[time-nuts] Re Danjon Astrolabe

Brooke Clarke brooke at pacific.net
Tue Sep 26 14:51:54 EDT 2006


Hi Bill:

When C-Band satellite TV first started the dish I used came from a Mills 
Cross array at Stanford consisting of maybe 16 each 10' diameter spun 
aluminum dishes used to study the features of the Sun at about 
2.something GHz.  This took up some number of acres of land.  The hour 
angle of all the dishes was driven by a central motor and line shafts to 
each dish and the elevation was set manually on each dish once a month.  
All the feeds were line stretched to have the same phase (it was done by 
turning on and off a florescent tube in front of the feed horn on each 
dish.  This system had resolution much smaller than the diameter of the 
sun and could make maps of it's surface.  This dish out performed much 
larger dishes when used for 3.7 - 4.2 GHz TV because it's surface was 
good to 10 GHz and because it had a 0.3 f/D ratio which is optimum for 
G/T whereas the commercial Satellite TV dishes were made with f/D around 
0.4 something that was the highest gain ratio.  But I doubt that you can 
get millisecond timing with such a wide beam from a single dish.  I no 
longer have that nice dish and mount.

The beam width for my X-band internet connection is maybe 4 times the 
diameter of the sun, see:
http://www.pacificsites.com/~brooke/Sundial.shtml#SSB
so it's unlikely you can get millisecond timing using one of them.

A telescope and PIN diode might be the best way to get precise timings.  
If a fixed scope is used with a reticule where the star crosses twice or 
more then a plot of brightness vs. time will have a flat top pulse for 
each line crossed.  The leading and trailing edges of these pulses can 
be measured to hopefully milliseconds.

Have Fun,

Brooke Clarke

w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com



Bill Hawkins wrote:

>I wonder if it is possible to use radio astronomy. You'd
>have to find an object that doesn't require a very large
>antenna to acquire its signal.
>
>Collins Radio once built a sun tracker for the Navy that used
>microwave radiation from the sun. The antenna was driven by a
>servo that maximized the signal.
>
>I understand that optical tracking of a star crossing a hair
>is more precise than the peak of a radio signal, but perhaps
>modern signal processing would fix that.
>
>Or maybe you don't have as many cloudy days as we do.
>
>Regards,
>Bill Hawkins
>
>
>-----Original Message-----
>From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
>Behalf Of Brooke Clarke
>Sent: Tuesday, September 26, 2006 12:48 PM
>To: Discussion of precise time and frequency measurement
>Subject: Re: [time-nuts] Re Danjon Astrolabe
>
>Hi Bruce:
>
>I'm interested in automatically measuring the earth's period by looking
>close to straight up with a fixed telescope.
>There are automatic star trackers that can "see" stars in the day time
>so the only time you would not see the star is when there's cloud cover.
>By using a reticule it should be possible to detect individual stars and
>discriminate other bright objects.  I've started a web page on Stellar
>Time Keeping at: http://www.pacificsites.com/~brooke/StellarTime.shtml
>although many of the patents linked on that page are aimed at satellite
>navigation there are a couple designed for surveying on the earth.  But
>if the position of the instrument is known then you can get the time of
>star meridian crossing.
>
>Have Fun,
>
>Brooke Clarke
>
>w/Java http://www.PRC68.com
>w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
>http://www.precisionclock.com
>
>
>
>Dr Bruce Griffiths wrote:
>
>  
>
>>>In article <44D8C0BB.1030608 at pacific.net 
>>><https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts>>, Brooke 
>>>Clarke <brooke at pacific.net 
>>><https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts>> writes
>>>   
>>>
>>>      
>>>
>>>>/Hi Geoff:
>>>>     
>>>>
>>>>        
>>>>
>>>/>/
>>>/>/Thanks for the reference to "Geodesy".  Do you know if the first 
>>>edition />/has the Danjon information?
>>>/>/I ask because the second editions are rather pricey and the 3rd and
>>>      
>>>
>
>  
>
>>>4th />/are not available at all.
>>>/>/Could someone on this list make a copy of the Danjon section?
>>>/>/
>>>/I'm quoting from memory about the workings of the Danjon Astrolabe - 
>>>the book was borrowed from my local library, and I don't remember 
>>>which edition it was.
>>>
>>>Nor can I quote chapter and verse, I fear - although I will attempt to
>>>      
>>>
>
>  
>
>>>re-borrow the book, and copy the relevant section. Later this week, I 
>>>hope.
>>>
>>>AFAIR, the discussion was of the workings of various instruments for 
>>>determination of time from the stars, together with ways to mitigate 
>>>the systematic errors in each instrument. "Personal equation" figured 
>>>large in all this, which is why the PZT was preferred.
>>>--
>>>Geoff Powell
>>>   
>>>
>>>      
>>>
>>The explanation of the operation of the Danjon Astrolabe is somewhat 
>>oversimplified.
>>There was a motor driven optical mechanism that was used to keep the 2 
>>stellar images superimposed for some time so that a sequence of 
>>observations could be taken on a single star.
>>This significantly reduced the personal equation on the measurements 
>>obtained with this impersonal version of the astrolabe.
>>
>>I remember a book on Astrometry (sorry cant remember the title) that 
>>had detailed cross sections of the astrolabe together with a very 
>>detailed description of how it worked.
>>A modified version of the Dajon Astrolabe is currently in use at 
>>Santiago Chile for Solar astrometry.
>>
>>
>>
>>Bruce
>>
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>>
>> 
>>
>>    
>>
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