy and NTP looks like it will fill the bill, so a GPS
>>>> receiver may not be required.
>>>> Have Fun,
>>>> Brooke Clarke
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