[time-nuts] Re Danjon Astrolabe

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Sep 26 19:06:46 EDT 2006

Poul-Henning Kamp wrote:
> In message <451976CA.1000602 at pacific.net>, Brooke Clarke writes:
>> 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.
> If you want to seriously play with this, study the Hipparcos
> astrometry missions telescope design carefully.
> You can probably find the docs if you start here:
> 	http://www.rssd.esa.int/Hipparcos/
> Historical tidbit:  The Hipparcos main catalogue contains only 100k
> stars, but Erik Høg (Copenhagen University) proposed some minor
> changes to the navigation sensors and have used the resulting data
> to make first the Tycho-1 catalogue of 1M stars at slightly lower
> precision, than the Hipparcos catalogue, and then reprocessing the
> data he subsequently generated the Tycho-2 with 2.5M stars.
> Poul-Henning
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One of the most important aspects of an astrometric telescope is 
stability of its focal length.
Your options are :
1) A refractor employing conventional glasses definitely not an 
apochromat using ED glasses as the temperature coefficient of the focal 
length of such apochromats is quite large.

2) A reflector using a single powered mirror with a field corrector such 
as a Schmidt, Maksutov, or even a a hyperboloidal primary with a small 
zero power refractive field corrector.

3) A Schupman Medial (uses a single glass objective together with a 
Mangin mirror and a field lens or mirror to produce a instrument with 
extremely small chromatic aberrations). However the filed of view may be 
too small. The 1m Swedish solar coronagraph uses a schupman medial design.

The equivalent focal length of mirror telescope designs using multiple 
powered mirrors such as Cassegrains, Gergorians and their variants 
depends critically on the spacing between the mirrors. Whilst heroic 
measures such as using invar or carbon fibre spacers can help its better 
to use a design which doesn't have this inherent instability. If you are 
use a red or yello filter an apochromat isn't necessary.

For observations during the daytime its also important to use a sunshade 
to avoid solar heating of the lens or mirror cell.
This avoids heating induced spherical aberrration due to radial 
temperature gradients in the lens or mirror.
It is possible to design a refractor objective with low sensitivity to 
radial temperature gradients, however special glasses are required for 
the best performance.
Baffling to minimise stray light is also critical, this is simpler with 
refractors and single powered mirror designs although using a Lyot stop 
can be effective even with a Cassegrain or Gregorian.

The design of the HST fine guidance sensors may also be of interest.

Atmospheric seeing will affect the accuracy of your measurements.
Techniques that work well in the vacuum environment of a satellite with 
no atmospheric seeing effects may be less effective when used at the 
bottom of the atmosphere when the stellar image isnt diffraction limited.


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